AMP block

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The information on this page supplements the official manuals.

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Contents


The Amp block

Scroll to the end of this page to read more about amp modeling.

Amp block: supported by which Fractal Audio products?

X/Y switching in the Amp block

The Amp block supports X/Y switching.

Switching an Amp block between X and Y causes a short gap in the sound.

Fractal Audio:
"Our modeling is very detailed and whenever the amp model changes the "virtual circuit" needs to be reconfigured. We can switch quickly but often this causes clicks and pops due to the reconfiguration. So the amp block needs to run silently for a bit so things will settle. Then we unmute the block. This mute period is what you are hearing. Most other products just use a fixed algorithm and change the input/output EQ and gain. With this type of algorithm you can switch quickly without clicks and pops but, of course, you don't get the realism and detail." source

Why no separate preamp and power amp blocks?

Fractal Audio:
"It's not impossible but it has implementation difficulties. The main problem is that the amp block is nonlinear and therefore oversamples the data. Any effect inserted between the virtual preamp and power amp would need to also run at the oversampled rate which means many times the CPU usage. For example, if the amp block is running 8x oversampled then the CPU usage for any effect inserted would by 8x as much (I'm not going to disclose our actual oversample rate). The other way is to dowsample back to native sample rate, run the effect(s), and the upsample again. No problem right? Except the no-free-lunch theory gets in the way. Downsampling and upsampling add latency." source

Fractal Audio:
"If you only need power amp modeling, for example when using an external pre-amp through the Axe-Fx, use the Tube Pre model. It uses the Vintage tonestack which is flat when the tone controls are at noon." source

Disable power amp modeling when using a tube power amp

Disabling power amp modeling in the unit is useful when using a separate (not neutral) tube power amp for amplification.

This can be done in the Global menu. It's not the same as bypassing the Amp block, because it applies to power amp modeling only, not preamp modeling.

Fractal Audio:

Owner's Manual (about switching off power amp simulation): "In this mode, MASTER works as a simple volume, DEPTH is deactivated, and PRESENCE turns into a simple shelving filter."

Authenticity of the Amp block controls

Firmware 6: "In general most knobs now behave exactly like the actual amp when possible. In a few instances there may be minor discrepancies between the knob position of the model and actual amp due to programming constraints and/or peculiarities of the actual amp (such as poor potentiometer tolerance). Due to variations in presence circuit topologies the taper of the Presence parameter, in particular, may vary between the model and the actual amp. In other words, a different setting on the model may be required to achieve the same response as the actual amp. In most cases however, the Drive, Treble, Mid, and Bass knobs will be accurate to within 10% of the actual amp."

If the real amp has two gain controls, the one which is the closest to the 1/4" input is modeled as Input Drive in the model. The other one is Overdrive.

If the real amp has two inputs (f.e. low and high), the model is based on the high input. Set Input Trim to 0.500 to get the equivalent of using the low input.

Fractal Audio:

Conversion chart for real amp settings versus amp models

The range of the gain taper in the Axe-Fx is 0-10. Volume on Fender amps go from 1 to 10. This translates to (source):

Frequency respons and polarity of the amp models

Fractal Audio:

Output level of the amp models

The amp models have varying output volume levels. Adjust the Level parameter in the Amp block to compensate.

Fractal Audio:
" If you put the MV at full blast the level is dependent upon the amount of preamp gain. The voltage at the onset of clipping does not determine the perceived volume. As soon as things distort the perceived volume is depended upon the amount of distortion which is determined by the amount of gain and the signal level into the amp (which is dependent upon pickup strength, string gauge, technique, drive pedals, etc., etc., etc.)." source

Re-select or reset: Amp block defaults

There are two ways to set the Amp block parameters to their default values: re-select the Amp Type (soft reset) or reset the block (hard reset). These methods have different results.

When a new amp type is selected within the AMP block (on hardware or in the editor), that amp type is loaded with its default parameters. Meaning: the default parameters applying to that specific amp type. But not all! The basic drive and tone controls are left untouched. These are: Drive, Overdrive, Bass, Mid, Treble, Output Level. This method of defaulting is referred to as "deselect-and-reselect" or "soft reset".

The affected parameters are: PRE: Input Trim, Cut/Fat/Brt switches, Bright knob PWR: everything except Output Level SPKR: everything EQ: all sliders and EQ Type PWR DYN: everything except Comp Type PRE DYN: everything except Output Level DYNEQ: everything except Char Type, Char Q ADV: everything except Input Select TREM/MIX: Trem Freq, Output Level, Balance, Byp Mode

If you want every parameter in the AMP block to return to its default value for the selected amp type, you need to reset the block, on the hardware or in the editor. This loads the default parameters for that specific amp type, including common parameters (Bass, Treble etc.). This is referred to as a "(hard) reset".

Wicked wiki

More or less amp gain

Here's a list of things you can do to increase or decrease amp gain. Also read this Wicked Wiki thread. Note: adjusting Input Level in I/O does not affect gain!

Cliff's Tech Note about the different gain controls

Switch between amp tones

Some ideas:

Number of Amp blocks

Each Axe-Fx II preset can contain one or two Amp blocks (the AX8 has a single one). One DSP inside the Axe-Fx II is devoted entirely to Amp block(s) processing.

Fractal Audio:
"There is actually a small amount of processing for the amp blocks done on the master DSP. That, along with inter-DSP communications, uses about 2% of the master DSP." source

When using a single Amp block, the Amp block always runs at double the internal sampling frequency. This happens automatically; there's is no parameter. Main benefit of this is less aliasing in high gain models.

Fractal Audio:

When using a single Amp block in a preset, always use AMP 1 (not AMP 2), or you might experience a "pop" when switching to that preset.

When using two Amp blocks, always run them in two parallel grid rows. Do not place them on the same row.

Fractal Audio:
"Don't put amp blocks in series. Especially when one is clean and the other is high-gain. Always put them in parallel and set the bypass mode to Mute." source

When switching between two Amp blocks in parallel rows, make sure to set their Bypass Mode to Mute, to prevent bleed-through of dry signal from the bypassed Amp block.

Adding a Drive block before just one of the two Amp blocks, may cause latency. Solution: add a bypassed Drive block to the other row. source

There are many ways to switch between two "amp routings". For example by using scenes, X/Y or two rows. If the routings also need to contain things like a Drive block or Compressor, here's how you do it, courtesy of Bakerman (source):

  1. Set IA switch to Vol/Pan 1 Bypass.
  2. At Axe-FX I/O: CTRL page set Vol/Pan 2 to the same CC# as Vol/Pan 1.
  3. Begin one path with Vol/Pan 1, the other path with Vol/Pan 2.
  4. Make these settings:
    1. Vol/Pan 1: Volume 10, Bypass mode = mute
    2. Vol/Pan 2: Volume 0, Bypass mode = thru
  5. Store the Vol/Pan blocks both on or both off per scene. You'll get signal from #1 only when they're engaged, #2 only when they're bypassed.

Each Amp block has X/Y sets which let you switch between sets of parameters. Switching between X/Y causes a short audio gap. Switching between two amp blocks in the same preset, instead of using X/Y, prevents the gap.

Minimize the audio gap when switching between amps

There are several ways to switch between different amp tones.

Fractal Audio (about the gap when switching between X and Y):
"Our modeling is very detailed and whenever the amp model changes the "virtual circuit" needs to be reconfigured. We can switch quickly but often this causes clicks and pops due to the reconfiguration. So the amp block needs to run silently for a bit so things will settle. Then we unmute the block. This mute period is what you are hearing. Most other products just use a fixed algorithm and change the input/output EQ and gain. With this type of algorithm you can switch quickly without clicks and pops but, of course, you don't get the realism and detail." source

Other methods:

The time to switch scenes and presets, causing the gap, decreases with lower CPU usage.

Pop when switching between amp models

Fractal Audio:
"Switching between certain amps can do this. We can eliminate it but then there would be a longer audio dropout when switching." source

Audio gap when entering the Amp block's Edit mode

If you press Edit to adjust Amp block parameters, there will be a short gap in the audio stream because of processing tasks. source

Noise, fizz, crackle, sizzle, intermodulation distortion

"Fizz" and "crackle" are terms to describe certain things people hear when playing an amp model:

People play through their Axe-Fx II or AX8, hear fizz and/or crackle and think there must be something wrong with the modeling or their settings. In fact, they are listening to the realistic equivalent of a real amp. Because real amps also generate fizz and crackle. You need to listen very closely to the guitar speaker to hear it. That's why you'll hear it better through the Axe-Fx II and AX8, because these emulate a close-mic'd guitar sound when using cabinet modeling.

Fizz and crackle attribute to the authenticity of amp modeling. They also help to make the guitar stand out in the mix. Don't fight them, accept them.

Fractal Audio:

More information:

Basic controls

AMP TYPES

Short description of all amp models.

Yeks Guide to the Fractal Audio Amp Models provides detailed information about each amp model.

On the Type screen, the ABCD knobs control Drive, Master Volume and Level, respectively.

Selecting another Amp Type resets many parameters to their default value for the amp type, see above.

INPUT DRIVE, OVERDRIVE, MASTER VOLUME

Input Drive and Master Volume:

Vintage amps don't have separate gain (drive) and master volume controls. Master Volume defaults to "10" in these models. Use Input Drive for volume and gain.

Fractal Audio:

Master Volume:

Firmware 10: "Amp models now default to a starting Master Volume setting when selected. Also, the proper setting for non-MV amps is now a Master Volume setting of 10.0. Non-MV amps, therefore, will default to a value of 10.0 when selected. If more MV drive is desired for non-MV amps, the new MSTR VOL TRIM parameter in the Advanced GUI page can be used to increase (or decrease) the Master Volume. The starting MV value for non-MV amps is roughly the “sweet spot” for the amp. This is the point where the power amp starts to contribute to the tone and feel of the amp. Decreasing the MV will typically cause the amp to get brighter and less compressed and increasing the MV will cause the amp to get more midrange focus and more compressed. As always, your ears should be your guide."

Firmware 15: "The amp modeling improvements have resulted in a significantly increased “sweet spot” for the Master Volume control. Previous advice to keep the Master Volume low for high-gain amp types no longer applies and, in fact, increasing the Master Volume can result in better tone (more bloom and swirl) and much better feel (due to power supply sag). Therefore most non-MV amps now default to a higher value than previously. This may result in louder preset volume which will necessitate reducing the Output Level to compensate."

The MSTR VOL TRIM parameter in the Advanced GUI page can be used to increase (or decrease) the Master Volume. Cliff: "Just multiplies the MV by the amount. You only need to use it if you want more power amp drive and your MV is already at 10. IOW, if MV is 10 and you set MV Trim to 2.0 then the MV will be 20." source

Fractal Audio:

Input Drive and Overdrive:

Some models have Input Drive and Overdrive controls. Input Drive is the same thing as Drive in earlier firmware versions. If the real amp has two gain controls, the one which is the closest to the 1/4" input is modeled as Input Drive in the model. The other one is Overdrive.

Firmware 10: The Amp block now differentiates amps that have both Input Drive and Overdrive controls, i.e. Mesa Mark series, Dumble, etc. When a model is selected for amps of this type, the menu shows both controls. For other types the menu shows only the Input Drive control (which was formerly called simply “Drive”). The Overdrive control defaults to noon when amps with this control are selected. As such, any presets based on these amps may need to be updated as this control was not present previously and the amount of drive may differ now. Note that these two controls are applied to the appropriate point in the circuit for the amp being modeled, i.e. for Dumble-style amps the Overdrive is prior to the last triode stage. In Mesa Mark amps the Overdrive is applied prior to the third triode."

Fractal Audio:

The range of the gain taper in the Axe-Fx is 0-10. Volume on Fender amps go from 1 to 10. This translates to (source):

You can attach a Scene Controller to Input Drive, and then use scenes to vary the amount of amp gain. Note that with some amps models this will temporarily increase CPU load a lot. Cliff: "Depending upon the amp model it can take a lot of CPU to calculate the Input Drive network. Some amps have simple networks that are rapidly solved. Others, like the Hook Lead and Rhythm models have complex networks that require more math. If you attach a modifier to the Input Drive it is constantly recalculating the network which increases CPU usage."

INPUT TRIM

This parameter lets you adjust the range of gain of the amp. It's the same thing as the Amp Gain parameter in the Global menu but Input Trim operates per preset.

Fractal Audio:
"They (Amp Gain and Input Trim) are basically the same thing. The global amp gain has a smaller range as it's designed to be for fine-tuning between guitars whereas the local trim allows you to radically alter the response of the model. The local trim is equivalent to -20 to +20 dB." source

If the real amp has two inputs (f.e. low and high) the model is based on the high input. Set Input Trim to 0.500 (= -6 dB) to get the equivalent of using the low input. source

Search this page using the keyword "trim" for information regarding specific amp types.

Input Trim can be attached to a controller in the Modifier menu for a variable boost. This includes scene controllers. Translate scene controller settings to Input Trim values.

Fractal Audio:
"You might want to convert the Input Trim parameter to dB if you're used to thinking of it that way. As a rule of thumb, every 2x multiplier = +6dB boost. In other words, Input Trim = 4.0 produces a +12dB boost." Here's a handy calculator" (source).

Fractal Audio:

BASS, MID, TREBLE

These are the usual tone controls: Bass, Middle and Treble. The exact behavior (range) depends on the amp model.

Fractal Audio:

If you don't like the effect of high gain on your low notes, turn down Bass, or increase Low Cut Freq, and then increase the low frequency level in the Amp's GEQ.

Fractal Audio:

BOOST

The Boost switch increases signal at the input of the Amp block with 12 dB.

Sometimes enabling the Boost switch works better than turning up preamp gain. It's a clean boost so it'll increase the gain across the entire frequency spectrum. BOOST is modifiable so it can be activated remotely.

Enabling Boost is the same as setting Input Trim at 4.

Boost can be assigned to an external controller.

Want an adjustable Boost: use a Null Filter block instead.

BRIGHT switch

Bright switch, located under the TREB knob.

The Bright switch is not to be confused with the BRIGHT knob which is shelving filter control between the preamp and power amp (see below).

The bright cap in real amps is used to compensate for guitars with weak pickup loads. Treble increases as gain is decreased. It changes the mids which affects gain too.

Turning up the amp's Drive or Master may decrease the impact of the Bright switch, depending on the amp type.

It's possible to change the effect of the Bright switch on the tone by adjusting the Bright Cap value.

In some amp types, such as Plexi, the Bright switch is set at a very high value and has a large impact on the amount of gain.

Fractal Audio:

BRIGHT knob

This high treble control is a shelving filter between the preamp and power amp and may be used to darken or brighten the output of the preamp. This control replicates the “Presence” control found in the Mesa Triaxis preamp when set to negative values (the Presence control in the Triaxis is actually a high frequency cut shelving filter).

This is not to be confused with the Bright switch (see above), which engages/disengages a capacitor across the drive pot.

Fractal Audio:
"The Bright Knob is an active fourth tone control at high frequencies. Think of it as "High Treble". You can use it to add a little zing to a preset or remove harsh high frequencies. You can also use it to simulate the behavior of the Presence control on a Triaxis (which is really just a high cut). Turn it down to simulate "Presence" settings less than 10." source

CUT

Located under the Bass knob. This reduces the amount of low frequencies into the amp simulation. This can be used to achieve a “tighter” tone or to reduce low-end “flub”.

Cut is similar to increasing Low Cut Frequency but still retains some low end so it doesn’t get thin.

Cut is a first-order shelving filter (high-pass) at 120 Hz.

Fractal Audio:

FAT

Located under the Mid knob. When engaged it shifts the center frequency of the tone stack, “fattening” the tone. It's similar to the FAT switch on a Mesa Boogie or Bogner amp.

Fractal Audio:

DEPTH

Depth is a low frequency tone control for the power amp section.

Fractal Audio:

MIMIC whitepaper: "Note however that the taper of the presence (and depth) control can deviate from the actual amp. In our tests we found that the presence control on many amps did nothing for the first 80% of its rotation and all the action occurred in the last 20%. We feel that this design anomaly is undesirable and therefore did not model that aspect."

Firmware 15: "Improved Amp block feedback network accuracy especially for those amps that have depth networks. This causes the Presence and Depth controls to interact (as they would on a real amp) but yields greater realism."

PRESENCE / HI CUT

Presence is a high frequency tone control for the power amp section.

If Negative Feedback (Damping) is zero, Presence turns into a HiCut control.

If Power amp modeling is off, Presence turns into a shelving filter where "5" is neutral.

The range (min/max values) of the virtual Presence knob in the model is the same as the range of the real Presence knob on the modeled amp. But the visual position of both knobs does not necessarily match, because the taper is not the same.

Fractal Audio:

Firmware 18: "Presence and Depth controls may not match the taper of the actual amp. On most amps the Presence control does nothing until you turn it almost all the way up. This seems a bit silly so we make the Presence behave more logically. Same goes for the Depth control." source

The Presence control in the Amp block behaves like the actual amp rather than an idealized version. The Presence Frequency parameter is now a frequency multiplier rather than an absolute frequency as the frequency of the presence circuit depends on the Presence control position. The Presence Frequency parameter works by scaling the value of the virtual presence circuit’s capacitor value. Setting the Pres/Depth Type parameter to Active or Active Pres will override the authentic modeling and implement an ideal presence circuit with fixed center frequency.

Presence control is set to a default value when an amp model is selected. This is done because many amps, i.e. Double Verb, Deluxe Verb, et. al., have no presence control and the value should be set to zero for best accuracy. On the other hand some amps, i.e. Jr. Blues, 65 Bassman, et. al, have fixed presence networks. The Presence control will default to the appropriate value for these amps. For amps that do have a presence control the Presence parameter will default to a value that is deemed typical for the model.

MIMIC whitepaper: "Note however that the taper of the presence (and depth) control can deviate from the actual amp. In our tests we found that the presence control on many amps did nothing for the first 80% of its rotation and all the action occurred in the last 20%. We feel that this design anomaly is undesirable and therefore did not model that aspect."

When switching off power amp modeling, check the Presence setting. Cliff: "If you turn off power amp modeling always check the presence control. It changes from a "classic" control to a shelving type where 5.00 is neutral. I just spent an hour trying to figure out why this preamp model I am working on wasn't matching. Forgot to set the presence control to 5.00." source

Firmware 15: "Improved Amp block feedback network accuracy especially for those amps that have depth networks. This causes the Presence and Depth controls to interact (as they would on a real amp) but yields greater realism."

With some USA amp models, the Presence knob turns into a "Presence Shift" button.

Other parameters

AC LINE FREQUENCY

See PWR SUPPLY TYPE.

AC VOLTAGE (VARIAC)

This parameter sets the relative AC line voltage into the amp simulation implementing a virtual “Variac”. Note that normally the volume would vary with the Variac setting in a real amp. The simulation compensates for the volume change by applying the inverse. Some additional level correction may be required.

A Variac is said to be required to get the Brown sound (EVH). Try it around 75%.

Fractal Audio:

B+ TIME CONSTANT

Firmware 10: "High values of Sag along with low B+ Time Constant values can cause “ghost notes” when the supply type is AC (as in a real amp). Lower B+ Time Constant values will make the amp feel “faster” but too low can cause ghost notes."

Fractal Audio:

Quantum 2.0 release notes: "For convenience the virtual power supply voltage (B+) can now be monitored on the PWR DYN tab of the amp block. When the Supply Sag control is selected the gain reduction meter will display the supply voltage in dB relative to the idle voltage."

Quantum 2.0: "B+ Time Constant controls the capacitance of the virtual power supply. The more capacitance the "slower" the supply and vice-versa. Most guitar players like a fast supply but too fast will cause excessive AC ripple and create ghost notes (although I think a little ghost note is cool). When the supply is fast it will sag rapidly accentuating the pick attack and compressing after. This parameter works in conjunction with Supply Sag parameter. The time constant remains constant so if you increase Supply Sag the virtual capacitance decreases." source

Tech Note on ghost notes:
"A phenomenon present in some vintage amps is an artifact known as "ghost notes". Ghost notes are the result of intermodulation distortion between the note being played and ripple on the power supply. The ripple is at 120 Hz because the AC voltage is full-wave rectified. So there are frequency components of 120 Hz and its harmonics in the power supply. These frequency components mix with the note being played and create new tones that are not harmonically related to the note being played. Since it is intermodulation distortion, tones are created at the sum and difference frequencies. For example, if you play a D at the seventh fret on the G string this is 294 Hz. The intermodulation will create new tones at 294 - 120 = 174 Hz and 294 + 120 = 414 Hz. The harmonics of the note being played also factor in. The aforementioned D will also produce tones at multiples of 294 Hz and these mix with the 120 Hz and its harmonics. The G string above the 5th fret is most prone to this because of the harmonic spectrum of those notes. The amount of ripple on the supply is a function of the supply impedance. More capacitance and less resistance will reduce the ripple. Conversely less capacitance and/or more resistance will increase the ripple. You can adjust these values in the Axe-Fx using the Supply Sag and B+ Time Constant parameters. Supply Sag adjusts the virtual resistance of the power supply. B+ Time Constant adjusts the resulting time constant of the supply resistance and capacitance, i.e. as you increase the sag the time constant stays constant (capacitance decreases). To counter this increase B+ Time Constant. Old 100W Plexis exhibit this the most of any amp I've seen due to the high resistance of the power supply transformer. Our reference 100W Plexi has so much power supply resistance that the power supply sags up to 120V! This along with only 50 uF of power supply capacitance leads to prominent ghost notes." source

BIAS EXCURSION

Fractal Audio:

BRIGHT CAP

See BRT (Bright switch).

CF (PREAMP) COMP, CF PREAMP TIME, CF PREAMP RATIO, CF PREAMP HARDNESS

See PREAMP COMP.

CHARACTER TYPE, CHARACTER FREQUENCY, CHARACTER Q, CHARACTER AMOUNT

These parameters control extremely powerful “inverse homomorphic filters”. When playing softly these dynamic filters have little effect on the sound. As the amount of distortion increases, the influence of these filters increases. The Character Frequency control sets the center frequency of the filters while the Character control sets how pronounced the effect is. For example, to darken the tone when playing harder, one might set the frequency to 10 kHz and the amount to -5. Setting the amount to +5 will make the tone brighter when playing hard. The amount defaults to zero whenever an amp type is selected.

This control is similar to Dynamic Presence and Dynamic Depth but the frequency is adjustable.

Fractal Audio:

Firmware Quantum 1.00: "Added a new mode to the “Character” controls in the Amp block. A Char Type of “Dynamic” engages an exciting new mode of tone control. This can be used to fatten or scoop the tone as a function of picking strength. For example, set the Type to Dynamic, Char Freq to 450.0, Char Q to 0.7 and Char Amt to 4.0. This will cause the tone to get fatter and thicker as you play hard but without getting honky when playing soft. "

CRUNCH

Introduced in firmware 18.

DEFINITION

This parameter allows changing the fundamental character of the amp from vintage to modern or vice-versa. Positive values increase the amount of upper overtone saturation whilst negative values reinforce lower harmonics. It's a treble boost/cut ("Tilt EQ").

DYNAMIC DAMPING

Firmware 13:

DYNAMIC PRESENCE, DYNAMIC DEPTH

Dynamic Presence models the output transformer leakage inductance that results in a brightening of the tone when the power amp is pushed. This control is set to a default value when the model is selected corresponding to the real amp, if applicable. Increasing this value results in a brighter response as the virtual power amp is pushed. When playing softly or at lower gains, the influence of this control is lessened. Note that this only affects the power amp modeling and is dependent on the degree of power amp overdrive. This control can also be set negative to cause the tone to darken when playing hard. This control can also be used to help “dial in” the sweet spot of an amp model. As the MV is increased an amp becomes more liquid, compressed and easier to play. However, the highs may get overly compressed causing the amp to sound too dark. The Dynamic Presence control allows you to get the desired power amp drive and liquid feeling and then bring the highs back without affecting the rest of the spectrum.

Another way to look at it: a distortion-dependent treble filter.

Fractal Audio:
"Transformer Grind is what you want to get that top-end sizzle. Dynamic Presence is one of my "Inverse Homomorphic" filters and only approximates the dynamic presence boost found in some amps. Transformer Grind is an authentic model of what actually happens in those amps." source

Dynamic Depth: analogous to the Dynamic Presence control, this increases or decreases low frequencies when the virtual power amp is being pushed. While real amps don’t display this behavior, it is a valuable tone-shaping tool.

Another way to look at it: a distortion-dependent bass filter.

Firmware 13: "Changed Amp block Dynamic Depth behavior so that frequency of action is set by the Depth Freq parameter rather than fixed."

EQ, EQ TYPE, EQ LOCATION

Press Enter to reset all sliders to 0 dB.

EQ TYPE selects the type of EQ.

HARMONICS

Introduced in Quantum 2. Do NOT use with Modeling set to firmware before Quantum 2. It controls the amount of interaction. Higher values yield softer distortion.

HIGH FREQUENCY SLOPE

This parameter allows fine adjustment of the high-frequency impedance of the virtual voice coil (which affects the slope of the impedance curve). A speaker voice coil is “semi-inductive” due to eddy current losses in the motor. This presents an impedance to the power amp that isn’t fully inductive nor fully resistive. The amount of resistive loss varies by brand and type. Reducing the Slope simulates a speaker that is less inductive, increasing Slope simulates a speaker that is more inductive. Typical speakers range from 3.0 to 4.5 with the median being about 3.7. Lower values yield greater midrange while higher values are more scooped and sizzly.

INPUT SELECT

Selects which side of the incoming signal should be processed by the Amp block.

Use this parameter with two Amp blocks in conjunction with Left and Right Cab blocks, when using a single preset for two separate guitar. Or for stereo separation when running dual Amp blocks with a single guitar. Or to keep the stereo spread of a stereo delay before the amps.

LOW CUT FREQUENCY, HI CUT FREQUENCY

Low Cut Freq controls the amount of lows the amp model sees. It's a blocking filter at the input (before distortion). Ranges from 10-1000Hz, with the lowest setting basically letting all the lows you feed it in. The main practical use for this is to tighten up a tubby bass end. Somewhere between 10-150Hz is generally where it will sound best for standard guitar tones.

Hi Cut Freq is a low-pass 2nd order filter positioned at the end of the preamp section that will chop all frequencies above the value you select. Ranges from 2000-40000Hz. This will make your top end sound smooth and silky, the lower the value, brilliant and defined, the higher the value. Try changing values from stock when you want to fine tune a sound.

Fractal Audio:

Quantum 3.00: "It's probably inaudible but there were a few amp models where the matching was indicating a loss of high-frequency response. This was traced to the high-cut filter. When the high-cut frequency is 20 kHz that means the response is 3 dB down at 20 kHz so you've still got some slight attenuation at, say, 15 kHz. So for the sake or accuracy it now goes to 40 kHz which pushes that pole well outside the audible range." source

LOW/HIGH RESONANCE FREQUENCY, LOW/HIGH RESONANCE Q, LOW/HI RESONANCE (SPEAKER IMPEDANCE)

"The speaker tab is not an EQ. It allows you to adjust the impedance that the virtual speaker presents to the virtual power tubes. In most cases the resulting EQ is quite different than the impedance curve since negative feedback flattens the response. If you turn the damping all the way down then the EQ will be close to the impedance curve (but still influenced by the transformer)." source

Fractal Audio:

MASTER VOLUME CAP

This parameter sets the value of the bright cap across the Master Volume pot.

Fractal Audio:
"Setting it to 1 pF defeats it." source

MASTER VOLUME LOCATION

Fractal Audio:

MOTOR DRIVE

This models the effect of high power levels on the speaker.

Keep it at zero when using a real cabinet (not a cab model).

It was revised in Quantum 7.0:

When using two Ultra-Res cabs in a preset, don't use Motor Drive on only one of them, because this will cause a hollow sound.

Fractal Audio:

AX8: Motor Drive is available in the Amp block only, not in the Cab block (unlike the Axe-Fx II).

NEGATIVE FEEDBACK (DAMPING)

Changing Negative Feedback (Damping) between 0 and another value will change the display on the hardware, and switches between Presence and HiCut.

Note that adjusting this parameter affects the amp's volume level. If the power amp is saturated, both damp and level must be increased to maintain level the same.

Fractal Audio:

Cliff talks about Negative Feedback (Damping) in this Tech Note. Excerpt: "The Axe-Fx II allows the user to fine-tune the amount of negative feedback in the power via the Damping parameter. The term damping refers to the fact that increasing negative feedback lowers the output impedance and therefore "dampens" the response of the speaker. Power amps often specify their output impedance in terms of "Damping Factor" which is the ratio of the load impedance to the output impedance. The higher the damping factor the less the speaker impedance influences the frequency response. Let's examine what happens as you adjust the Damping parameter. As we increase the Damping we increase the negative feedback. This does several things:

As we decrease Damping we decrease the negative feedback which does:

Many guitar players like the sound of amps with no negative feedback. The Vox AC-30 is the classic example of an amp with no negative feedback. The power amp distortion is soft and the scooped response along with lots of harmonic distortion give a bell-like tone for high frequencies and warm low frequency response. The drawback to this is that the low end can get muddy as the low frequencies clip readily due to the frequency response distortion. These types of amps typically do not work well for high-gain tones although there are notable exceptions, i.e. the Dual Rectifier which uses a high-power power amp and bass reduction in the preamp to compensate for the increased bass response. Fender and Marshall amps (and their derivatives) use varying amounts of negative feedback. The amount of feedback in Marshall amps was all over the map in the early years and seems as though the builders didn't really adhere to rigorous documentation and revision control. As such there can be quite a bit of variation in the sound of these early Marshalls. So what is the correct amount? There is no definitive answer however there are some guidelines. For more vintage tones less Damping is typically desirable. This gives softer power amp breakup and more "baseline" distortion. For modern, high-gain tones more Damping may be desirable as these tones typically rely on preamp distortion and the power amp is desired to be neutral (which many players describe as "tight"). As stated before the Dual Rectifier in modern modes is a bit of an enigma. The power amp in this mode uses no negative feedback. You can hear this as an increase in volume when you flip the switch to Modern (remember that negative feedback reduces the gain so turning it off will increase the gain). I have read some users recommending increasing the Damping to reduce the amount of power amp distortion in, for example, Fender models. I do not endorse this viewpoint. The distortion is primarily reduced because the gain is reduced but the power amp will sound more "sterile" due to the increased linearity and flatter frequency response. A better solution is to simply lower the Master Volume. This drives the power amp less while retaining the baseline harmonic distortion, softer transition into clipping and more scooped frequency response. Modification of the error signal is commonly employed in guitar amps. This was first employed as the ubiquitous Presence control. The presence circuit reduces the amount of feedback at higher frequencies. Since the gain of the power amp is inversely proportional to the amount of feedback, reducing the amount of feedback over only a certain range of frequencies will therefore increase the gain of the amp at those frequencies. The presence control therefore boosts high frequencies. This concept was extended to low frequencies via the Depth or Resonance control. Basically these controls are bass and treble controls for the power amp but operate by reducing the feedback for those bands. On most amps setting these controls fully CCW will basically remove them from the circuit. Turning them CW will reduce the feedback in the prescribed bands thereby increasing the gain of these bands. An exception are Boogie (Mark series) power amps. The presence control in these amps is more complex and the flattest frequency response is achieved with the knob set to noon. Turning the knob CCW will reduce the treble response. Turning it CW will increase it. Note that these all interact with the Damping control. To hear this reduce the Damping to a value just greater than zero. Note that the Presence and Depth controls will have almost no effect. This is logical since B (beta) is nearly zero and we can't reduce it further."

OUT COMP TYPE, OUT COMP AMOUNT, OUT COMP CLARITY, OUT COMP THRESHOLD

This parameter controls a compressor specifically tailored to reducing the output dynamic range of the Amp block. Note that this compressor runs in the master DSP and if set to a non-zero value will increase CPU usage. The Out Comp parameter controls the amount of compression (compression ratio). In the Advanced menu the user can also adjust the compression threshold via the “Comp Thrshld” parameter, if desired. The bar graph at the bottom of the menu displays the amount of gain reduction.

Firmware 19: "The Output Compressor in the Amp block now features two modes of operation: Output and Feedback. These modes are set using the new Comp Type parameter. “Output” is the previous type where the compressor acts on the output of the block. “Feedback” also compresses the block output but applies dynamics to the input of the block based on the output compression. The “Comp Clarity” parameter adjusts the bass response of the input dynamics and can be used to add clarity to the bass."

Firmware Quantum 7.02:

Fractal Audio:

A modifier can be attached to Output Comp.

Output Comp Clarity affects OUTPUT COMP. It adjusts the bass response of the input dynamics and can be used to add clarity to the bass.

OUTPUT LEVEL

The Output Level parameter in the Amp block (appears on several pages) controls the overall volume level of the preset, enables you to match it to other presets and prevents clipping the digital signal.

It's the best place to set the preset's overall level. Unless you want define the volume level per scene, in which case you should use gain in the Output block.

Fractal Audio:
"The amp block is always the place to set your volume. The Level control is repeated at several places in the amp block menus for convenience so you don't have to keep switching pages. The Level control has no affect on the tone." source

PI BIAS SHIFT (PHASER INVERTER BIAS SHIFT)

Added in firmware Quantum 7.00: "New algorithm also includes bias shifting which results in more harmonic spectrum variation with input amplitude. This improves feel, “knock” and creates sweeter single note soloing. The new “PI Bias Shift” parameter controls the amount of phase inverter bias shift. Note that some real amps are “spitty” in nature due to PI bias shifting, i.e. Trainwrecks, and the new algorithm is designed to replicate that behavior accurately. If you find the behavior undesirable reduce the PI Bias Shift value as desired although this will reduce authenticity."

PICK ATTACK

Controls a sophisticated dynamic range processor that operates on leading edge transients. Negative values reduce pick attack while positive values enhance it.

Not available on AX8.

Fractal Audio:
"It doesn't have any particular frequency. Pick attack is impulsive so, by definition, it contains all frequencies. The standard approach to reducing attack is to use dynamics processing. The Axe-Fx II has a Pick Attack parameter which can be used to reduce the attack but the AX-8 does not have this parameter. You can try using the Gate/Expander to soften the attack." source

POWER AMP CATHODE RESISTANCE

Fractal Audio:
"Sets the cathode resistance of the power tubes. Should only be used with "Class A" amps, i.e. AC-20 DLX, etc. Lower values increase the bias current. Note that some amps have separate bias resistors while others have a shared bias resistor. The choice of split/shared is not exposed to the user." source

POWER AMP HARDNESS

This parameter controls the hardness of the virtual power tube grid clipping.

Fractal Audio:

POWER SUPPLY TYPE

AC = Alternating Current. DC = Direct Current.

Firmware 10: "Amp block power supply modeling now models AC rectification and resulting supply ripple (if Pwr Supply Type is set to ‘AC’). The power supply type can be selected between AC and DC with the Pwr Supply Type parameter. The line frequency can be selected with the AC Line Freq parameter. Note that high values of Sag along with low B+ Time Constant values can cause “ghost notes” when the supply type is AC (as in a real amp). Lower B+ Time Constant values will make the amp feel “faster” but too low can cause ghost notes."

Fractal Audio:

POWER TUBE GRID BIAS (PA GRID BIAS)

This parameter can be used to adjust the offset voltage of the virtual power amp (this should not be confused with the Power Tube Bias parameter which sets the quiescent operating current of the virtual power tubes). Power Amp Bias allows the user to vary the symmetry of the clipping of the virtual power amp. A value of zero produces nearly symmetrical clipping which will produce very little even harmonics. Higher values will produce increasingly asymmetrical clipping which increases the amount of even harmonics. Small amounts of even harmonics can make the power amp distortion sound “warmer” and more bell-like while higher amounts will give a “fuzzier” tone. Most amps have some amount of offset and the amp models will default to a typical value. Note that this parameter is only applicable for push-pull power amp types. For single-ended power amps the Power Tube Bias parameter sets the symmetry (as always).

Fractal Audio:

POWER TUBE TYPE

Firmware 10: "Added Tube Type parameter to amp block. This allows selecting Tetrode (i.e. 6L6, KT66, etc.) or Pentode (i.e. EL34, 6BQ5, etc.) power tube types. The type defaults to the appropriate value when a model is chosen but may be overridden by the user."

Firmware 13: "Added selectable power tube types for Amp block. Available types are: EL34, EL84, 6L6, 6V6, KT66, KT88, 6550, 6973, 6AQ5 and 300B (triode). Also available are an ideal tetrode and ideal pentode. The power tube type defaults to the appropriate type when the amp type is selected but may be overridden by the user. The power tube type presets the Dynamic Damping parameter as well as several internal parameters."

Fractal Audio:

PREAMP BIAS

PREAMP BIAS sets the bias point of the last triode (cathode follower not counted). Depending on the bias points of the previous stages increasing or decreasing this value can alter both the harmonic content and the attack characteristics. Typically, if the previous stage has a negative bias then increasing this value will be more noticeable and vice-versa. This value is set to a default value for the model whenever the type is changed but can be overridden by the user.

Fractal Audio:

PREAMP CF COMP TYPE, PREAMP CF COMP, PREAMP CF TIME, PREAMP CF RATIO, PREAMP CF HARDNESS

Cathode follower algorithm.

The CF Comp parameter has been renamed Preamp Comp to better explain its function.

Preamp Comp Type selects between “Authentic”, which accurately models the compression in a tube amp, and “Ideal” which is an idealized distorting compressor. The idealized type is more focused and has tighter bass whereas the authentic type is bolder and looser. High gain players may prefer the ideal type due to its tight character."

PREAMP DYNAMICS

Dynamics processor that can be used to alter the dynamic response of the amp algorithms. When set below zero the amp compresses resulting in a smoother, less dynamic sound. When set greater than zero the amp expands resulting in a punchier, crunchier and more dynamic sound. Note that extreme values can have undesirable side-effects such as pumping and clipping.

Fractal Audio:

PREAMP HARDNESS

Previously called: Triode Hardness (before firmware 16). source

This parameter controls how sharply the triodes enter saturation and can be used to simulate softer or harder tubes. The default value is 5.0 and is set to this value whenever the type is changed. The effect of this is subtle and most apparent at edge of breakup. Lower values give softer saturation, higher values give a more aggressive breakup.

Fractal Audio:

PREAMP SAG

This parameter allows turning Preamp Sag modeling on or off. Turning it off replicates the behavior of separate preamp and power amp. Turning it on replicates the behavior of an integrated tube head or combo amp.

PREAMP TUBE TYPE

Firmware 18.04: "Added “Preamp Tube Type” parameter to Amp block. “Modern” (default) selects a triode characteristic representative of modern production tubes. “Vintage” selects a characteristic typical of tubes produced in the 50’s and 60’s. “Long Plate” replicates the softer saturation characteristic of so-called “Long Plate” triodes."

Firmware 18.06: "There are now only two Preamp Tube Type options in the Amp block: Short Plate and Long Plate. The Vintage type has been removed. Short Plate is similar to the previous Modern model but has the improved saturation characteristics that were developed for the Long Plate model."

Firmware 18.08: "There are now four Preamp Tube Types:

Firmware Quantum 1.00: "New RTS triode models. There are three new triode models based on our new algorithms: 12AX7A (default), ECC83 and 7025. The previous models are still available and may be selected with the Pre Tube Type parameter."

Firmware Quantum 2.01: "Further improvement of preamp tube models based on measurements. The existing theoretical models, i.e. “Modern”, “Vintage”, etc., have been removed. There are now six extremely accurate preamp tube types: 12AX7A, ECC83, 7025, 12AX7B, ECC803 and EF86. Note that the EF86 type has been normalized to have roughly the same gain as the triode types."

Firmware Quantum 6.01: "Updated all models to use an appropriate Preamp Tube Type when selected. I.e. British models will now use the ECC83 when selected."

Fractal Audio:

PRESENCE FREQUENCY, DEPTH FREQUENCY

Sets the frequency range of the Presence and Depth controls.

SATURATION SWITCH, SATURATION DRIVE

Enabling the SAT switch decreases power amp smoothing which results in meaner distortion. This switch is enabled by default in certain models such as Cameron ch. 2 amp. Try it with amp types such as Plexi, JCM800, Friedman and Mesa Mark.

Firmware 15.01: "Amp block Sat Switch now has three settings: Off, On (Auth) and On (Ideal). On (Auth) replicates authentic saturation circuit behavior and will lower the volume out of the virtual preamp. On (Ideal) replicates the idealized behavior present in Version 14.xx and earlier firmware."

Firmware 18: "The parameter Sat Drive controls the amount of "saturation" when the Sat Switch is on."

Fractal Audio:

Sat Drive controls the amount of "saturation" when the Sat Switch is on. The value is different for each model.

SPEAKER DRIVE

This parameter models speaker overdrive. It interacts with Master Volume. A value of 0.5 already commensurates with a small amount of speaker breakup. If you crank it, you'll get the sound of a blown speaker.

Keep it at zero when using a real cabinet (not a cab model).

Firmware 6.02: "Improved speaker overdrive modeling in Amp block. New algorithm captures the “throaty” sound of an overdriven speaker along with the gentle compression. The “Spkr Drv” (Speaker Drive) parameter has been moved to the Spkr tab on the Amp menu.

Fractal Audio:

SUPPLY SAG (MAINS IMP.)

There are two dynamics controls for the power amp section. SUPPLY SAG controls how much the virtual power supply sags. This is a complex interaction between the master volume (MSTR), transformer matching (XFRMR MATCH) and screen network. Depending upon the amp you may even feel the screen voltage bounce if the screen network is underdamped (amps with chokes can often be underdamped). The screen network parameters are automatically set when the model is selected and cannot be altered by the user. DYNAMICS is an idealized dynamic range processor which controls the power amp response independently of the aforementioned parameters although it is still somewhat dependent on master volume. In general, the more heavily driven the power amp section, the more effect the SUPPLY SAG and DYNAMICS controls have.

Turning Sag to 0 disables the entire power amp modeling for the Amp block in the preset. The Sag control has no effect at all when Power Amp Modeling is switched off in the Global menu. Cliff: "If you shut the power amp modeling off from the Global menu it is not exactly the same as turning it off by setting Supply Sag to zero. This is because the virtual power amp always runs. So if you shut the power amp modeling off from the Global menu the supply will still sag resulting in a more compressed response. If the Master is set high the sag can be quite pronounced." source

High values of Sag along with low B+ Time Constant values can cause “ghost notes” when the supply type is AC (as in a real amp). Lower B+ Time Constant values will make the amp feel “faster” but too low can cause ghost notes.

Owner's manual about switching off power amp simulation: "In this mode, MASTER works as a simple volume, DEPTH is deactivated, and PRESENCE turns into a simple shelving filter".

You use Supply Sag to simulate different rectifiers. Cliff: "Reduce to simulate solid-state, increase to simulate tube." source

Fractal Audio:

Quantum 2.0 release notes: "For convenience the virtual power supply voltage (B+) can now be monitored on the PWR DYN tab of the amp block. When the Supply Sag control is selected the gain reduction meter will display the supply voltage in dB relative to the idle voltage."

TONESTACK TYPE, TONESTACK FREQUENCY, TONESTACK LOCATION

The tonestack is the set of tone controls on an amplifier. Use the ADV page in the Amp block to select a different tonestack.

Firmware 6: "Reworked most tone stacks based on amp matching results. In general most knobs now behave exactly like the actual amp when possible. In a few instances there may be minor discrepancies between the knob position of the model and actual amp due to programming constraints and/or peculiarities of the actual amp (such as poor potentiometer tolerance). Due to variations in presence circuit topologies the taper of the Presence parameter, in particular, may vary between the model and the actual amp. In other words, a different setting on the model may be required to achieve the same response as the actual amp. In most cases however, the Drive, Treble, Mid, and Bass knobs will be accurate to within 10% of the actual amp."

With some amp sims, such as the Lonestar, moving the tonestack location results in loss of volume.

Fractal Audio:

TREMOLO FREQUENCY, TREMOLO DEPTH

Not supported on the AX8.

This is a true bias tremolo and works by varying the bias of the virtual power tubes. The tremolo action is therefore different than other types of tremolo and the amount of tremolo varies with a multitude of variables, most importantly the tremolo is “self-ducking” and decreases at higher signal amplitudes. Note particularly that bias tremolo is a somewhat crude tremolo circuit and it’s interaction with the power amp depends on many things including damping, bias, etc. On some amps high values of bias trem depth can result in excessive crossover distortion. On other amps the amount of tremolo can vary greatly between loud and soft playing. All this, however, is part of the allure of bias tremolo as it results in a particularly “organic” sound. Control of the bias tremolo is afforded by the Trem Freq and Trem Depth parameters. A modifier can be attached to Trem Depth to facilitate engaging and disengaging the tremolo via footswitch or for other applications.

When power amp modelling is disabled, the amp tremolo won't function.

Fractal Audio:

TRIODE1 PLATE FREQUENCY, TRIODE2 PLATE FREQUENCY

Fractal Audio:

Groovenut: "It will give you some control over the high harmonics that are created during clipping. The cap in question forms a low pass filter with the plate resistor on the triode stage. In English, it will allow you to control the buzziness that sometimes occurs with higher gain settings. It can also serve as a gain dependent tone control of sorts." source

Firmware 2.0 and up also expose Triode1 Plate Freq. and Triode2 Plate Freq. Release notes: "This parameter sets the cutoff frequency of the plate impedance for the next-to-last triode in the chain. Many amps have a capacitor across this triode’s plate resistor. This capacitor is used to smooth the response and reduce noise. You can adjust the amount of capacitance, and the resulting frequency, using this parameter. The last triode plate capacitor is also exposed: Triode2 Plate Freq."

XFRMR GRIND (TRANSFORMER GRIND)

Quantum 3.0: "Improved Amp block output transformer modeling. New model more accurately simulates dynamic core losses and leakage inductance. The “Xfrmr Grind” knob controls the intensity of the effect. Higher values result in more high frequency response and a more “open” sound. Very high values can yield a raspy, spitty tone common in vintage and/or low wattage amps. Modern “big iron” amps tend to have low values. Note that the audibility is dependent upon how hard the virtual power amp is driven and is more noticeable as the MV is increased. Also note that the effect in real amps is highly dependent on the speaker. Some speaker/transformer combinations exhibit significant high frequency dynamic boost while other combinations yield almost none. As always use your ears as the final determinant. Note: The Transformer Grind parameter will be set to a default value and the Dynamic Presence parameter will be reset to 0.0 for any presets created with previous firmware."

Fractal Audio:

XFRMR LF, XFRMR HF, XFRMR DRV (TRANSFORMER FREQUENCY, TRANSFORMER DRIVE)

Fractal Audio:

Firmware 10.10: "Transformer distortion modeling is now independent of transformer match value. Before the amount of distortion was also dependent on the match value making adjustment more difficult."

XFRMR MATCH (TRANSFORMER MATCH)

This parameter controls power amp clipping. Similar to adjusting MV and it can be used to mismatch a real amp and speaker. Decrease to make the amps sound more broad.

Firmware 3.0: "This is an extremely powerful parameter that sets the relative output transformer primary impedance which in turn controls how easily the power tubes are driven into clipping. The higher the Master Volume setting the more pronounced the effect of this parameter. Decreasing the matching causes the power tubes to clip later and therefore the phase inverter and grid clipping becomes more predominant. Increasing the matching causes the power tubes to clip sooner. At lower settings the speaker resonance will be more pronounced, at higher settings the speaker resonance will be less pronounced. For optimum results bring up the Master until the desired amount of power amp distortion is achieved, then adjust the matching until the character of the distortion is as desired. The various LF and HF resonance parameters interact strongly with this parameter so be sure to experiment with those as well when crafting your ideal tone. The value of this parameter is relative to the actual transformer matching which is set internally and not directly exposed. The value is reset to 1.0 whenever they amp type is selected."

Fractal Audio:

About amp modeling

General

Fractal Audio:
"The hardest part of modeling an amp is getting the various controls to match the actual amp. If you don't care if the tone, drive, etc. controls behave the same it's much easier as we have software that learns the input EQ, output EQ and gain. The problem is then people go "the model doesn't sound the same as my amp if I turn the drive all the way up and bass all the way down". So to accurately model the control behavior we need a schematic and the actual amp (as the schematics often don't indicate the pot tapers). Truth is amps are more similar than people think. You can make almost any high gain amp sound like any other high gain amp with a few EQ tweaks which is basically what the designers do. For example a Bogner is basically a boosted Marshall with a different treble pot taper. Another popular new amp is basically just a JTM45 clone with a couple minor changes. In fact the schematic I got from the designer was a JTM45 schematic with markups. The scary thing I've learned is that a lot of these amp "designers" don't really even understand what they are doing. They don't have degrees in engineering and lack even basic circuit theory. They take existing designs and tinker with them changing circuit values. The basic topology of the amps are unchanged. So many of these new amps are nothing more than clones of old designs with some minor changes. Things you can do in the Axe-Fx with all the EQ options available. There are only a handful of guys that really understand circuit theory and know what they're doing: Alan Phillips from Carol-Ann, Stevie Fryette, John Suhr, and several others. The vast majority are glorified technicians that are just making clones of existing designs with minor modifications. A good example is the Marshall 18W. There are numerous clones and amps inspired by this design. The problem is that the original design is flawed. You can make that amp sound much better with some minor changes to the phase inverter (or grid stoppers) but none of these amps do that. They all use the same PI design which overdrives the snot out of the power tubes making the amp shift into Class-B operation resulting in fizz and crackling on the decay." source

MIMIC

Multi-Point Iterative Matching and Impedance Correction (MIMIC) is a technology that identifies deviations in the response of the simulated amplifier to the actual amplifier and generates corrective data bringing a level of accuracy that has been heretofore unachievable. MIMIC was introduced in firmware 10.

MIMIC whitepaper (PDF)

If Power Amp Modeling is off, the pertinent aspects of MIMIC are defeated. source

Fractal Audio:

Firmware 4 and earlier

Amp modeling in the Axe-Fx II was originally called G2 and Virtual Vaccuum Modeling modeling (see Fractal Audio website). Part of it was ported back to Standard/Ultra firmware 11.

Fractal Audio:
"As you listen to clips from modelers what you start to recognize is a certain "stationary" aspect to the tone compared to the every-changing tonality of a tube amp. Another thing is finger response. With a good tube amp you can vary the tone quite a bit just by how you fret the note and attack it. Modelers tend to make every note sound the same. So I tested some hypotheses and came to the conclusion that it's because a real vacuum tube has a transfer function that is not static. The transfer function is dependent on time, frequency and amplitude. Where you really hear it is in the in-between regions where the tube is just starting to distort. At first I tried some dynamic transfer functions but that was a lesson in futility. So then I created the VVT stuff. In VVT there is an actual vacuum-tube replica in software. You enter the values of the resistors and capacitors on the grid, cathode, etc. and it behaves just like a tube complete with Miller effect, cathode memory, etc. The problem is that it requires an obscene amount of horsepower so the only solution was a dedicated DSP. The other big part of the G2 sound is the output transformer modeling. The OT distorts and as it distorts its inductance decreases which changes the bandwidth and loop characteristics." source

Firmware 5

Fractal Audio:

Firmware 6

Firmware 7

Firmware 8

Firmware 9

Firmware 10

Firmware 11

Firmware 12

Firmware 13

Firmware 14

Firmware 15

Firmware 16

Firmware 17

Firmware 18 (G3)

Firmware 19

Firmware Quantum 1.x

Firmware Quantum 2.x

Firmware Quantum 3.0

Firmware Quantum 4.x

Firmware Quantum 5.x

Firmware Quantum 6.x

Firmware Quantum 7.x

Firmware Quantum 8.x

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