From Axe-Fx II Wiki
- See MIMIC.
Splitting preamp and power amp
- The pre-amp and power amp sections of an amp model cannot be split.
- Power amp modeling can be disabled, more information.
- Cliff's comments:
- "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
- If you only need power amp modeling, for example when using an external pre-amp through the Axe-Fx II or AX8, use the Tube Pre model. It uses the passive Vintage tonestack which is flat when the tone controls are at noon. source
Amp block resolution: using one or two Amp blocks on the Axe-Fx II
- Each Axe-Fx II preset can contain one or two Amp blocks. One DSP inside the Axe-Fx II is devoted entirely to Amp block(s) processing.
- Cliff: "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.
- "In high-res mode the internal sampling rate is doubled so as to provide greater fidelity and resistance to aliasing. This mode is automatic and is selected whenever there is only one amp block in the layout grid. Adding a second amp block will revert to normal resolution. Note that switching between presets with differing number of amp blocks may introduce an additional delay as a “soft reset” of the amp blocks must be done whenever changing the resolution."
- "The oversampling rate is cut in half when running two amps. It's probably not noticeable. Even when running at half, it's as fast or faster than every other product available." source
- Cliff: "I don't hear a difference but some claim they can. A single amp block runs the amp simulation at 16x oversampling. Two amp blocks run each simulation at 8x." source
- When using a single Amp block in a preset, use Amp 1 (not Amp 2), or you might experience a "pop" when switching to that preset.
- Each Amp block has X/Y sets which let you switch between sets of parameters. Switching between X/Y causes a short audio gap. Alternatively you can switch between two amp blocks in the same preset, instead of using X/Y, which doesn't cause the audio gap.
- When using two Amp blocks:
- Run them in two parallel grid rows. Do not place them on the same row.
- Cliff: "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
- Switch between Amp 1 and Amp 2 using MIDI CCs. Or morph between them using an expression pedal. Example: add a Mixer block, input both signal chains into it, use the modifier menu to go from 0-100% on one amp and 100-0% on the other, adjust the curve responses for a smooth crossfade. Clarky's guide to amp morphing.
- Adding a Drive block before only only one of the two Amp blocks causes latency. Solution: add a (bypassed!) 2nd Drive block to the other row. source
- There are many ways to switch between two "amp routings". For example by using scenes or 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):
- Set IA switch to Vol/Pan 1 Bypass.
- At Axe-FX I/O: CTRL page set Vol/Pan 2 to the same CC# as Vol/Pan 1.
- Begin one path with Vol/Pan 1, the other path with Vol/Pan 2.
- Make these settings:
- Vol/Pan 1: Volume 10, Bypass mode = mute
- Vol/Pan 2: Volume 0, Bypass mode = thru
- 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.
Fizz, crackle, sizzle, intermodulation distortion
- "Fizz" and "crackle" are terms to describe certain things people hear when playing an amp model:
- Fizz: high-frequency "noise", which almost seems detached from the basic sound, floating above it. It's only there briefly after striking a chord or tone, like some kind of interference. Best heard when playing a model with power amp distortion.
- Crackle: side-effect of the initial attack, which halts but not gradually, it kind of stutters before it stops. Best heard when playing a clean amp model at high master volume.
- 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.
- Cliff's comments:
- "Fizz is desirable. Almost all high gain amps use a lot of feedback on the last triode stage to make it as "hard" as possible. This is typically done by using no cathode cap or a small cathode cap. You never see big cathode caps on the last stage of a high-gain amp. This gives lots of negative feedback and makes it clip harder. Another technique is to use negative feedback to the grid. This is done in 5150-based designs which makes the stage clip even harder. The last stage of a 5150 basically looks like a diode clipper. The harder the clipping the more overtones that are created and the better the tone cuts. Without those overtones your sound is lost in the mix. Amp designers know what they are doing. Trust them." source
- "If you start trying to dial out what you call "fizz" and "artifacts" you're going to end up with a tone that doesn't cut. It might sound good to you but it won't fit in the mix. That fizz and sizzle is what makes those classic rock tones work. Listen to some isolated tracks of VH and AC/DC and you'll hear a ton of high-end sizzle. In the mix, however, it's not noticeable. If you remove it then the guitar sounds dead." source
- "Fizz on the decay is natural and it's what tube amps do. If you don't like it you can reduce the Triode Hardness but then it won't cut in the mix as well." source
- "The "fizziness" of clipping is determined by how "hard" the clipping is. There are three primary places that clipping occurs in a tube amp: the preamp tubes, the phase inverter and the power tube plates. Preamp tube clipping can range from soft to hard depending upon the design. Phase inverter (PI) clipping, which is actually the power tubes grids clipping, is very hard. Power tube clipping ranges from soft to hard depending upon the amount of negative feedback in the power amp. Preamp tube clipping is comprised of cutoff, which is soft, plus saturation, which tends to be hard. Actual saturation rarely occurs because most preamp stages are designed such that the grid clips before the tube enters saturation. Grid clipping is hard. Local negative feedback is used in the form of cathode caps to shape the response of a preamp stage. If there is no cathode cap then there is negative feedback at all frequencies which increases the hardness of the clipping. The last stage usually dominates the clipping. Some amps have no cathode cap on this stage, e.g. JCM800, and therefore have hard preamp clipping. The Axe-Fx II does not expose the negative feedback settings for the preamp stages to the user, these are hard-coded. Reducing the Triode Hardness parameter will soften the clipping more-or-less depending upon the particular amp model. In a typical tube amp the power tubes start to clip right about the same time the PI/grid clipping occurs. This is intentional so as to get the most power from the tubes. However some amps are intentionally mismatched as the designer's intent was to get more power tube clipping than PI clipping (i.e. Trainwrecks). The Transformer Match parameter adjusts the relative onset of power tube vs. PI/grid clipping. Lower values will cause the PI/grid clipping to occur before power tube clipping. Higher values will cause the power tubes to clip before the PI. Note that the power tube plates follow the impedance curve of the speaker so while the PI/grid may be designed to start clipping first, this only occurs in the midrange. At frequencies above 1 kHz or so the power tubes clip first since the voltage on the plates increases as a function of the speaker impedance. The first thing to clip tends to dominate as once you enter clipping the effect of clipping elsewhere is diminished. Negative feedback around the power amp attempts to linearize the transfer function. The more negative feedback the more the power amp is linearized. However this also causes the clipping to become harder. A power amp with no negative feedback will go into clipping softly. As you increase the negative feedback the "knee" gets sharper. The Damping parameter is the negative feedback control. Higher values give more feedback and harder clipping. Presence and Depth work by modifying the negative feedback. As you increase them the feedback gets less so by turning up the Presence you get softer clipping in the power amp. Therefore to decrease the hardness of the power amp clipping: reduce Damping, increase Presence, increase Transformer Match. To reduce preamp clipping hardness reduce Triode Hardness. There is no parameter exposed to adjust the PI hardness. HOWEVER, the relative hardness of clipping is not all that audible. You have to listen closely. The IR is far more important in the final result. Some IRs let through a lot more high frequencies and therefore sound more fizzy. Furthermore overanalyzing this is inadvisable. Many amps are specifically designed to clip hard as this gives a more aggressive tone that fits better in the mix. Some amps actually attempt to increase the hardness of the clipping as much as possible by using diode clipping or using very high values of negative feedback (i.e. Modded Marshalls, Camerons, 5150 III). Listening at low levels fools your ear. Our ears are more sensitive to midrange at low listening levels. This means we hear the clipping differently than when listening at the actual level the real amp would be generating." source
- "When listening to things at a low volume fizz will be more noticeable. Power amp distortion can be particularly fizzy but we don't notice it because the amp is usually really loud when the power amp is distorting." And: "I was sitting in my chair about 3 feet from a 4x12 A/B'ing a JCM800 to the prototype Axe-Fx II some years ago. I was astonished at all the grit and fizz coming from the amp. The model was way too smooth. I sat there just playing a major 3rd interval listening to the crackle from the amp whereas the model did not have this. The big thing was the crackle on the note decay. The amp sounded like frying bacon as the note died out. It would go "gcchhhh crackkklleee bzzzzzz". I grabbed other amps and noticed the same thing from all of them. It was then that I realized I had a lot of work to do and spent a couple years researching how to make a digital amp modeler replicate all that stuff. All that stuff, though, gives you note separation and helps you cut in the mix. It turns out that tube amps do strange things when overdriven and those old concepts of soft clipping circuits are wrong. Tube amps get nasty and clip very hard and very asymmetrically." source
- "I prefer the "fizz" on probably because I grew up with it. I just like the extra grit. When you're playing in a group context that grit seems to make the guitar cut better and fills out the sound. Without it things sound sterile. I spent months trying to capture that. One day one of my employees came by the lab while I was working on the new algorithms. I was trying to explain the grit to him that I heard in my JCM800. "Hear that sizzle on top of the notes? Hear that raspy, bacon frying sound? That's what modelers are missing." So I spent months figuring out where that came from and how to replicate it. All IMHO..." source
- "The "buzziness" of IM is a function of the "hardness" of the clipping circuit. The first thing you need to do is determine what is distorting: preamp or power amp. Power amp distortion is typically harder than preamp distortion due to global negative feedback. If you are using a modern amp model, i.e. something with a Master Volume, then there is a very good chance you might be overdriving the virtual power amp. In this case simply lowering the Master Volume will reduce IM. If it's preamp distortion then you can reduce Preamp Hardness or experiment with the Preamp Bias to change the distortion character. The Axe-Fx II is extremely accurate. Understanding how real amps work will allow you to adjust things to achieve your ultimate tone. To put this to rest (again, for the umpteenth time): http://www.fractalaudio.com/tmp/blues_jr.mp3. One clip is the amp, the other is the model. All controls set the same. The reason that it's more "noticeable" with modelers is that there is a lot more high frequency content in a typical IR. When you listen to your tube amp through its speaker you are in the far field and usually somewhat off-axis. There is less high frequency content reaching your ears compared to what a microphone records. Go put your amp in an isolation room and listen to it through the control room monitors. You'll be astonished at what you hear." source
- "The "buziness" of IM is a function of the clipping hardness. As I stated before you need to determine whether you are getting power amp or preamp distortion. In general preamp distortion is softer but not always. It depends on the circuitry. You can reduce the clipping hardness with the Preamp Hardness and Power Amp Hardness parameters. This ONLY changes the shape of the virtual tube though. The final clipping behavior is also dependent on the surrounding circuitry which the user has no control over. For example a JCM800 preamp actually clips pretty hard because there is no bypass cap on the last triode's cathode. Therefore there is a lot of local negative feedback on that stage which makes the resulting transfer function "harder". In contrast a 1959SL Plexi has a bypass cap which softens the transfer function. Power amp clipping behavior is dependent upon the amount of global negative feedback. The less feedback the softer the distortion. That's why "Class A" amps have softer clipping as they have no negative feedback. Fenders, otoh, typically have 3-6 dB of gain reduction which makes the power amp clip harder. Therefore you can reduce negative feedback to soften the clipping behavior if your distortion is coming from power amp overdrive." source
- "Crossover distortion occurs when you play very lightly. To really hear the effect of crossover distortion turn the Power Tube Bias parameter all the way down and play lightly. You'll hear a scratchy sound. This occurs because the waveform gets a kink in it at the point where one tube stops conducting and the other starts. Some people actually like the sound and I've heard rumors that EVH liked his amps biased cold to get some crossover distortion. Output transformers can create a type of crossover distortion as well due to the BH curve being nonlinear at the origin." source
- (crackling sound through FRFR) "That's what amps sound like. Go put an amp in an isolation room with an SM57 on it. Listen in the control room. You'll hear the same thing. You don't hear it as much using a real guitar cab because the high frequencies are rolled off which softens the sound." And: "You're probably just hearing "tube crackle". This is most noticeable when letting chords ring out as the sound decays. It's more noticeable with FRFR because of the extended high frequency response of near-field IRs. Tube crackle occurs when playing more than one note typically. When you play multiple notes (as in a chord or even a diad) the amplitude "bounces" around as the multiple notes reinforce each other or cancel each other. The result is an envelope that is not uniform. The peaks of the waveform clip but the troughs do not. This causes a crackling sound when you get on the edge of distortion because the points at which distortion occurs are far enough apart in time to be audile. You can reduce it by softening the distortion. Reducing Preamp and/or Power Amp Hardness will reduce the crackling but this will deviate from authenticity." source
- ""Tube crackle" is most noticeable when letting chords ring out as the sound decays. It's more noticeable with FRFR because of the extended high frequency response of near-field IRs. Tube crackle occurs when playing more than one note typically. When you play multiple notes (as in a chord or even a diad) the amplitude "bounces" around as the multiple notes reinforce each other or cancel each other. The result is an envelope that is not uniform. The peaks of the waveform clip but the troughs do not. This causes a crackling sound when you get on the edge of distortion because the points at which distortion occurs are far enough apart in time to be audile. You can reduce it by softening the distortion. Reducing Preamp and/or Power Amp Hardness will reduce the crackling but this will deviate from authenticity." source
- More information:
Frequency respons, polarity
- "The frequency response is identical to the actual amps modeled. That is part of the modeling process. The models are EQ matched to the amps and the data stored in the firmware." source
- "Even without matching the modeling is so accurate now that any deviations are less than 1 dB." source
- "All amp models have the same polarity so that you can mix them without weirdness." source
Authenticity of the Amp block's main controls
- See AMP block parameters.
Amp modeling: Axe-Fx II 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.
- Cliff's comments:
- "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
Amp modeling: Axe-Fx II firmware 5
- Cliffs comments:
- "The new firmware (V5) is all about dynamics. The power amp modeling was totally rewritten. If you listen to those early VH albums you can hear the amps knock and ring when he hits them hard. Lots of work went into discovering why that happens and replicating it. It's due to several factors: the power supply sagging, the bias point shifting as the supply sags, the screens ringing and power compression in the speakers." source
- "Much improved grid modeling in Amp block preamp and power amp stages. New modeling very accurately replicates grid conduction and resulting bias excursion. This results in a more dynamic, thicker and bouncier tone. The power tube grid conduction parameters are exposed to the user in the GUI. The Bias Excursion parameter controls how much the grid voltage droops when the grids conduct. The Excursion Time and Recovery Time parameters control the time constants associated with the excursion."
- "Added dynamics processing to Amp block. A new tab, “DYN”, in the amp block, allows adjusting various parameters of the dynamics processor along with several other parameters related to amp dynamics. The Dynamics parameter controls the amount of dynamics processing and models the interaction between the power amp, power supply and loudspeaker under high power-level conditions. The Dynamics Time parameter (ADV tab) controls the time constant of the associated processing."
Amp modeling: Axe-Fx II firmware 6
- Reworked power amp modeling based on new research. The power amp modeling has been totally rewritten based on “amp matching” studies. This includes improved output transformer saturation modeling.
- Most amp models have been matched to their respective physical amps. Exceptions are those models which don’t have a physical counterpart, i.e. FAS MODERN, etc. The matching data is integral to the amp block and is transparent to the user. This has increased the size of the firmware slightly and also uses the master DSP for part of the calculations which increases the CPU load associated with the amp block slightly.
- Cliff's comments:
- "Almost everything has been reworked. Almost all the "real" amps are "matched" to the actual amp. This involved much more than just Tone Matching. It also included gain matching, harmonic content matching, tone-stack matching, etc." source
Amp modeling: Axe-Fx II firmware 7
- The dynamics processing of the amp block was totally rewritten for this release. A complex set of formulas was developed that completely describe the various voltages in a tube amp. Unlike other modelers that simply model an amps dynamics as a first-order compressor, the Axe-Fx II now accurately models the complex interaction of the power tubes with the surrounding circuitry including the power supply and screen voltage network. You may notice a difference in the feel of the various amp models. Please refer to the descriptions below to understand the operation of the various controls. Doing so will enable you to adjust the dynamics to your personal preferences.
- Improved preamp modeling. Harmonics now move more with input level which results in a more open and less congested tone.
Amp modeling: Axe-Fx II firmware 8
- Improved phase inverter modeling provides “juicier” tone when PI is driven hard (MSTR set high).
- Doubled resolution of internal amp matching data.
- Added input matching data to many amp models. While this may not be audible in many cases, especially for higher gain amps, it does affect the feel.
Amp modeling: Axe-Fx II firmware 9
- New power amp modeling with improved dynamic response. This new modeling features improved transformer/plate interaction modeling resulting in better feel and a punchier response. The Supply Sag parameter is more responsive as a result. Additionally, crossover and transformer hysteresis distortion modeling is improved resulting in more overtones when playing softly. This improves controlled feedback performance and yields a more aggressive tone at lower Power Tube Bias settings. Note that Version 9.xx is slightly quieter so don’t be swayed by Fletcher-Munson effects when evaluating differences.
- Greatly improved cathode follower modeling. The cathode follower modeling now varies the amount of distortion in addition to compression. This results in a more dynamic attack, improved feel and more “punch” and “thunk” (since this also creates low frequency energy into the power amp). The amount of cathode follower affect is controlled, as always, by the COMP parameter in the Amp block. Note that the higher the COMP value, the more effective distortion on sustained notes. Therefore as you increase COMP, you may want to decrease Drive. Also note that excessive values can result in pumping or blocking distortion. Note that the “cathode follower effect” occurs even in common cathode stages as well so even amps that don’t have cathode followers may exhibit some cathode follower-like effect.
Amp modeling: Axe-Fx II firmware 10
- Implemented “Multi-point Iterative Matching and Impedance Correction” technology (MIMICTM P.A.F.) to amp models. MIMIC applies analytic signals to an amplifier and captures the fine nuances of each amp at various points in the circuit and corrects each model vs. its theoretical implementation. In some cases the difference can be substantial, in other cases the difference is minimal. This depends on the layout of the amp and the various parasitics involved. MIMIC has the advantage of applying these corrections at the appropriate location in the amp model rather than as just an output EQ so that the various controls of the model behave virtually identically to the actual. For example, the Modern modes of a Dual Rectifier are highly sensitive to Master Volume with the tone becoming thicker as the MV is increased. MIMIC preserves this behavior rather than just getting louder as the Master Volume is increased. The equalization correction portion of MIMIC processing can be turned off in the Advanced menu tab of the Amp block, if desired. Note that the nonlinear correction and other aspects of MIMIC are integral to a model and cannot be turned off. In many cases the equalization correction can be subtle and many not be immediately audible when switched on or off.
- Updated numerous amp model details in light of MIMIC's identification of deviations between the models and actual amps.
- Tweaked power amp modeling slightly to increase even-order harmonics. This makes most models “sweeter”.
- Improved triode modeling removes “glare” from distortion yielding greater clarity and string separation.
- Improved power tube modeling gives more punch and pop, especially to tones that rely on power amp distortion. Additionally this provides a more dynamic response, better touch sensitivity and improved pick attack.
- Cliff's comments:
- "The models are statistically matched to the amps in both frequency response and distortion profile. In some cases this gives a certain sizzle. Some may not like this but it is accurate." source
- "The amp modeling in V10 is very similar to V9. There are only a few changes to the algorithms. The big difference between V9 and V10 is the inclusion of the MIMIC data and many of the cabs were remixed. Of those two the remixed cabs will be far more audible. So to get that V9 sound the most important thing is to go to the cab block and select the equivalent V9 cab(s). These are indicated by "(V9)" in the name. For example, if you preset is using "1x12 Brit G12H30" it has a remixed cab. Therefore change the cab to "1x12 Brit G12H30 (V9)". source
Amp modeling: Axe-Fx II firmware 11
- Improved pre-amp algorithms.
- Improved cathode follower algorithm. There are three parameters exposed for the cathode follower algorithm: Cathode Comp (which is also the COMP knob), Cathode Time and Cathode Ratio. Cathode Comp sets the amount of compression. Cathode Time sets the attack time of the compressor. Cathode Ratio sets the maximum amount of compression with lower values giving more compression.
- Improved power amp algorithms. New algorithms yields smoother highs and more open sound.
- Exposed “MV Cap” parameter in Amp block. This parameter sets the value of the bright cap across the Master Volume pot and is located in the Advanced menu.
Amp modeling: Axe-Fx II firmware 12
- Improved Amp block power supply current draw modeling which improves “feel”.
- Improved Amp block phase-inverter-to-power-tube-grid interaction modeling which improves transient response resulting in faster attack. This also improves the clarity of notes especially when using heavily overdriven power amp settings.
- The Amp block “Grid Modeling” parameter has been renamed and repurposed. The new name is “Modeling Mode” and offers the following choices:
- Authentic – Replicates a tube amplifier with the utmost accuracy.
- Grid Cond Off – Turns off grid conduction modeling in the power amp modeling. This reduces blocking distortion.
- Smooth – Sets Triode Hardness to the minimum value essentially creating an ideal preamp and turns off grid conduction modeling in the power amp. This removes most of the “nasty” distortion that tube amps create.
- Ideal – Removes most of the “warts” from the modeling which includes grid conduction, output transformer distortion, bias shifting and AC power supply modeling. Supply sag, screen voltage effects and crossover distortion are still modeled.
- Ideal/Smooth – Same as #3 but also sets Triode Hardness to its lowest value. This is in essence an ideal preamp plus an ideal power amp.
- The audibility of these settings is dependent upon the particular amp model and various parameters. The use of these modes in conjunction with other parameters can yield idealized tones not achievable with real tube amps. The first two choices duplicate the previous behavior of the deprecated “Grid Modeling” parameter so no changes are required to existing presets. Note that when “Ideal” is chosen the global Modeling Version parameter is irrelevant.
- Greatly improved “cathode squish modeling” for cathode biased power amp models. This improves the feel for affected amp types, i.e. Class-A, Mr Z, etc. Two new parameters have been added which allow the user to alter the pertinent variables: Cathode Squish and Squish Time. Cathode Squish sets the amount of bias shift due to cathode voltage rise and Squish Time sets the time constant of the cathode network. These parameters are set to default values upon selection of an amp type. Setting Cathode Squish to zero defeats the cathode squish modeling.
- Preamp modeling now uses screen voltage from power amp in calculations rather than operating independently. This improves feel as preamp voltage will drop with power amp sag. The effect is more noticeable as Supply Sag is increased. Note that preamp sag has a long time constant and, as such, the initial pick attack is relatively unaffected while sustained sounds undergo compression. This results in a “chewier” sensation.
- Improved single-ended power amp modeling.
- Cliff's comments:
- "The harder the virtual power amp is driven the more noticeable the improved preamp modeling will be. Before I was treating the models as separate preamps and power amps as though you were using a rack system with a preamp unit and a power amp. I did some tests and noticed that there is quite a bit of interaction between the power amp and preamp in an integrated amp (combo or head). So now the modeling feeds the power supply voltage from the power amp algorithm back into the preamp algorithm. As the B+ sags (and the screens droop and bounce) the preamp is affected as it will in a real amp. What happens is that as you hold a note or chord the B+ sags. The preamp voltage sags as well but at a slower rate due to all the extra capacitance and resistance between the screen voltage and the preamp tubes. Eventually the preamp voltages sag enough to compress 10-20% depending on the amp. This is a couple dB or so of compression. It's a slow compression though so your pick attack is unaffected. I don't think I hear much, if any, difference but the feel is definitely different. It feels a lot easier to play like I don't need to press on the strings as hard. I imagine the low end will be affected as the compression will cause less drive into the power amp and therefore less saturation at the resonance of the speaker." source
Amp modeling: Axe-Fx II firmware 13
- Improved power amp modeling via improved modeling of the plate impedance of the power tubes. This gives tighter bass (less flub) and warmer highs when the virtual power amp is heavily driven (higher Master Volume settings). This also improves the feel and dramatically increases the “3-dimensionality” of the tone. The plate characteristics are adjustable via the new Dynamic Damping parameter. This parameter defaults to the appropriate value when an amp model or power tube type is selected.
- 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.
- Added Preamp Sag parameter to Advanced menu in Amp block. This parameter allows turning the 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.
Amp modeling: Axe-Fx II firmware 14
- Improved Amp block triode model. New model more accurately replicates behavior when triode enters saturation. This results in a warmer and punchier tone with smoother distortion characteristics.
- Improved tone stack accuracy at extreme knob settings (i.e. when Treble is near maximum) by accounting for impedance loading. In most cases this is barely audible but for certain tone stacks that are heavily loaded there may be a small change in tone, especially as the tone controls are set near their minimum or maximum values.
- Improved power tube grid conduction modeling. This more accurate modeling improves dynamic response resulting in a more powerful tone and feel. This also improves post-PI Master Volume behavior.
- Removed “Dynamics” control from Amp block. This control is no longer deemed necessary due to the improvements in the underlying modeling.
- Renamed Amp block “Comp” knob “CF Comp” since this knob controls the amount of compression in the Cathode Follower (CF) modeling.
- Improved Amp block power amp dynamic impedance modeling.
- Improved Amp block X/Y switching time for presets with high CPU usage.
- Improved Amp block power amp dynamic impedance modeling.
Amp modeling: Axe-Fx II 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.
- Improved Amp block virtual power amp algorithm. The new algorithm improves realism and offers much more “punch” and a “crunchier” tone when overdriven.
- Improved cathode bias modeling. This improves so-called “Class A” amps (e.g. Mr Z Maz 38, AC-20 Dlx, etc.).
- Improved Amp block output transformer modeling by improving the accuracy of the flux density vs intensity curve (B-H curve) and resulting primary inductance. Because of this, the default Transformer Drive value for many amp models has been changed. Existing presets will be automatically updated to the new default value upon recall.
- 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.
- Improved Amp block power supply accuracy. This provides more open, less compressed response. Because of this, the default Supply Sag for many amp models has changed. Existing presets will be automatically updated to the new default value upon recall.
- Improved Amp block triode modeling. Models now incorporate reactive plate loading effects.
- Improved Amp block Hi Cut control accuracy.
- Changed Amp block so that 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.
- Added Power Amp Bias parameter to Amp block. 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).
- Added exact tone stack solutions for all amp models with treble roll-off networks, e.g. PVH 6160, Triptik, Tucana, 5153, etc.
- Changed Amp block Sat Switch operation. Sat Switch now operates like actual amp. Note that this will cause a reduction in level when the switch is engaged (as in the real amp).
- Added “Out Comp” (Output Compression) control to Amp block. 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.
- Removed Amp block Thunk parameter. This processing operation is no longer deemed necessary due to the improved Amp block algorithms.
- The Amp block Modeling Mode choices have been reduced to “Authentic”, “Smooth”, “Ideal” and “Ideal/Smooth”. In “Smooth” mode Triode Hardness is set to minimum. In Ideal mode grid conduction is defeated. In Ideal/Smooth mode grid conduction is defeated and Triode Hardness is set to minimum.
- Renamed “Damping” control in Amp block to “Neg Fdbk” (Negative Feedback) as this is a more accurate term.
- 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.
- Improved Negative Feedback calculation accuracy by including virtual feedback of phase inverter.
- Improved Amp block Dynamic Depth algorithm. The new algorithm uses the low- frequency speaker information from the speaker page to set the inverse-homomorphic filters resulting in a more musical control.
- Improved Amp block cathode follower. New algorithm includes loading on previous triode plate and resulting effect on distortion characteristics.
Amp modeling: Axe-Fx II firmware 16
- Improved Amp block cathode follower model. New model includes effects of grid resistance which results in more accurate clipping characteristics.
- Improved Amp block virtual power tube model. Improved algorithm more accurately models plate current vs. grid voltage yielding a more accurate transfer function which results in more pleasing power amp distortion and improved feel.
- Improved Amp block virtual power tube interaction with virtual output transformer. This yields improved low-frequency accuracy and feel. Furthermore this improves the output waveform accuracy resulting in much improved interaction with the cabinet block. IRs should now sound fuller and more resonant with more “thunk” and “knock”. The Low Res parameter in the Spkr tab can be used to increase or decrease the amount of knock.
- Added Power Amp Hardness parameter to Amp block. This parameter controls the hardness of the virtual power tube grid clipping. Models default to “Medium” and any presets created with earlier firmware versions will be set to Medium upon recall.
- Improved Amp block output transformer modeling. New algorithm more accurately simulates effects of inter-winding capacitance and the resulting load on the virtual power tubes. This results in improved high frequency response. Tones have more “chime” without being harsh. This is especially noticeable for amps with low negative feedback or when the Presence is turned up high.
- Added Slope parameter to SPKR page in Amp block. 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.
- Added “Dynamics” control to amp block. This parameter controls a 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.
- Added AC voltage control to Amp block. This parameter, called “AC Voltage (Variac)” in the Advanced menu 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 but the simulation compensates for the volume change by applying the inverse. This mitigates having to manually compensate using the Output Level.
Amp modeling: Axe-Fx II firmware 17
- Added “DYN EQ” GUI page to Amp block. This page now holds the Dynamic Depth, Dynamic Presence, Character Amount (Char Amt, previously named “Character”) and Character Freq parameters as well as two new parameters: Character Type and Character Q. Character Type selects between a shelving behavior or a peaking behavior. The previous behavior was always shelving. Character Q controls the bandwidth of the response when the peaking behavior is chosen. The Character parameters now control a powerful dynamic equalizer that can be used to achieve tones and feel impossible with a real amplifier. Negative values of the Character Amount along with a peaking behavior result in midrange scoop where the scoop increases as you play harder. Positive values result in a midrange boost as you play harder. Experiment with the frequency, Q and amount to achieve interesting dynamic response.
- Improved Amp block power supply modeling by accounting for DC sag due to quiescent bias currents.
- A “Presence Shift” switch has been added to certain “USA” amp models (those based on amps with a “Pull Shift” on the Presence knob). This switch is found under the Presence control and replicates the behavior when the Presence knob is pulled out on these amps. Note that the behavior of this switch is authentic and may result in volume reduction when active since the negative feedback is increased which lowers the loop gain.
- Improved power amp screen grid and anode voltage dependence calculations. This results in more “open” tone and improved dynamic response especially when the virtual power amp is overdriven. This has necessitated changes to the default Cathode Squish value for the some models. Existing presets will have the Cathode Squish value updated automatically upon recall.
- Improved power amp cathode squish accuracy by accounting for dependency on bias point.
- Improved power amp modeling. This results in better touch sensitivity, more explosive dynamics and improved low note clarity when the virtual power amp is driven hard.
Amp modeling: Axe-Fx II firmware 18 (G3)
- This firmware incorporates all-new amp modeling algorithms. With this update the Axe-Fx II now incorporates our “Generation 3 (G3) Amp Modeling Technology”. G3 modeling improves upon our previous modeling with better touch sensitivity, better dynamics, better distortion texture (or “grain”) with less fizz, more crunch and improved frequency response accuracy.
- 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.
- 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.
- Improved power tube grid conduction modeling. This yields better dynamics and edge-of-breakup tones, particularly when the virtual power amp is being overdriven.
- Improved Amp block phase inverter model by incorporating effects of power supply sag.
- Improved Amp block output transformer model by incorporating secondary losses.
- Improved Amp block power tube transfer functions.
- Cliff's comments:
- "G3 modeling is a proprietary technique that learns the characteristics of a nonlinear circuit." source
- "I have three units here in my lab. One has 17.04 and the other two have 18.00. A/B'ing them it's immediately apparent and as Manny noted the most apparent thing is the punch in the chest aspect. That's exactly what I mentioned about a week or two that I noticed being the biggest difference. You can feel the speakers "jump" for lack of a better term. One caveat, though, is that punch is dependent upon MV. The higher the MV the more punch but too high and things can get stuffy and/or raspy. You have to find the sweet spot and that depends on the other controls as well as your guitar and playing style. The higher the MV, the more the power supply sags and bounces. That power supply sag and bounce is partially what contributes to the punch." source
- "You should notice better midrange and treble clarity on certain high gain amp models compared to the beta. I did a lot of critical listening and comparing of the new algorithms over the last few weeks and noticed a tiny bit of "signature" to the sound. I traced that to the triode cutoff formula being too "soft". The new formula removes that signature which also yields increased clarity and string separation for models that rely on preamp distortion." source
- "In general the improvements are subtle but there should be improvements in the "bounce" and slightly smoother power amp distortion. I realized I wasn't accounting for the fact that bias excursion is a function of the B+ voltage. I also derived more accurate formulas for the behavior of the various power tubes when they enter saturation. The improved OT modeling results in fewer ultrasonic overtones which, while inaudible on their own do result in more intermodulation "garbage" when playing multiple notes so you should hear better string separation and chord clarity." source
Amp modeling: Axe-Fx II firmware 19
- Improved Amp block power tube plate current calculation. This results in increased clarity, improved feel/attack and better edge of breakup distortion texture. The default Power Tube Bias values have been updated to reflect this change and existing presets will be updated automatically to the new values.
Amp modeling: Axe-Fx II firmware Quantum 1.x
- This is the initial release of our “Quantum” firmware. Quantum represents a milestone in amplifier modeling technology. Our new Real-Time Spice (RTS) algorithms offer a degree of accuracy not found in any other product.
- 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. Note that the models have all shifted and if you were using a model other than default previously you will need to adjust your preset to the desired model.
- The “CF Comp” parameter has been renamed “Preamp Comp” to better explain its function.
- Added new Preamp Compressor Type “Comp Type” parameter to amp block. There is a new menu page that contains Preamp Comp, Comp Type, Dynamics, Preamp Bias and Output Level (the last repeated from other menus for convenience). 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.
- 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.
- Cliff's comments:
- "Personally I consider Quantum the biggest improvement from previous firmware than any other release. The day I first got it working I was so shocked at how good it sounded I jumped out my chair. I then crafted an email to Matt with the title "I did it!!!". I'm sure he'll be happy to verify." source
- "Q1.01 models the power tube plate current slightly more accurately than Q1.00. It's a subtle difference but some may prefer the more idealized sound of Q1.00 so I made it a menu option." source
- (1.03) Improved triode grid conduction model. This improves edge-of-breakup tones as it keeps the distortion on the edge of breakup more and improves high-gain tones as it tightens up the bass. Also improves feel and dynamic response.
- (1.03) Improved cathode follower model. This improves harmonic accuracy yielding less “glare”, more detailed treble (more “chime”) and more bite. The previous cathode follower model can be selected from the Global menu by choosing a Modeling Version other than 1.03.
- Cliff's comments:
- "Tube amps are amazing in some ways. The just naturally do the right things. One of these things is that the grids sort of "self-align" so that the distortion stays on the edge. You can tell when a modeler isn't modeling this right as you will get fizzy decay (some people erroneously call this "crossover distortion") and flubby bass. If you can't boost treble without it getting harsh then it's not modeling the grids properly. If you need to put a shelving EQ to roll off some bass before the model then it's not modeling properly. With this latest version of Quantum you can put an EQ after the amp or cab and crank the highs as much as you want without it getting harsh and causing ear fatigue." source
- (1.04) Improved triode saturation modeling based on new measurements. The new modeling results in more accurate saturation behavior which, in turn, results in smoother overall distortion characteristics and more accurate harmonic content. Due to the new algorithm the default Preamp Hardness value is now 6.0.
Amp modeling: Axe-Fx II firmware Quantum 2.x
- Improved tube modeling. New algorithms uses more accurate plate current formulas based on actual measurements rather than theoretical values. This results in smoother, thicker distortion and better dynamic response.
- Improved power supply modeling. New algorithms improve sag and feel. 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.
- Improved cathode bias algorithm for “Class-A” amp models (i.e. Class-A 30W, AC- 20 Dlx, etc). In conjunction with this the Cathode Squish parameter has been repurposed as “Cathode Bias” and controls the value of a virtual cathode resistor. A value of 50% is “optimum” and biases the power tubes at true Class A operation (neglecting any bias shifting due to supply sag, screen droop, etc.). Values greater than 50% increase the resistance and therefore bias the power tubes “colder”. Values less than 50% bias the tubes beyond Class A. In a real amp this would probably destroy the tubes but that limitation does not exist in our virtual amp. Most real amps of this type actually operate far below Class A and the default values for the models will reflect this. Note that the Power Tube Bias value should be set to 1.00 for these amp types (since that parameter controls the grid voltage and the grid voltage is at a maximum in these types of amps).
- 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.
- Improved power tube saturation modeling. This yields warmer, “tubier” distortion. The PA Hardness parameter is automatically set for each power tube type but may be overridden if desired.
- Improved tube interaction modeling. A new parameter, “Harmonics”, controls the amount of interaction. Higher values yield softer distortion. The default value is set for each amp model but may be overridden.
- Improved virtual output transformer saturation modeling.
- Cliff's comments:
- "The previous firmwares were based on theoretical tube models. One thing I began to notice is that the theoretical models simply weren't agreeing with measurements. So after much research we realized that the theoretical models are just that, theoretical and that real tubes do not behave exactly as theory predicts. I can't reveal the details of what aspects don't agree and why those aspects are important. Q2 is based on the actual tube curves which gives a different character to the distortion and a more bouncy and responsive feel." source
- "Many of the amp models had their speaker parameters updated based on new measurements." source
Amp modeling: Axe-Fx II firmware 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.*
- Improved triode plate modeling for cases when plate load is complex.
Amp modeling: Axe-Fx II firmware Quantum 4.x
- Improved power tube plate current vs. grid voltage accuracy. New models more accurately reflect how the response is higher order at lower grid voltages and “relaxes” into a lower order response at higher grid voltages. This improves feel when the virtual power amp is overdriven and improves harmonic content accuracy.
Amp modeling: Axe-Fx II firmware Quantum 5.x
- Improved preamp tube modeling for tubes driving cathode followers. In models that use cathode followers this results in warmer distortion with smoother decay. The shape of the cathode follower distortion can be adjusted with the new “CF Hardness” parameter found on the Advanced tab in the Amp block.
- Increased maximum amount of virtual power amp sag in Amp block. The maximum amount of sag is now twice what was previously available. Existing presets will automatically be adjusted to compensate for the increased range of the Supply Sag control, i.e. if the Supply Sag was set to 8.00 it will be adjusted down to 4.00 so as to yield the same amount of sag. High values of Supply Sag can be used to make the feel “easier” and/or replicate the behavior of other products that intentionally exaggerate compression.
Amp modeling: Axe-Fx II firmware Quantum 6.x
- Improved Amp block output transformer to power tube interaction modeling. This results in less “smear” in the overtones when the virtual power tubes are driven into clipping.
- Improved Amp block plate clipping modeling. New algorithms more accurately replicate the gradual onset of clipping and never fully hard clip resulting in a more dynamic and “open” tone. Note that the increased dynamics may result in some amp models having higher peak volumes which can result in clipping with existing presets. To remedy this simply turn down the Level control in the Amp block.
- Recalculated Amp block preamp tube parameters. The “12AX7A SYL” (Sylvania 12AX7A) is now the default type as we think it sounds the best.
- Improved background calculation accuracy for cathode follower parameters.
- Cliff: "The preamp modeling in 6.00 is the same as 5.xx except the parameters for the default tube type (12AX7A SYL) are different. The Sylvania 12AX7A is more nonlinear than other 12AX7As which results in more dynamics but will also result in more "background" distortion because the waveform is being distorted even when it isn't being clipped. The JJ version is more linear which will result in a tighter tone and less background distortion but less dynamics. For 6.01 I've also added back the old 12AX7B type which is the most linear of the types and clips hard. People who play with lots of gain tend to like this as it results in tighter tone and more aggressive harmonic content. There are two primary parameters associated with our preamp tube models. "Preamp Hardness" determines how abrupt the tube clips when it enters the saturation region. There is another parameter that determines how nonlinear the tube is between cutoff and saturation. This is currently not exposed to the user but I've been contemplating adding it. I've also changed the default type for British amps to the ECC83 model as these amps typically were equipped with ECC83s (duh). The ECC83 was the European equivalent of the 12AX7A and tended to be a bit more linear and clip a little harder." source
- 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. NOTE: If you are using Speaker Drive in existing presets you should audition these presets and adjust the parameter as necessary as the sound and behavior of the algorithm is considerably different than before. NOTE: Setting this parameter to higher values will simulate a blown speaker.