Amp block

User contributions

Relevant Amp block topics

• AMP (block): all parameters
• AMP (block): list

About amp modeling

• The amp modeling in the Axe-Fx II is called G2 and Virtual Vaccuum Modeling modeling (see the Fractal Audio website). Part of it is ported to Standard/Ultra firmware 11.
• Cliff: "Almost all the amps in the AFXII are based on actual amps." Source1 Source2
• Cliff: "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
• Cliff: "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
• Firmware 5 release notes:
  • "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. The Level parameter is duplicated on the DYN page for convenience."
• Firmware 6 release notes:
  • "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."
  • "Many amps have received a complete overhaul based on the matching studies. The most notable are the 65 Bassguy, Deluxe Verb, 1987x Normal and Treble, Hipower Normal and Treble, all USA models, all Recto models, Euro Blue, Red and Uber, ODS-100 Clean and Lead, and Mr Z 38 Sr."
• Cliff: "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
• Firmware 8.00: Improved phase inverter modeling provides “juicier” tone when PI is driven hard (MSTR set high).
• Cliff: "If you set the Global to 7.xx the modeling code is IDENTICAL to the actual 7.00 firmware. All I do is call the old function instead. However, the presence calculation is based on the new (and far more accurate) calculations. Source
• Firmware 9 release notes:
  • 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. The Global menu allows the choice of Version 9.xx, 8.xx or 7.xx modeling to suit individual tastes. 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.

Preamp and power amp modeling

• The pre-amp and power amp sections of the amp sim cannot be split.
• The power amp can be disabled though, either globally (Global menu) or per preset (turn Sag to zero). Note that turning off power amp simulation doesn't work well with amp sims that fully rely on power amp distortion such as the Wrecker sim.

What happened to Global Amps

• The Global Amps feature in the Standard/Ultra is now part of Global Blocks in the Axe-Fx II.

Using 1 or 2 Amp blocks (HiRes mode)

• Each preset can have up to two Amp blocks. One DSP is devoted entirely to the Amp block(s). Adding a second Amp block just adds about 2% CPU usage. 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
• Each Amp block can use the X/Y feature to switch between sets of parameters.
• When using a single Amp block in a preset, the Amp block (automatically!) runs at double the internal sampling frequency (firmware 3.03 and higher). Main benefit of this is less aliasing in high gain models. Firmware release notes: "Amp block now has high-res mode. In this 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 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." And Cliff: "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
• You can use two Amp blocks simultaneously by using two grid rows.
• You can switch between Amp 1 and Amp 2 using MIDI CCs. Or morph between them using an expression pedal: 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.

Audio gap when entering an Amp block's Edit mode

• If you press Edit in the Amp block (or using X/Y on the front panel), there will be a short gap in the audio stream because of processing tasks. Source

Resetting Amp block settings

• Doubleclicking Bypass resets an Amp block. This will reset all parameters to default. If firmware earlier than 6 is installed, the amp type wll be set back to 59 Bassman. If firmware 6 or later is installed, the current amp type stays the same.