Amp block

User contributions

Amp block parameters

All Amp block parameters are explained here: Amplifier (FX block): controls and parameters.

Preamp and power amp

• 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 (Trainwreck).

CPU utilization

• Each preset can have two Amp blocks. Adding a 2nd Amp block just adds about 2% CPU usage (because of of the DSP is dedicated to amp 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
• Each Amp block can use the X/Y feature to switch between sets of parameters.

Modelling

• The amp modeling in the Axe-Fx II is called G2 and Virtual Vaccuum Modeling modeling, see 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
• "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:
  • Cliff: "The new firmware 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."

Global amps

The Global Amps feature (Standard/Ultra) has become part of Global Blocks in the Axe-Fx II.

Using one or two Amp blocks

• 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."
• You can use two Amp blocks simultaneously by using two grid rows.
• You can switch between Amp1 and Amp2 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.
• You can use one Amp block and use X/Y switching.

Audio gap when entering 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

Tonestack curves

Forum member DonPetersen generated a frequency curve graph of the tonestack of each amp type in the Standard/Ultra, using white noise. You can view the graphs in this thread. Note that the graphs for the Axe-Fx II models will be different.

Added amp types

Firmware upgrades may add amp types. The owner's manual is updated periodically to incorporate those additons. A detailed list of all amp types is here.