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Understanding All the Different Gain Controls


Fractal Fanatic
I'm guessing the buffer is what prevents the Input Trim from interacting with the Input Drive and Bright Cap to affect the tone.

So logically the Input Trim can be thought of as coming "After" input drive because it is not affecting the tone / bright cap circuit.
Makes more sense thinking of it in this way.


Fractal Audio Systems
Now confused here also - so is Input Trim before or after Input Drive / Bright cap. To fit Cliff's description, would Trim not have to be after Drive (contrary to Paco's diagram)?

Been fiddling with these controls on the USA Lead model and I can't seem to detect a brighter tone when replacing Drive Input gain with Trim Input gain - maybe I am not listening correctly.
It doesn't matter. It's linear so it's commutative. I.e. a x b = b x a


Power User
The amp block in the Axe-Fx has a variety of gain controls that change depending upon the amp model selected. These controls are:
Input Drive
Input Trim
Master Volume

These various controls are located at fixed points in the virtual amplifier circuit as follows:

Input Drive:
This is the modeled amp's gain, drive, volume, etc. control. It adjusts the attenuation at the input to the amplifier gain stages after the input buffer. On a Marshall Plexi, for example it is the "Loudness" control. On a typical Fender amp it is the "Volume" control. On many high-gain amps it is called either "Gain" or "Drive".

On a real amp this is implemented using a variable resistor called a potentiometer. Many amps include a "bright cap" on the drive control which is a small value capacitor placed across the terminals of the pot that bleeds treble frequencies through as the gain is reduced. Sometimes this bright cap is switchable via a switch on the amp. Sometimes it is fixed.

Input Trim:
The Input Trim control allows you to adjust the input attenuation without changing the frequency response. If you turn down the Input Drive and the model has a bright cap the amp will get brighter. Now you may like the brighter tone but wish there were more gain. Input Trim allows you to increase the gain without changing the tone. Conversely you may like the darker tone with Input Drive set high but wish there were less gain. In this case you can lower Input Trim.

Most real amps do not possess an Input Trim control. Instead they usually have a switch or two input jacks that select between a high-gain and low-gain input. Almost invariably the difference between these two jacks is 6 dB. All the Axe-Fx amps are modeled using the high-gain input or switch position (if any). To simulate the low-gain input set the Input Trim to 0.5 which is 6 dB less.

Some amps possess an attenuation control between the later gain stages. Examples of the are the Mesa/Boogie Mark series, Dumble ODS and others. This control allows the user to vary the gain staging. The Input Drive can be turned up and the Overdrive turned down so that the earlier stages distort more and the later stages distort less and vice-versa.

Master Volume:
The Master Volume (MV) controls how much signal level is sent to the power amp. Many vintage amps have no MV control and the power amp runs "wide open". Modern amps often get their distortion from the preamp and the Master Volume then allows the user to control the volume of the amp.

The Master Volume in the Axe-Fx II, as well as on real amps, is probably the singular most powerful control in the amp block. As the Master Volume is increased the virtual power amp begins to distort. The virtual power amp also begins to sag and all sorts of beautiful magic occurs. The tone becomes more focused, the dynamic response changes, the note attack is accentuated, etc.

The key to crafting the ultimate tone involves understanding these various controls and learning how to balance them.
Yet another Golden Thread from Ol' Saint Cliff! :encouragement::encouragement::encouragement:


Power User

The key to crafting the ultimate tone involves understanding these various controls and learning how to balance them.

Bolded for emphasis.
Ha! I was going to ask Cliff to elaborate on these last five words of this last sentence to his opening post ... until I read his subsequent post urging us to experiment first, rather than seek additional descriptors ... but they would be welcome indeed!:encouragement::encouragement::encouragement:


Fractal Fanatic
My post was not ment to start any confusion here, just to avoid any confusion regarding the input trim and the first gain stage, which is always before the input drive control! As Cliff said - it dosen't matter as long you get the basic idea about the difference between input drive, overdrive, master & mastervolume-trim. There is no input trim control at a traditional amp rig, so don't get confused about this parameter. But it's there for a reason and should be used.....

Personally, I love to add some extra boost (+2dB) more signal to my patches. Why? Why not ;) That is what makes the axefx at least one step forward against all other guitar amplification products. You can do it - because it's all in there!

There are various amps, that also have a 3rd paradigm diagram. JUMP-Amp types amp using two input drive controls, because there are two separate "first" gain stages, one voiced to bring in more brillant tones, the other more for low end "called normal" tones, which were mixed together in the following gain stage. You can blend them to your personal taste, so they push the following gain stage a bit harder than a single first gain stage would do - the result is more gain/drive.

Have fun!



Power User
Wow... thank you Master!!!

I'm talking about just me... but... before the Axe for me an amp was only a musical box with knobs.
I know how tweak that knobs in the different kind of boxes or what kind of sound I can have from a low or high input or from a normal or bright input or mixing those inputs in a bridge config (i.e. 4 inputs amps).
BUT... i know NOTHING about WHY and HOW those things interacts together.

When you Cliff explain those things the way you explain... you Sir turn on a light in my medieval brain!!!


Fractal Fanatic
An saturation switch? Where how and what kind of gain does this add? Cliff? Thx

Sent from my iPhone using Tapatalk
The saturation switch, became famous as the Jose Arredondo Mod - a nice feature by a Ex-Van Halen amp tech who passed away years ago, also available in the Friedman BE amp is a special switchable diode clipping stage / split load feature of the cathode follower in marshall amp designs (which introduces a huge signal drop but brings some extra clipping - more dirty tones).

before MV as Rex said.... ;)


Fractal Fanatic
I find I only have to use the Input Trim if the Bright Cap is engaged since the Input Drive then doubles as a tone control. Like the notes say, I dial in the Input Drive to find the overall tone I like, then adjust the gain, if required, with the Input Trim.


I find I only have to use the Input Trim if the Bright Cap is engaged since the Input Drive then doubles as a tone control. Like the notes say, I dial in the Input Drive to find the overall tone I like, then adjust the gain, if required, with the Input Trim.

Input Drive up = darker
Overdrive down = cleaner

E.g. Dark clean = Input Drive Up -> Overdrive Down (to cut the gain and clean it up)


Fractal Fanatic
You're such a high-gain lover and you don't know about the Saturation parameter in the Amp block?
You're in for a treat.
? I know of it of course? I press it all the time so I'm not sure there will be any sonic surprise. I just wanted the same type of detailed info to better utilize. [emoji4]

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Sent from my iPhone using Tapatalk
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