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Understanding All the Different Gain Controls

FractalAudio

Administrator
Fractal Audio Systems
Moderator
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
Overdrive
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.

Overdrive:
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.
 

BillyZeppa

Forum Addict
Awesome.

Overdrive:
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.


Apart from the effect of the bright cap on the input drive level, what sort of tonal changes can usually be expected by moving more of the distortion from earlier stages to later stages, or the other way around?
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
Awesome.

Overdrive:
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.


Apart from the effect of the bright cap on the input drive level, what sort of tonal changes can usually be expected by moving more of the distortion from earlier stages to later stages, or the other way around?
Depends on the amp. It's too complicated to simplify to a handful of descriptors. The only thing to do is experiment. Probably the reason why Boogies have a reputation for being hard to dial in.
 

Stratoblaster

Fractal Fanatic
I spend a lot of time working the Input Drive, Overdrive (if there), and the Master Volume when I'm creating presets or just experimenting. I go back and forth between the preamp/power amp drive controls to find the qualities/feel each stage reveals and the best 'mix' between them.

In many cases, I get most of my dirt from the preamp, and find the Master setting that brings the amp alive, sometimes the other way around...depends on the amp. Very important controls indeed, especially the Master volume. Great info and writeup...
 

AminorZmajor

Veteran
Learn something everyday from the forum. I now have a better handle on the amps with Input drive and Overdrive. Thanks.
 

Rex

Legend!
i'm interested in blending clean tones to gain tones in the one amp using a controller, which is easy enough

...if i pick a clean tone and gain tone by adjusting both master volume and input gain and transition between the both on linear a path, would expect there to be a volume difference?
There almost certainly will be a volume difference. How much of a difference depends on the amp, and what you've dialed in for clean and gain tones.


Assuming these controls aren't linear, is there anyway to predict the controller curve id need...
Yes, but it involves multiple measurements inside the circuit, some tricky math, knowledge of how the ear responds to sound pressure and distortion, and a whole bunch of your time. Even then, it would only get you in the ballpark. You'd still have to nudge it by ear to make it sound right.


what would be the best way to find the curve needed on the controller for such shenanigans, besides the obvious "use your ear"
"Use your ear" is by far the easiest way to do it. You could spend hours trying to predict the curve, or you could spend ten minutes dialing it in by ear.

It's like merging into traffic on a highway ramp. No one parks on the side of the road and does the research and math to predict when and how far to turn the wheel or press the accelerator. And even if you did, you'd better be ready to make some corrections. :)

Check out clarky's threads on tone morphing. He's a morphmaster, and he explains what he does in detail.

Hint: dial in your dirty and clean tones, then tie the same modifier to the LEVEL parameter in the Amp block. That way, you can make all your corrections on one modifier curve.
 

chucma

Forum Addict
Big thank you for the wisdom, it really taught me a lot! :)

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.
I love this feature, it gives a dimension that you cannot easily get on real amps for tone shaping (especially in bedroom conditions). Am I correct that this is the better control to use to also compensate for different strength pickups? I.e. so I would normally nudge the Input Trim up if I wanted to simulate hotter pickups?
 

Smilzo

Forum Addict
Am I correct that this is the better control to use to also compensate for different strength pickups? I.e. so I would normally nudge the Input Trim up if I wanted to simulate hotter pickups?
To my ears there is more tremble in a weak pickup... :roll
 

Fl7x

Veteran
I never get tired of this revealing posts.
The geek inside of me love this stuff!

Thank you.
 

vestapol

Inspired
Thanks Cliff! Very helpful information and clear explanation. I look forward to your mini lessons almost as much as FW updates.
 

Noriuky73

Forum Addict
Many thanks Cliff for this super useful information !

I have a question : In "real world " I have a triple recto and too many guitars . The old one play and sounds wonderfully , the newer ( I must say the ones that have hi output p.u.'s ) not so much ... I thinks the big output from my guitars seems to "override " ( if this word make sense ... I'm italian [emoji33]) the front end of the amplifier making things TOO overdrived even if I lower strongly the gain of the Red/orange channels 2/3 .
Even the John Petrucci 7 that I see played by so many PROs sound like sh... compared with my Les Paul , Tele and Stratos .

Of course right now we are talking of the Axe , were this difference in Bad sounds does'nt exist .
It is correct or better desirable , simulating the behaviour of different output guitars , to lower the input trim on med to high gain axe-amps ? As a rule ?
Of course I like different guitars sound different , but the very quality of "pushing" the input of the amp less as a rule for having always thing tight and defined for hi-output guitars

Many thanks
 
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