Understanding All the Different Gain Controls

FractalAudio said:
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:

amp_block_gains.jpg



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.
Click to expand...
Great explanation!
 
FractalAudio said:
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:

amp_block_gains.jpg



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.
Click to expand...

Thanks!
 
And the Level control in the amp block affects the overall tone how? (Or none at all...?)

I ask b/c most of the presets in my Axe clip the output. "Level" is my go-to control for correcting that issue. Backing it off stops the clipping but, of course, I lose overall volume. To put that volume (and/or "feel" in the amp) back, should my go-to control be the amp's MV? Or is the answer basically "Experiment...whatever works for you and sounds good to you *is* the right choice!"?

Thanks.
 
jshirkey said:
And the Level control in the amp block affects the overall tone how? (Or none at all...?)
Click to expand...

Not at all assuming all things that come after the block in the chain are linear effects (chorus, delay and even the CAB block unless you use motor drive).

To put that volume (and/or "feel" in the amp) back, should my go-to control be the amp's MV? Or is the answer basically "Experiment...whatever works for you and sounds good to you *is* the right choice!"?
Click to expand...
Use the analog volume out to compensate.
 
Rex said:
Not at all. Listen, and you'll hear for yourself.



To put the volume back, turn up the little black "Output Level" knob on the front panel. :)
Click to expand...

No...that screws with my overall levels, and they are set based on the output of my hottest guitar. I need to adjust on a per-preset basis.
 
iaresee said:
Not at all assuming all things that come after the block in the chain are linear effects (chorus, delay and even the CAB block unless you use motor drive).


Use the analog volume out to compensate.
Click to expand...

Hard output knob on the front of the Axe (if so...see my other reply)? Or...?
 
jshirkey said:
No...that screws with my overall levels, and they are set based on the output of my hottest guitar.
Click to expand...
But most of your presets are clipping, so they all need to have their levels reduced...so they'll all benefit from turning up the output knob.
 
jshirkey said:
That's the parameter I'm using. Was just wanting to make sure that was the best choice. I will watch your video, too, though. Thank you.
Click to expand...
there is a reason the other guys mentioned the Physical Out knob on the front panel though.

if you drop the level using that Amp Level parameter to avoid clipping, you are changing the INTERNAL level, which of course affects the volume coming out of the Axe itself. because you dropped the level, you probably want to increase it somewhere to have the same amount of volume coming out of the Axe. however, if you increase the levels anywhere internally you are causing the same issue of clipping.

you mentioned the MV control - but no you can't increase that because you are basically un-doing what you did when reducing the Level parameter to prevent clipping.

therefore what you want to do is increase the Physical Out knob on the front panel, after you have reduced all your preset levels to prevent internal clipping.
 
  • Like
Reactions: Rex
let's say you have 5 presets, and most of them are clipping:

Screen Shot 2016-09-18 at 5.30.26 PM.png
regardless of where i turn the Physical Output Knob, the chart above represents what is happening INTERNALLY with levels. it's clipping the Internal limit. i can turn the Physical Output Knob up or down and it will still always clip the same way. don't think of the individual bars in the chart going up and down with the Physical Output Knob; think of everything just simply getting louder or softer with the Physical Output Knob. the clipping sound just gets louder and softer as a whole when you turn the Physical Output Knob - nothing internal is changing. nothing changes relative to that Clipping point line. you just hear more or less of whatever is programmed inside.

let's work on Preset 4, and reduce it so it stops clipping. we'll reduce the Amp Level parameter by using the VU meter page and the quick knob until it stops clipping:
Screen Shot 2016-09-18 at 5.30.31 PM.png

so now we adjusted Preset 4 so it has some headroom before the clipping point, just like Preset 5. but guess what. Preset 4 is QUIETER now, but why? because we reduced the Level! so since we reduced Preset 4, we need to increase something so the next item in our signal path - ultimately a speaker - produces the same amount of volume. the thing we increase is the Physical Output Knob. remember that makes everything in the Axe just get louder or softer all at the same time. so we turn up the Physical Output Knob and now Preset 4 is just as loud as it was before, with no tone change.

but what about Presets 1 - 3? they are STILL clipping. well we need to adjust those too:

Screen Shot 2016-09-18 at 5.30.36 PM.png

you can see that i adjusted all the presets now for much more headroom before clipping. i chose to make Preset 2 louder than the others, yet still not clipping. and presets 4 and 5 are much quieter than the others because i want them to be. but now EVERYTHING is much quieter than before.

well as you know, since we just spent all that time reducing the internal volume, we can't use anything internal to make it louder. so again, that leaves the Physical Output Knob, and we simply turn it up.
 
Last edited:
Chris@AxeFxTutorials said:
let's say you have 5 presets, and most of them are clipping:

View attachment 35081
regardless of where i turn the Physical Output Knob, the chart above represents what is happening INTERNALLY with levels. it's clipping the Internal limit. i can turn the Physical Output Knob up or down and it will still always clip the same way. don't think of the individual bars in the chart going up and down with the Physical Output Knob; think of everything just simply getting louder or softer with the Physical Output Knob. the clipping sound just gets louder and softer as a whole when you turn the Physical Output Knob - nothing internal is changing. nothing changes relative to that Clipping point line. you just hear more or less of whatever is programmed inside.

let's work on Preset 4, and reduce it so it stops clipping. we'll reduce the Amp Level parameter by using the VU meter page and the quick knob until it stops clipping:
View attachment 35082

so now we adjusted Preset 4 so it has some headroom before the clipping point, just like Preset 5. but guess what. Preset 4 is QUIETER now, but why? because we reduced the Level! so since we reduced Preset 4, we need to increase something so the next item in our signal path - ultimately a speaker - produces the same amount of volume. the thing we increase is the Physical Output Knob. remember that makes everything in the Axe just get louder or softer all at the same time. so we turn up the Physical Output Knob and now Preset 4 is just as loud as it was before, with no tone change.

but what about Presets 1 - 3? they are STILL clipping. well we need to adjust those too:

View attachment 35083

you can see that i adjusted all the presets now for much more headroom before clipping. i chose to make Preset 2 louder than the others, yet still not clipping. and presets 4 and 5 are much quieter than the others because i want them to be. but now EVERYTHING is much quieter than before.

well as you know, since we just spent all that time reducing the internal volume, we can't use anything internal to make it louder. so again, that leaves the Physical Output Knob, and we simply turn it up.
Click to expand...
Nice work Chris. that was a great pictorial for the OP
 
Rex said:
But most of your presets are clipping, so they all need to have their levels reduced...so they'll all benefit from turning up the output knob.
Click to expand...

No. I said most of the factory presets clip. Or at least that's what I meant to say, if that's not what I actually wrote. lol
 
Okay...you're right. I did just say "presets" in my first post. *My* presets are fine. It's all the stock factory stuff that is way too loud and clips. So, the first thing I do (if I happen to want to use the amp that's being modeled...although most of the modern high-gain stuff I couldn't care less about, and they are often the worst offenders) is start turning things down just to make them usable.
 
If most of the factory presets are clipping, then they are louder than your own presets, which aren't clipping. So we're back to the little black level knob to make up the volume, because you'll have to do that anyway when you switch back to your own presets.
 
You must log in or register to reply here.
Back
Top Bottom