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Understanding All the Different Gain Controls

sprint

Fractal Fanatic
And don't forget; on some amps, you have a Master Volume Trim control which allows you to multiply your MV amount - ie, if MV is at 10 and MV Trim is at 1.1, then your amp now goes to 11 !!!!!!, thus allowing you to rock out with Nigel Tufnel!
 

Teej

Member
And don't forget; on some amps, you have a Master Volume Trim control which allows you to multiply your MV amount - ie, if MV is at 10 and MV Trim is at 1.1, then your amp now goes to 11 !!!!!!, thus allowing you to rock out with Nigel Tufnel!
Excellent :lol
 

joegold

Fractal Fanatic
I'm quite surprised to see that the Input Trim is *after* the Input Drive and the Bright cap.
I always thought that Input Trim would be the very 1st gain stage so that it could be used w/o affecting the tonal characteristics at all, just the distortion/gain characteristics.

Are you doing it this way to more closely be able to replicate the way that real amps with Hi and Low channels are actually wired?
Or is it an LTI thing where the order of these gain stages doesn't matter because everything's still in the linear range at this point in the gain staging?
Or is it some other reason?

Just curious.

I do find that several of the clean amp sims break up earlier on the Input Drive setting than I would expect their real-world counterparts to do (e.g. Doubleverb) and I'm usually bringing down the Input Trim to clean it back up.
Then again I've never played extensively through a real vintage blackface Twin.
My experience is all based on blackface reissues and/or silverface amps I owned back in the 70s.
I'm sure the modelling is spot on though.
It sure sounds good.
 

Rex

Legend!
I'm quite surprised to see that the Input Trim is *after* the Input Drive and the Bright cap.
I always thought that Input Trim would be the very 1st gain stage so that it could be used w/o affecting the tonal characteristics at all, just the distortion/gain characteristics.
If the Input Trim were in front of the Input Drive circuit, the behavior of the bright cap circuit would change when you changed the trim. This would cause weirdness and frustration as you tried to dial in just the right amount of gain, and then had the treble go all wacky on you if you adjusted the trim. With the Input Trim following the Input Drive, the behavior stays constant.


I do find that several of the clean amp sims break up earlier on the Input Drive setting than I would expect their real-world counterparts to do (e.g. Doubleverb) and I'm usually bringing down the Input Trim to clean it back up.
Then again I've never played extensively through a real vintage blackface Twin.
The Silvers break up later than the blackfaces.
 

yek

Moderator
Moderator
I'm quite surprised to see that the Input Trim is *after* the Input Drive and the Bright cap.
I always thought that Input Trim would be the very 1st gain stage so that it could be used w/o affecting the tonal characteristics at all, just the distortion/gain characteristics.

Are you doing it this way to more closely be able to replicate the way that real amps with Hi and Low channels are actually wired?
Or is it an LTI thing where the order of these gain stages doesn't matter because everything's still in the linear range at this point in the gain staging?
Or is it some other reason?

Just curious.

I do find that several of the clean amp sims break up earlier on the Input Drive setting than I would expect their real-world counterparts to do (e.g. Doubleverb) and I'm usually bringing down the Input Trim to clean it back up.
Then again I've never played extensively through a real vintage blackface Twin.
My experience is all based on blackface reissues and/or silverface amps I owned back in the 70s.
I'm sure the modelling is spot on though.
It sure sounds good.
About the early breakup: some models are based on modded real amps. For ex the Double Verb is based on such an amp, which breaks up early. See wiki. Also, the 'high' input is always modeled, not the 'low' one.
 

Zelja

Inspired
If the Input Trim were in front of the Input Drive circuit, the behavior of the bright cap circuit would change when you changed the trim.
I can't see why that would be the case. My understanding is that the bright cap function is dependent on the setting of the Input Drive knob - in real circuits, the position of the knob (and not a function of how much signal is at that point n the circuit). If set low, the overall signal is attenuated but certain highs still pass fully due to the bright cap, giving a brighter overall mix. As the pot is increased, less of the overall signal is attenuated (i.e. more overall volume) but the same amount the highs pass through the bright cap, giving an overall less bright sound. With the pot at max the bright cap is effectively bypassed and has no effect.

I too thought the input trim was right at the beginning of the circuit i.e. where the normal input resistors would be on an amp but I can see the value of putting it where it is on the Axe as explained by Cliff.

In a real amp the Low input attenuates the signal from the guitar right before the first gain stage. I'm assuming that the very first gain stage of most amps doesn't distort (even with a bit of an input boost due to the 100 plus volts of headroom). If that is the case then placing it where it is on the Axe would not have much of a different effect than on a real amp but would have other advantages.

I have seen some instructions on how to "goose" the 1st gain stage of an amp by putting in a large clean boost in to the amp & then turning down the volume (input drive) control in order to get overdrive from the the 1st gain stage for the purpose of some perceived sonic bliss. Maybe that's a furfie and the 1st gain stage isn't overdriven at all & it's actually happening later in the circuit?
 

GotMetalBoy

Power User
Wow this is really helpful and will make dialing in my tone so much easier!

Cliff, can you explain how Drive pedals work in a Cliff Notes/Tech Notes? Do they have similar gain staging and tone shaping? How do they affect an amplifiers gain staging and tone shaping when placed in front of them?
 

Rex

Legend!
I can't see why that would be the case. My understanding is that the bright cap function is dependent on the setting of the Input Drive knob - in real circuits, the position of the knob (and not a function of how much signal is at that point n the circuit).
That's correct. The function of the treble bleed is determined by the capacitor and the Input Drive pot. If you put the Input Trim in front of the Input Drive, then portions of both pots are parallel with each other, and both pots affect the way the circuit performs.
 

Hyper Planet

Power User
Another piece of an informative subject about The Axe-Fx ll by Cliff,

well I'm a huge fan of Mesa Mark series amps in The Axe-Fx ll, N I always used Input Drive, Overdrive, Master Volume in order to get my favorite tone, I never used Input Trim, but now when I read this topic especially about Input Trim, it changed all my recipes for creating a massive tone, It's time to Re-creating every thing with this highlights.


Thank you Cliff !.
 

joegold

Fractal Fanatic
If the Input Trim were in front of the Input Drive circuit, the behavior of the bright cap circuit would change when you changed the trim. This would cause weirdness and frustration as you tried to dial in just the right amount of gain, and then had the treble go all wacky on you if you adjusted the trim. With the Input Trim following the Input Drive, the behavior stays constant.
I guess that makes sense.
Thanks.
 

PacoCasanovas

Fractal Fanatic
Dear Cliff

Much thanks for explaining the basic gain control paradigm of guitar amps to all axefx users. I think this will help most people to understand all these different controls and their relationship to each others a lot better. Even there is a slight difference from your explanation of the controls and your basic paradigm drawing, which made me quickly put together another drawing (handpainted, because I'm really suck on computer equipment when it comes to drawings.....took me hours to get the stuff right....uugh!)



Explaning input trim:

Input trim as it's inside your axefx is something you normally don't have on your amp, because you're not only able to attenuate the signal, you can also boost it! So therefore is the buffer amp shown in cliffs paradigm diagram, now shown as controllable input buffer in my drawing. The input drive on all amps is located behind the first gain stage - on all guitar amps.

Explaning Mastervolume Trim (MV-Trim):

Mastervolume Trim is something which you wouldn't find in a real amp too.
(There are possibilities to change the phase splitter design when in- or decreasing the tail resistor in the long pair tail splitter design which results in sending more or less signal from the phase splitter to the input grids of the power tubes.....but you won't find this as a dedicated control inside an amp)

It in- or decreases the signal coming to the power amp section, it's not a fine tuning master volume control or even a multiplier for the master volume....

Hint: The Master volume position can be altered in the advanced parameters (MV location). In the drawing it's PRE-PI (before the phase splitter also can moved after the phase splitter which is called PPIMV or Post-PI, see Axe-Wiki or your AxeFx User manual)

MV Cap: Same as the bright switch - but over the Master volume potentiometer - shown here as a switch, in the axefx you can control the capacity of capacitor in picofarads (which is value multiply 10^-12 ) with MV Cap . This does the same as the bright cap, on lower master volume settings. This unique master volume bright cap feature is often seen in Dumble type of amps.


Cheers
Paco(sipulami)
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
Dear Cliff

Much thanks for explaining the basic gain control paradigm of guitar amps to all axefx users. I think this will help most people to understand all these different controls and their relationship to each others a lot better. Even there is a slight difference from your explanation of the controls and your basic paradigm drawing, which made me quickly put together another drawing (handpainted, because I'm really suck on computer equipment when it comes to drawings.....took me hours to get the stuff right....uugh!)



Explaning input trim:

Input trim as it's inside your axefx is something you normally don't have on your amp, because you're not only able to attenuate the signal, you can also boost it! So therefore is the buffer amp shown in cliffs paradigm diagram, now shown as controllable input buffer in my drawing. The input drive on all amps is located behind the first gain stage - on all guitar amps.

Explaning Mastervolume Trim (MV-Trim):

Mastervolume Trim is something which you wouldn't find in a real amp too.
(There are possibilities to change the phase splitter design when in- or decreasing the tail resistor in the long pair tail splitter design which results in sending more or less signal from the phase splitter to the input grids of the power tubes.....but you won't find this as a dedicated control inside an amp)

It in- or decreases the signal coming to the power amp section, it's not a fine tuning master volume control or even a multiplier for the master volume....

Hint: The Master volume position can be altered in the advanced parameters (MV location). In the drawing it's PRE-PI (before the phase splitter also can moved after the phase splitter which is called PPIMV or Post-PI, see Axe-Wiki or your AxeFx User manual)

MV Cap: Same as the bright switch - but over the Master volume potentiometer - shown here as a switch, in the axefx you can control the capacity of capacitor in picofarads (which is value multiply 10^-12 ) with MV Cap . This does the same as the bright cap, on lower master volume settings. This unique master volume bright cap feature is often seen in Dumble type of amps.


Cheers
Paco(sipulami)
Your drawing is more accurate. I put Input Trim where I did in my drawing as I thought it would make more sense to people.
 

joegold

Fractal Fanatic
Your drawing is more accurate. I put Input Trim where I did in my drawing as I thought it would make more sense to people.
*Now* I'm confused.
Paco's drawing (which doesn't show up on my system in his post but does show up in Cliff's post for some reason) seems to show the Input Buffer and Input Trim ganged together *in front* of the Input Drive.
So was I right in my original supposition about where Input Trim would be placed in the audio flow?
 

barhrecords

Axe-Master
*Now* I'm confused.
Paco's drawing (which doesn't show up on my system in his post but does show up in Cliff's post for some reason) seems to show the Input Buffer and Input Trim ganged together *in front* of the Input Drive.
So was I right in my original supposition about where Input Trim would be placed in the audio flow?
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.
 

sprint

Fractal Fanatic
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.
 
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