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The "Damp" Control is Strange

BK-Amps

Inspired
Assumption: The Damp control is essentially a power amp negative feedback control. Lower the Damp to zero and you essentially remove the negative feedback from the power section, ala Vox AC-30 and Recto Modern.

If this is assumption is true, then the control does not behave like a real amplifier. Remove negative feedback in, say, a real JCM800 and the volume jumps dramatically, the sound is more raw and the bass is more woofy. Do this with the model in the AxeFX and set the "Damp" control to zero I find the volume drops. Also, taking the JCM800 example, increasing negative feedback (by either using the 16Ohm tap instead of the 8Ohm tap or reducing the feedback resistor) will lower the volume, linearize the output stage, and make the sound somewhat anemic.

The AxeFX with the "Damp" control on 10 will bey ~12db higher in volume than "Damp" at 0! This is not right. Either the assumption is wrong (which if the case, we need a better desciption of what Damp does) or the implemention of the Damp control is wonky.

BK
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
The Axe-Fx attempts to normalize the volume as you change the damping. Since the overall gain of an amplifier with negative feedback is

A = Ao / (1 + Ao * B)

the closed-loop gain can be calculated and corrected for. So when you change the damping the Axe-Fx calculates the resulting closed-loop gain and compensates accordingly. However, if you are driving the "power amp" hard the equation falls apart because it assumes linear operation. Therefore there may be some volume change.

This is done since otherwise you would constantly have to adjust your output volume as you change the damping.
 

Smilzo

Power User
FractalAudio said:
This is done since otherwise you would constantly have to adjust your output volume as you change the damping.
I have already to adjust volume, as BK observed, as damp increase. It is possible to lower a bit the adjustment, so that with a "light" saturation the level remains nearly the same?
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
Unfortunately it is impossible to predict how saturated the power amp is since that depends on input level. The compensation isn't perfect, the idea is to minimize the volume fluctuations since without compensation the volume would fluctuate wildly.
 

Smilzo

Power User
FractalAudio said:
the idea is to minimize the volume fluctuations since without compensation the volume would fluctuate wildly.
It is a great idea, because our minds judge louder sounds as better ones! I like it very much... maybe a few lines on the wiki to warn about level loss when damp is high and poweramp saturate will help people to better use damp.
 

yek

Moderator
Moderator
Smilzo said:
FractalAudio said:
the idea is to minimize the volume fluctuations since without compensation the volume would fluctuate wildly.
It is a great idea, because our minds judge louder sounds as better ones! I like it very much... maybe a few lines on the wiki to warn about level loss when damp is high and poweramp saturate will help people to better use damp.
Done:
http://axefxwiki.guitarlogic.org/index. ... parameters
 

Smilzo

Power User
Well done. It must be underline that if poweramp is saturated, both damp and level must be increased to maintain level the same.
 
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