Amp Block Discovery

Jay Mitchell

Fractal Fanatic
This may or may not turn out to be significant, but it's certainly interesting. In the Advanced settings in the amp block, there are three possible selections for the "Input Select" parameter. The three settings are "Sum L+R," "Right," and "Left," and the default is "Sum L+R." The default setting is presumably so the block will pass signal if you're using a right-channel input, but the front input feeds the left channel only.

Last week, just for grins, I changed the setting from the default to "Left" and got a surprise (which, in retrospect, should not have been a surprise). The gain increased, and the character of the sound was quite different. Now, since the amp block is nonlinear, the gain increase at its input produced several collateral tonal changes, as well as increasing the drive/saturation of the block, so what I was hearing was the result of multiple variables being changed together. Reducing the Drive parameter brought things back in line, but I think there is still a difference (which I prefer, BTW), due to the fact that the response of the amp block can vary with the setting of the Drive parameter.

Having thought about this some more, it dawned on me that it only makes sense to introduce attenuation at a summing block. Two uncorrelated signals of equal level, when added together, produce a summed signal that is 3dB higher in level than either signal alone. Attenuating the summed signal helps prevent output clipping in the Axe-Fx.

However, if you're not sending two signals through the amp block, there's no need to have this attenuation, and the added drive level may change things in a desirable way. I've changed the amp block input select parameter in all my presets to "Left" and adjusted the Drive parameter accordingly. If you're experiencing trouble getting enough drive with a given amp block, this may get you where you want to be without having to add a block just for some signal boost.

This is all FWIW. I'm not claiming to hear huge differences, but I'm pretty sure that there is a difference. If you're not feeding the amp block with two independent signals, it certainly won't hurt to select the left input.
 
I think it just reduces level by ~6 dB when set to sum. IOW when the L & R channels are identical at that point the setting makes no difference.
 
I've done that for a long time (changed to 'left input') because it seemed 'better' (subjective) though I never pinned down why.
 
Last edited:
On further checking, I see you can accomplish the same thing by setting the global I/O to "Left only." This puts the front input on both sides equally, and the amp block's input setting then does not matter.
 
On further checking, I see you can accomplish the same thing by setting the global I/O to "Left only." This puts the front input on both sides equally, and the amp block's input setting then does not matter.

I do that too; though again... never thought about why. :D

So I guess I was doubly safe no matter what I did in the amp block input. ;) :D
 
Choosing left in the I/O menu is generally a good idea if In 1 Right isn't being used. Otherwise you'll be adding noise if anything (drive, amp w/ InSel = L+R) sums the channels, or creating more hassle in muting the noise or when you want signal in both channels before any amp(s).
 
On further checking, I see you can accomplish the same thing by setting the global I/O to "Left only." This puts the front input on both sides equally, and the amp block's input setting then does not matter.

That is the way I run. It explains why I never noticed :)
 
On further checking, I see you can accomplish the same thing by setting the global I/O to "Left only." This puts the front input on both sides equally, and the amp block's input setting then does not matter.

ok, this explains why I coudn't notice any difference.
 
That is the way I run. It explains why I never noticed :)
LOL -me 2. Just checked and mine is set up on Global I/O as "Left only" already...
Since I don't remember [consciously] changing it to that value from "Sum L+R", makes me wonder how it got that way ?
Default setting for front input ?
 
Last edited:
....if I only knew what I didn't know.....

I'll settle for the realization that I don't know it all and that what I don't know can in fact hurt me, thereby creating an opportunity for learning!
 
Last week, just for grins, I changed the setting from the default to "Left" and got a surprise (which, in retrospect, should not have been a surprise). The gain increased, and the character of the sound was quite different.

I've noticed the exact same thing, and have always set the Input Select to Left only since I do indeed hear a difference, and find I liked what I heard over the Sum L+R.

Like you say, the difference isn't huge, but is indeed there...I also thought that setting shouldn't make a difference with only a single input, but I just went with what I heard...
 
Last edited:
I just checked mine, and it looks like both of my inputs in I/O were set to "Left Only", but I don't remember changing them. I'm running firmware 10.3. I wonder if this was changed to a default at some point?
 
Back
Top Bottom