Expression pedal to control gain?

Doug Notter

Inspired
I saw someone mention in another thread that it's not a good idea to hook an expression pedal up to the gain control. The reason stated was it's too much of a CPU hit and to use input trim instead. I can't find it now and wouldn't want to hijack the thread anyway but can anyone expand on this a bit? The threads I have found by searching have been on older units, not the III so I'm wondering if it applies here. I have mine setup this way and haven't noticed any performance issues (yet). I like the way the character of the tone is altered when using the gain versus input trim. To get the same effect with the input trim it seems like I'd have to also be controlling eq and presence a little bit which seems like it would be an even bigger cpu hit but that's why I'm asking the question. I don't know for sure.

This is my favorite element of a particular preset of mine but I want to make sure I'm not setting myself up for a problem down the road. Any input appreciated! :cool:
 
I’m wondering this too! I’d like to get an Axe FX iii but I need this functionality. Is it only recommended that you stay away from doing this on really high CPU presets? Does it cause audio glitches?
 
I’m wondering this too! I’d like to get an Axe FX iii but I need this functionality. Is it only recommended that you stay away from doing this on really high CPU presets? Does it cause audio glitches?

None that I've heard personally. I just spent the past hour thrashing on it. I saw no spike in the CPU meter at all, regardless of how much or how fast I was moving it. This preset's CPU usage is about 62%. I've had this preset setup this way for a couple months and haven't heard a single glitch and haven't noticed the CPU spiking at all but until I saw that post I was never paying much attention. Today I was paying very close attention and still saw nothing.

I wouldn't let this give you pause on getting the Axe III by the way. Even if someone chimes in with a good enough reason to change the way I have this setup, there are other ways to get the same effect. The suggestion is just to use the input trim instead. The difference apparently being is that input trim has no effect on tone but I like that little bit of a thinning effect that turning down the gain control has. I could do the same thing with the eq and/or presence control, combined with the input trim. I think anyway. Haven't actually tried it yet but I see no reason it wouldn't work.
 
I wouldn’t worry about it. Large CPU spikes only matter if they put you close to too much CPU usage. Otherwise, if you have enough CPU, why not?
 
I believe this was discussed in another thread. The issue addressed in the thread was when attempting to use an expression pedal there were audio spikes or pops as the gain is turned up in real-time.
 
I was the person who posted in the other thread.

As mentioned there, the recommendation was regarding the Axe Fx II and came from Cliff. He said you should not attach a modifier to Input Drive.

No idea if this applies to the Axe Fx III, but I suspect it might because from what I remember of the issue the calculations for that are intense.
 
It sounds more like it is something that causes a CPU spike not so much something that should be avoided in general.
 

Thanks for that. That first thread... OP is hilarious.

Relevant post from that thread: Cliff: "Depending upon the amp model it can take a lot of CPU to calculate the Input Drive network. Some amps have simple networks that are rapidly solved. Others, like the Hook Lead and Rhythm models have complex networks that require more math. If you attach a modifier to the Input Drive it is constantly recalculating the network which increases CPU usage."

This might be why I haven't been able to duplicate it. I've only tried it with the Mesa Recto models. I'm going to try some of these other models and see if I can reproduce the spike on the III. I also saw a post where it was mentioned that the CPU spike came when attaching the controller, not when using it which is also something I haven't experimented with yet. Maybe it did spike when I attached it and didn't notice.

On the older units, was it common for there to be audio problems from doing this or did that only happen if you exceeded the maxCPU threshold? I haven't noticed any glitching of any kind with my III.
 
I used to do that with my old Axe II, and for me, it seemed to work fine, although my presets at that point weren't very CPU intensive. I never noticed any problems with it, but I don't have golden ears. 🤷‍♂️
 
So, what's wrong with just using a volume block in front of the amp, or attaching a modifier to the input trim of the amp? I don't get why you would want to control the input drive in real time, since it often changes the character and EQ of the amp too. Especially on amps with a large treble cap. Doing this on say, a Marshall would work less then ideal.
 
It sounds more like it is something that causes a CPU spike not so much something that should be avoided in general.
A large CPU spike while performing could be really bad... IMO that risk should be avoided.

Feel free to draw your own conclusions for your own use :)
 
A large CPU spike while performing could be really bad... IMO that risk should be avoided.

Feel free to draw your own conclusions for your own use :)
It sounds like the CPU hit is pretty consistent. Im not sure this is really something that is a risk any more than running a reverb in high quality mode.
 
It sounds like the CPU hit is pretty consistent. Im not sure this is really something that is a risk any more than running a reverb in high quality mode.
The Reverb doesn't change CPU use dynamically, a modifier on Input Drive does.

Yes, probably if you've carefully tested the full range of the modifier in your existing preset and it exhibits no issue and it works fine then you are safe.

However, if you're running presets that are already high CPU then maybe not so much.

Anyway, as I've said already, I don't even know if this is an issue at all on the Axe Fx III... I was merely sharing historical information and Cliff's related advice (which I tend to trust ;))
 
Thanks for that. That first thread... OP is hilarious.

Relevant post from that thread: Cliff: "Depending upon the amp model it can take a lot of CPU to calculate the Input Drive network. Some amps have simple networks that are rapidly solved. Others, like the Hook Lead and Rhythm models have complex networks that require more math. If you attach a modifier to the Input Drive it is constantly recalculating the network which increases CPU usage."

This might be why I haven't been able to duplicate it. I've only tried it with the Mesa Recto models. I'm going to try some of these other models and see if I can reproduce the spike on the III. I also saw a post where it was mentioned that the CPU spike came when attaching the controller, not when using it which is also something I haven't experimented with yet. Maybe it did spike when I attached it and didn't notice.

On the older units, was it common for there to be audio problems from doing this or did that only happen if you exceeded the maxCPU threshold? I haven't noticed any glitching of any kind with my III.
Let us know what you find!
 
So, what's wrong with just using a volume block in front of the amp, or attaching a modifier to the input trim of the amp? I don't get why you would want to control the input drive in real time, since it often changes the character and EQ of the amp too. Especially on amps with a large treble cap. Doing this on say, a Marshall would work less then ideal.

That bold part actually is the reason I want to use the drive control and not input trim. That effect is desirable on some amps. On the Mesa Rectifier model I'm using as you roll the gain down it thins out a little bit naturally and, to my ears anyway, in a way that's actually pretty nice. Roll on the gain and it warms up a bit and the transition is seamless. I've got it set to adjust from around 2 to a little over 6. The other scenes in the preset, the pedal is still active but I usually have it heel down as those scenes have drive blocks active. I'm doing the same thing on another preset using a Friedman model and no issues with that one that I've noticed but I don't use it as much.

I'll be experimenting with it and I'll keep the thread updated with what I find. I want to try to duplicate the cpu spike just to see if the III does it at all. It seems to not even notice me using it but I don't know if the cpu load was lower before I activated it in the first place or by how much if so. We'll see soon.
 
Update #1: Just spent some more time thrashing away on it while staring at the CPU meter. The CPU fluctuation in this Mesa Recto preset is about 61.1% - 61.9% at idle (volume down on the guitar). When playing it will spike up to mid 63% and I never saw it hit 64% even once. I was even working the gain pedal and the wah at the same time trying to hit 64% but I couldn't get it there.

Then I tried disabling it completely by setting that control parameter to None. I was watching the CPU meter as I did it. Nothing. It didn't react at all. I then thrashed on it some more just using the wah and the fluctuation range was exactly the same. Got up to 63.9% but never did hit 64% and didn't drop below the previous idle minimum of 61.1%.

So far at least with the Rectifier model, it seems to not be affected by this even slightly. I did the same on the Friedman model I'm using and same result but I didn't watch it quite as closely. I'll try some of the models that were known to have issues as soon as I have time.
 
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