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The "Modelers Don't Clean Up with the Volume Knob" Myth

FractalAudio

Administrator
Fractal Audio Systems
Moderator
One often hears pundits proclaim "Modelers don't clean up when rolling off the volume knob". While this may be true of some products we actually test and compare this to our reference amps. We measure the THD and output volume at different stimulus levels to ensure that the response is the same.

The reason for this myth stems from acoustic feedback. Real amps are LOUD. Modelers are usually played at much lower volumes.

Consider the following diagram:
volume_myth.png

This is a block diagram of a model of what happens when playing a guitar with a speaker. Vg is the signal generator (your guitar). Sound waves from the speaker are fed back to your guitar and add to that signal. This signal is then attenuated by the the volume pot, k. The signal is amplified by the amp gain, A. Some portion of that signal is fed back, B.

The formula for a closed loop system like this is Acl = kA / (1 - kAB), where Acl denotes the closed loop gain. The open loop gain is given by Aol = kA.

Let's consider some examples.

In the first example let's assume the amp gain, A = 10, the volume knob is wide open, k = 1 and a mere 2% of the signal is fed back, B = 0.02. Using our formula we get:
Acl = 10 / (1 - 10 * 0.02) = 12.5.

The open loop gain is Aol = 10.

That tiny 2% of feedback has INCREASED the effective gain by 25% (!!!). If the amp is approaching distortion then it will get more distorted.


Now consider what happens if we roll of the volume knob a bit. Let's assume everything else is the same but we set our Log10A volume pot to halfway which means k = 0.1. Now we get:
Acl = 0.1 * 10 / (1 - 0.1 * 10 * 0.02) = 1.02
and Aol = 1

Rolling our volume knob to halfway now only gives a paltry 2% of gain increase for the same amount of acoustic feedback. So when the volume knob is wide open the amp has effectively almost 25% more gain than when rolled off halfway!


Now let's look at what happens when we lower the amount of feedback which would occur if we turned down the volume of our speaker. Let's leave everything the same but reduce our feedback to 1%.

Our first example with the volume pot wide open now becomes:
Acl = 10 / (1 - 10 * 0.01) = 11.1
Aol = 10

And our second example becomes:
Acl = 1.01
Aol = 1


So we see that the closed-loop gain is highly dependent upon the speaker volume. Simply reducing the speaker volume by 6dB lowers the effective gain increase considerably.

When playing with a loud amp the positive feedback from the speaker into the guitar effectively increases the gain of the amp when the volume control is wide open. As you roll the volume control off the amount of gain increase is lower. This gives the ILLUSION that the amp cleans up more when you roll of the volume but it's not the amp that is cleaning up, the signal into the amp is lowered more than if there were no feedback.

When using a modeler people almost always have the volume lower because amps are too loud. Lowering the volume reduces the feedback which in turn lowers the gain enhancement. To compensate people raise the gain of the model but now when you roll off the volume it doesn't clean up as much because the gain is higher. IOW, to compensate for the reduced feedback the user increases the gain, say, 25% to get the same effective gain as the loud amp but when rolling off the volume the amp gain is still 25% higher so it doesn't clean up as much.

P.S.
An interesting result occurs if we let B = 0.1:
Acl = 10 / (1 - 10 * 0.1) = 10 / 0 = infinity.
This is what happens for controlled feedback. The closed loop gain approaches infinity and the loop becomes unstable and oscillates. That's why controlled feedback is easier to obtain at higher volumes, the feedback coefficient is greater. Another way is to move closer to the speaker. Since sound pressure is inversely proportional to the square of the distance moving 50% closer results in four times the feedback!
 
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chucma

Power User
It won't simulate controlled feedback. It does a good job of simulating gain enhancement.
Aah I was just about to ask if this was the same as controlled feedback. So does this mean that controlled feedback is only really achieved by playing the amp at higher volumes?
 

chris

Legend!
volume is a huge part of live audio in many aspects. physics is physics. a recent thread discussed how someone's axe used for live volume didn't "feel" the same to everyone as the other guy's regular amp that was "too loud." well. it's the too loud part that causes it. we need volume to produce some of the effects we're used to from amps that are too loud on stage. it's why our heroes ran their marshalls at 10. the volume did something magical to the guitar/amp interaction. same can happen with the axe at similar volumes. and same "non-magic" happens with real amps turned down to bedroom levels.
 

madmarcus1960

Inspired
Quite the bash-fest going on over at TGP. For starters, the name John Mayer brings out the TROLLS, and then this threads argument on whether or not the volume knob issue is real. It's amazing how much animosity there still is with modeler vs analog, and anything John Mayer!

Personally, call me a rube, I never or more accurately hardly ever roll my volume back.
 

Rex

Legend!
So does this mean that controlled feedback is only really achieved by playing the amp at higher volumes?
Yes. How loud it has to be depends on a lot of things (your guitar, how far away the speakers are, room acoustics...), but it still takes volume to do the job. If you play at whisper-quiet levels, you’ll never achieve that singing feedback
 

chris

Legend!
Aah I was just about to ask if this was the same as controlled feedback. So does this mean that controlled feedback is only really achieved by playing the amp at higher volumes?
feedback is literally audio "feeding back" into the source creating it. if the pickups can't "hear" the speaker, it can't feed back. that requires volume. different things happen at different levels of "loud volume." some play just loud enough to get a bit of feedback. some play much louder causing the "bloom" of notes with everything you play - that's the entire system ready to feedback at any moment. they've learned to control it and it's part of their playing. and if they don't feel that, they can't play how they want.

a person like this picks up a modeler, puts on headphones - which sound "loud" because they're millimeters away from your eardrums - and say huh modelers can't feedback. well of course not. the guitar isn't hearing anything.
 

Joe Bfstplk

Power User
a person like this picks up a modeler, puts on headphones - which sound "loud" because they're millimeters away from your eardrums - and say huh modelers can't feedback. well of course not. the guitar isn't hearing anything.
Exactly this ^^^

It depends on getting a positive feedback loop going with enough loop gain to keep the oscillator formed by guitar, strings, and amp oscillating. The gain can come in the form of excessive volume, excessive overdrive, excessive compression, or any combination of those three that is able to get the loop gain high enough to do the thing. Each way of getting there adds its own quirks to the feel/behavior/sound of the interaction.

I have gotten it going at about dinner table convo or TV volume, but it was really noisy, did not behave/feel very good, and was hard to control. Theoretically (but probably not in practice), you could achieve enough loop gain to get there off headphones if one was open to the air and close-ish to the guitar (i.e., not cupping the ear, but turned toward the guitar), but the amount of gain needed to get there would be insane and very noisy, and the noise level would seriously affect the feel/behavior/sound of the result rather badly.
 

reclavea

Power User
It won't simulate controlled feedback. It does a good job of simulating gain enhancement.
Great insight and I’ve always wondered about simulating the loudness phenomenon with the Axe set at lower volumes.

So when tweaking to achieve this ....moderate gain levels at first? ....work the Output Compression?
 

Smittefar

Fractal Fanatic
Would it be possible to use the Send and Return blocks with a little delay to simulate feedback. A little EQ shaping would probably be needed too.

It could even just be a dedicated feedback block that feeds directly back into one or more input blocks.

Edit: Of course this setup would go behind the volume pot, so maybe you would have to use an expression pedal rather than the volume pot to control the gain
 
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Solarfire

Experienced
For me it’s due to the fact that the Axe cleans up so well with the volume control (I’m a volume/tone control player) that got me hooked on the FAS-gravy-train.

Cliff I can’t thank you enough that my old ass finally reached his guitar heaven with my Axe III, a recapped and serviced Carver TFM-45 and my JBL 4425 studio monitors in my home-studio-setting, truly a guitar-nirvana combination.
 

Lax

Inspired
Yo, I allow myself this comment from my amateur point of view :D
I'm not using my guitars pots much and use lot of cleans or high gain amps but recently I was noodling testing ares 2.0 on my Axefx2 and I was like "wonder if it cleans up rolling back volume on this recto patch", and it did!
Proceeded to try other amps and it's so dynamic and flexible... Mindblowing
So yes, from a guitarist pov, volume pot totally works with Axefx2 :)
Gratz again FAS team
 

solo-act

Fractal Fanatic
Crazy thought: I wonder if anyone has ever used a small guitar mounted speaker to boost that gain/feedback loop, like an extreme version of bringing your guitar close to a speaker to induce more vibration/feedback.

If you could get the physics working OK, find a way to get signal and power to it and control its volume with an expression pedal...instant Santana! LOL.
 
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