• We would like to remind our members that this is a privately owned, run and supported forum. You are here at the invitation and discretion of the owners. As such, rules and standards of conduct will be applied that help keep this forum functioning as the owners desire. These include, but are not limited to, removing content and even access to the forum.

    Please give yourself a refresher on the forum rules you agreed to follow when you signed up.

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

REDD

Forum Addict
I'll have to try this tomorrow. I see people posting it might not be good for live use, I use everything live, in ears, 4x12, CLR's and to FOH. Would this not be good to use?
 

NeoSound

Fractal Fanatic
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:
View attachment 58779

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!
What would be the best way to get that extra (exponential) spike in volume that comes with the extra gain? The thing I've noticed with tube amps is to pick softly and all is calm, no changes to setting and picking hard rattles the windows! I know most of this comes from volume only, but it seems there could be a way to simulate this? I believe some players think they are squeezing tone when they are actually squeezing volume (and slightly gain/tone)? That probably doesn't translate much in a recording but in the room it's pretty massive,
 

Tom Morris

Forum Addict
I have been using a bass shaker lately along with my in ears for practice at home and live. Home I use the ButtKicker gamer shaker attached to my chair and live I have a wood platform with a Sonic Immersion IBEAM transducer bolted to it. Both setups just the AXE FX running through it. With either if you have a good connecting to them say holding the guitar against your hip bone it adds that little extra needed to get feedback. That IBEAM shaker will rattle fillings loose LOL.

Another idea would be to shock mount a transducer to your mic stand and then lean the guitar body against it when you need that little extra.

https://www.parts-express.com/sonic-immersion-vt200-ibeam-vibro-tactile-transducer-bass-shaker--300-920

IMG_3491.jpg
 

Stratoblaster

Fractal Fanatic
The acoustic coupling effect has been documented and talked about for some time now but it's very eye-opening to see the hard math and how small parameters changes affect the system gains as a whole as outlined here. Interesting write up and again, I love how the acoustic coupling effect has been quantified here...thanks for taking the time Cliff.
 
That's exactly what you should use. It's basically a speaker motor without the frame and cone.
I've messed with this quite a bit, hoping it's something I could market eventually. I had some success, it works but haven't yet found a way to work with multiple resonant frequencies. It seemed that no matter the placement or note played it would only feedback on a single frequency. More testing is needed, but also my goal was for it to be in a sense clip-on clip-off or so, so no additional cables were needed and it was fully self contained.
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
I've messed with this quite a bit, hoping it's something I could market eventually. I had some success, it works but haven't yet found a way to work with multiple resonant frequencies. It seemed that no matter the placement or note played it would only feedback on a single frequency. More testing is needed, but also my goal was for it to be in a sense clip-on clip-off or so, so no additional cables were needed and it was fully self contained.
It can't be self-contained. The signal to the motor needs to come from the output of the amplifier and should ideally be delayed a bit. Also, there needs to be EQ to simulate the absorption coefficient. Clamping a motor to the body causes nearly unity absorption whereas in air the body is going to absorb different frequencies more.
 

benvigil

Veteran
You can simulate this by using the Output Compression control and setting the Compressor type to "Feedback".
Output compression is one of my favorites FAS features for mid-gain presets... makes them squishy when you dig in and they clean up all sparkly-like.

In general, though, it really is problematic that many guitarists today have never played through a ripping loud half-stack to know how an amp interacts with the guitar -- it must be super frustrating for Cliff. PhilX calls it "playing the amp"
 

iaresee

Moderator
Moderator
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.
Why not just use a sustainer or sustainiac pickup setup?
 

iaresee

Moderator
Moderator
2) It’s not the same.
As jamming a speaker or motor into a guitar body? Sure it is. Arguably better because the detection for the frequency and change to the physical driver for the strings is such a faster loop than it'd be with a motor. I mean, make it harder than it needs to be if you want to... :D
 

Rex

Legend!
As jamming a speaker or motor into a guitar body? Sure it is. Arguably better because the detection for the frequency and change to the physical driver for the strings is such a faster loop than it'd be with a motor. I mean, make it harder than it needs to be if you want to... :D
Nope. Magnetic induction between an electrical signal and the strings ain't the same thing as exciting body resonances with motor or other mechanical feedback mechanism. With those setups, frequency detection isn't even involved.

Side note: the delay in the feedback mechanism is a factor in the voicing of the feedback.
 

iaresee

Moderator
Moderator
Nope. Magnetic induction between an electrical signal and the strings ain't the same thing as exciting body resonances with motor or other mechanical feedback mechanism. With those setups, frequency detection isn't even involved.

Side note: the delay in the feedback mechanism is a factor in the voicing of the feedback.
Means to an end. Your pickups detect a change o in the field. You do that by moving the strings relative to the pickups. If you want to achieve that by trying to vibrate those two components through the body go for it. Result is the same.
 

Rex

Legend!
Means to an end. Your pickups detect a change o in the field. You do that by moving the strings relative to the pickups. If you want to achieve that by trying to vibrate those two components through the body go for it. Result is the same.
I guess we'll have to agree to disagree.
 

unix-guy

Legend!
Means to an end. Your pickups detect a change o in the field. You do that by moving the strings relative to the pickups. If you want to achieve that by trying to vibrate those two components through the body go for it. Result is the same.
I'm not saying you're wrong, but if that is correct then why would Cliff be trying to design his own solution?
 

iaresee

Moderator
Moderator
I'm not saying you're wrong, but if that is correct then why would Cliff be trying to design his own solution?
¯\(ツ)

Maybe he wants a driver where he can control the frequency? It's certainly easier to play around with motors than it is to play around with pickup-shaped magentic drivers if you're noodling.

No one wants the sound of a revving motor here, they want the sound of a resonanting guitar-amp loop.

Or maybe I'm wrong? Wouldn't be the first time, right @Rex? :)

Guess I could stick a drill to my guitar and see what sounds like?
 

Rex

Legend!
No one wants the sound of a revving motor here, they want the sound of a resonanting guitar-amp loop.
Not a spinning kind of motor. The loudspeaker kind. The part of the speaker that's not the cone or the basket...

1570848352735.png



Or maybe I'm wrong? Wouldn't be the first time, right @Rex? :)
You and I could have a "Who's wrongest?" contest. I know I'd win sometimes. :)


Guess I could stick a drill to my guitar and see what sounds like?
Actually, passing a running drill over your pickups can sound pretty cool.
 
Top Bottom