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:
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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!
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.That's exactly what you should use. It's basically a speaker motor without the frame and cone.
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.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.
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.You can simulate this by using the Output Compression control and setting the Compressor type to "Feedback".
There were guitars in the 80s with a built in speaker under the scratchplate, beneath the strings - but I don't remember the name ....I have a design for that but haven't had time to build it. You don't need a speaker, just a motor.
Why not just use a sustainer or sustainiac pickup setup?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.
1) You have to give up a valuable pickup position.Why not just use a sustainer or sustainiac pickup setup?
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...2) It’s not the same.
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.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...
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.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.
I guess we'll have to agree to disagree.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?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.
¯\(ツ)/¯I'm not saying you're wrong, but if that is correct then why would Cliff be trying to design his own solution?
Not a spinning kind of motor. The loudspeaker kind. The part of the speaker that's not the cone or the basket...No one wants the sound of a revving motor here, they want the sound of a resonanting guitar-amp loop.
You and I could have a "Who's wrongest?" contest. I know I'd win sometimes.Or maybe I'm wrong? Wouldn't be the first time, right @Rex?
Actually, passing a running drill over your pickups can sound pretty cool.Guess I could stick a drill to my guitar and see what sounds like?