wknight2 said:
On a side note, I spent a good deal of time fiddling. Using the JCM800, if the master is set near all the way up 9.75, controlled feedback is easier. I really didn't see any difference flipping the phase (used the Bogner and JCM800 as test cases). Is the phase something that works better on certain amp models?
The Atomic is supposed to arrive today, so I'm itching to do more tests tomorrow with it.
fremen said:
I'm able to have controlled feedback at very low volume on crunch sounds, and my guitar sustain is weak.
Just a question, how do you set your input1 ?
What nobody is chiming in here with is the very nature of feedback. Feedback happens when you create a loop between an audio input and an audio output in a circuit in which the gain of the input is increased. When signal goes into the input and comes out the output, if it gets picked up again by the input and passed through again, with its gain increasing, eventually you will reach a point where conditions are right for oscillation. If you want the science, and the debates, look up Larsen Effect, Barkhausen Stability Criterion, Nyquist, et al.
What you will find is that when we are dealing with audio signals being fed to a microphone (which in this case would be a guitar pickup) and from the microphone into a circuit that increases the gain, unless all the frequencies are completely flat (impossible to achieve), as the gain is increased some frequencies get their gain increased faster than others. Ultimately, one of the frequencies will trigger oscillation (in theory, provided, its phase is an integer multiple of 360 and its gain equals 1) . Which one that is depends on many factors. Without going into the issues of angularity and such - which I understand just enough to be really dangerous - the factors when it comes to electric guitar feedback boil down primarily to these (oversimplifying): resonance frequency, gain, output volume, directionality, and the frequency distribution of the output device (the speaker).
To vastly oversimplify even further - If you send a signal from a guitar pickup into the Axe, and it ultimately comes out the speaker, if that speaker sends out a frequency that resonates with a string in front of the guitar pickup and causes it to send that frequency again - you'll get feedback. By the same token, if the speaker sends out a frequency that causes the pickup to vibrate (resonate), then the pickup could send its resonance frequency back through the chain, and you'll get feedback. If the frequency causes the guitar to vibrate enough to vibrate the pickup, if the frequency causes the pickguard to vibrate, if the frequency can cause anything to vibrate such that the pickup sends that frequency back through the amplification, you'll get feedback.
Now, if we WANT to get feedback, what are the problems? Well, the less inclined the pickup is to oscillate, the less inclined it is to send the frequency in question back through the circuit. So if it's bolted to wood, and the wood isn't terribly resonant, and your processing chain isn't kicking the gain up much and the volume isn't terribly loud, and you don't point the pickup directly at the speaker, and the distribution of the frequencies from the speaker is pretty flat, you aren't going to get feedback. On the other hand, if the pickup is mounted to a pickguard, and the pickguard is screwed into resonant wood, and the volume is pretty loud, and you point the pickup at the speaker, and the speaker emphasizes a range of frequencies that include the resonant frequency of the pickguard (or the guitar such that IT triggers the resonant frequency of the pickguard, that pickups gonna start oscillating and you're going to get feedback.
So controlling feedback is about getting things to oscillate at a frequency that you like and keeping it stable. Boost the volume enough and it's a strong possibility that you're gonna get feedback at some point. Use a guitar cabinet instead of a FRFR speaker, and you're going to increase the likelihood because guitar cabs and speakers make certain frequencies louder and kill others, whereas an FRFR solution is trying to evenly amplify all the frequencies it gets from the level it gets them up to the limitations placed on it. Use a Strat instead of a Les Paul and you'll increase the likelihood. Use a high-gain preamp rather than a low-gain preamp and you'll increase the likelihood.
I keep saying likelihood because while Barkhausen's criterion arguably dictates necessary conditions for oscillations, it doesn't necessarily define sufficient criteria.
All right, so what do I - with my limited and dangerous knowledge - think getting controlled feedback out of the Axe and FR speakers is going to take? Finding the frequencies you'd like to get the loop to oscillate at and boosting the hell out of them. Factors that are going to be involved: volume is number one, because no matter how much you boost some frequency, it's going to have to come at the guitar with sufficient magnitude to trigger sympathetic vibration. Gain is number two, because that's obviously the first step in getting to oscillation - increasing the gain of the signal. People have mentioned phase, and technically phase is important because Barkhausen's criterion says oscillation can occur if the phase of the frequency in question is an integer multiple of 360 and the gain of the frequency = 1. So inverting the output phase will, in theory, get us closer.
From there you want to start fiddling with frequencies, and you get lots of possibilities with that. The tone controls of the amp model. The model itself. The center frequency of the preamp. GEQ blocks. PEQ blocks. Global EQ. It's about finding that frequency that you want to feed back, giving it enough gain and volume, establishing the proper phase relationship, and letting nature take it's course.
Again, I'm oversimplifying all this, and Cliff and others hear will likely howl at my dunce's attempt at explanation. But they hadn't chimed in yet, so I did.
I just wanted to make sure you realized that it isn't about just inverting the phase, or just doing this, or just doing that. Yes, the environment has a big impact, but that impact is not a restriction. It's a relationship of various factors that have to be addressed to get what you want.
