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previous: dale ok, i tried the phase test. i sent ... -- 4/24/2000 10:01 PM

Re: Phase and Polarity

4/24/2000 10:42 PM
Ray Ivers	Re: Phase and Polarity Dale, I read your first reply post, too, so maybe I can answer both (or neither?) with this reply. Dale, here's the only thing I can think of to be done to solve this problem, and after you hear it you may decide it's not really too much of a problem after all (by the way, your test would still be valid even though the two speakers are different - there will be enough parts of the frequency spectrum that are shared equally by both speakers that the phasing 'swoosh' would be heard). A signal generator and 'scope will be needed. There's a more sophisticated test called the Lissajous test which I'm NOT going to describe - this one should be ballpark enough. First, you would feed the input of the signal generator to both channels of the scope and align both traces to exactly overlap each other, then you would move them apart (up and down). You would then have to put both amps on the 'scope, and feed them both the same signal, and monitor the outputs simultaneously. Move one signal on top of the other (using only the vertical control, not the horizontal). If they both perfectly overlap, both amps' electrical outputs are perfectly in phase. If they don't overlap, but aren't 180 degrees out (which could kinda sorta be fixed by speaker cable polarity swapping), you would have to add or subtract RC networks in the amp. To eliminate them, you have to usually eliminate whole gain stages, or DC-couple stages together. The first step is a bummer, the second one is real difficult to do correctly, even with a CAD program doing the math for you. There's one place, though, where a 60 positive phase shift can be had really easily - the input to the tone stack. Most every amp I've ever seen runs the output from the plate directly to the tone stack - after all, you can't escape the stack except through a cap. If you were to capacitor-couple to the tone stack (through at least a .22 or .47 uF cap), you would get 60 degrees phase shift no problem. There may well be other opportunities as well - would you like me to look for them? I have the Jubilee print, but not the Peavey. I mean, we can add RC networks to different places in the amp to add phase shift and try to get both amps perfectly in phase. I don't know if I can wholeheartedly recommend doing it myself, but it's your amp - you be the judge. All this stuff is really important in hi-fi amps using heavy negative feedback. Three 60-degree phase shifts equal one 180-degree one, and all of a sudden your negative feedback has turned positive and your amp's an oscillator. I know that whenever I get the chance to advise someone about a stereo rig, I always tell them to use identical EVERYTHING in both sides - amps, speakers, cabs, tubes, you name it. This is for a stereo rig; the reason most people use multi-amp non-stereo setups is to obtain just the sorts of phase-related tonal changes you're trying to eliminate. Here, you would want different amps, cabs, speakers, grille cloth, amp placement, etc. - but if you're going to throw everything in the pot, you had better like stew. I would think there would be a maximum of 60 degrees phase shift between the two amps, coming out of one amp and back in thru both amps' effects returns. To say for sure, I'd need the Peavey print. Let me know what you decide to do! Ray

4/24/2000 10:42 PM

Ray Ivers

Re: Phase and Polarity

Dale,

I read your first reply post, too, so maybe I can answer both (or neither?) with this reply.

Dale, here's the only thing I can think of to be done to solve this problem, and after you hear it you may decide it's not really too much of a problem after all (by the way, your test would still be valid even though the two speakers are different - there will be enough parts of the frequency spectrum that are shared equally by both speakers that the phasing 'swoosh' would be heard).
A signal generator and 'scope will be needed. There's a more sophisticated test called the Lissajous test which I'm NOT going to describe - this one should be ballpark enough.

First, you would feed the input of the signal generator to both channels of the scope and align both traces to exactly overlap each other, then you would move them apart (up and down). You would then have to put both amps on the 'scope, and feed them both the same signal, and monitor the outputs simultaneously. Move one signal on top of the other (using only the vertical control, not the horizontal). If they both perfectly overlap, both amps' electrical outputs are perfectly in phase.

If they don't overlap, but aren't 180 degrees out (which could kinda sorta be fixed by speaker cable polarity swapping), you would have to add or subtract RC networks in the amp. To eliminate them, you have to usually eliminate whole gain stages, or DC-couple stages together. The first step is a bummer, the second one is real difficult to do correctly, even with a CAD program doing the math for you. There's one place, though, where a 60 positive phase shift can be had really easily - the input to the tone stack. Most every amp I've ever seen runs the output from the plate directly to the tone stack - after all, you can't escape the stack except through a cap. If you were to capacitor-couple to the tone stack (through at least a .22 or .47 uF cap), you would get 60 degrees phase shift no problem. There may well be other opportunities as well - would you like me to look for them? I have the Jubilee print, but not the Peavey. I mean, we can add RC networks to different places in the amp to add phase shift and try to get both amps perfectly in phase. I don't know if I can wholeheartedly recommend doing it myself, but it's your amp - you be the judge.

All this stuff is really important in hi-fi amps using heavy negative feedback. Three 60-degree phase shifts equal one 180-degree one, and all of a sudden your negative feedback has turned positive and your amp's an oscillator. I know that whenever I get the chance to advise someone about a stereo rig, I always tell them to use identical EVERYTHING in both sides - amps, speakers, cabs, tubes, you name it. This is for a stereo rig; the reason most people use multi-amp non-stereo setups is to obtain just the sorts of phase-related tonal changes you're trying to eliminate. Here, you would want different amps, cabs, speakers, grille cloth, amp placement, etc. - but if you're going to throw everything in the pot, you had better like stew.

I would think there would be a maximum of 60 degrees phase shift between the two amps, coming out of one amp and back in thru both amps' effects returns. To say for sure, I'd need the Peavey print.

Let me know what you decide to do!

Ray

Replies:

dale ray,you know, i thi... -- 4/24/2000 11:05 PM