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previous: dale marshall jubilee-out of phase ? -- 4/24/2000 3:30 PM

Phase and Polarity

4/24/2000 7:36 PM
Ray Ivers	Phase and Polarity Dale, First off, so people will know what I mean: Polarity - absolute phase relationship to the original signal through a single transmission system (i.e., you pull up on the guitar string and let it go 'spang!' against the pickup, and the speaker moves out, towards the listener). Phase - relative relationship of signal timing and speaker motion through multiple, parallel-connected transmission systems (amps/effects/speakers/etc.). Also - inverting polarity (reversing speaker leads) is NOT the same as a 180 degree phase shift. This IS true for a completely symmetrical waveform, like a pure sine wave, but is untrue for any real-life musical (at least somewhat asymmetrical) waveform - draw it out on paper with a half-sine/half triangle waveform or something, and try it yourself. The triangle and sine portions will occur at the same moment in time with a polarity-reversed waveform; combine these two and they'll cancel. But the phase-shifted waveform will have the sine portion combined with the triangle part, and you'll get a bunch of even harmonics with no fundamental when the signals are combined. Every time a signal passes through a an RC or RL network, the phase shifts by 60 degrees, in opposite directions. Two amps being fed an identical signal, driving identical speakers whose cones both move outward when a 9 volt battery is connected to the speaker terminals, + to + and - to -, can have any number of phase relationships to each other, changing with frequency and possibly other factors, depending on how many capacitors and/or inductors are involved with the signal path. The diode clipping circuits that are used in the 2550 and 2555 Jubilee heads may also affect signal polarity and thus apparent phase in a multi-amp setup, but I don't know this for sure. So, it's quite possible that you could have two amps that were 120 degrees out-of-phase (about 30 percent) or 300 degrees out (75 percent), or anything in between. Are there any phase shifter pedals that allow you to stop the sweep like a flanger? If so, you could get a pretty good idea of what phase shift sounds like. Can you run in through the effects/power amp in, which is bound to eliminate a bunch of RC networks? If you already are, try going in through the front. Try having someone else play through the amp, or better yet, use a CD as a signal source - full-range music is a much better diagnostic tool for subtle problems than guitar is, and you can concentrate fully on the amp's sound rather than your playing. Then, turn the volume on one amp up and down relative to the other, and listen for the 'swoosh' of a phaser. Testing absolute phase response of an amp is hard without a scope, Dale. I guess it wouldn't be too hard to construct a box with six .1 uF caps and 2.2M resistors, and switches to switch them in and out, to put in your loop to test phase response using the above volume-control technique. Does any of this help at all? Ray

4/24/2000 7:36 PM

Ray Ivers

Phase and Polarity

Dale,

First off, so people will know what I mean:

Polarity - absolute phase relationship to the original signal through a single transmission system (i.e., you pull up on the guitar string and let it go 'spang!' against the pickup, and the speaker moves out, towards the listener).

Phase - relative relationship of signal timing and speaker motion through multiple, parallel-connected transmission systems (amps/effects/speakers/etc.).

Also - inverting polarity (reversing speaker leads) is NOT the same as a 180 degree phase shift. This IS true for a completely symmetrical waveform, like a pure sine wave, but is untrue for any real-life musical (at least somewhat asymmetrical) waveform - draw it out on paper with a half-sine/half triangle waveform or something, and try it yourself. The triangle and sine portions will occur at the same moment in time with a polarity-reversed waveform; combine these two and they'll cancel. But the phase-shifted waveform will have the sine portion combined with the triangle part, and you'll get a bunch of even harmonics with no fundamental when the signals are combined.

Every time a signal passes through a an RC or RL network, the phase shifts by 60 degrees, in opposite directions. Two amps being fed an identical signal, driving identical speakers whose cones both move outward when a 9 volt battery is connected to the speaker terminals, + to + and - to -, can have any number of phase relationships to each other, changing with frequency and possibly other factors, depending on how many capacitors and/or inductors are involved with the signal path.
The diode clipping circuits that are used in the 2550 and 2555 Jubilee heads may also affect signal polarity and thus apparent phase in a multi-amp setup, but I don't know this for sure. So, it's quite possible that you could have two amps that were 120 degrees out-of-phase (about 30 percent) or 300 degrees out (75 percent), or anything in between. Are there any phase shifter pedals that allow you to stop the sweep like a flanger? If so, you could get a pretty good idea of what phase shift sounds like.

Can you run in through the effects/power amp in, which is bound to eliminate a bunch of RC networks? If you already are, try going in through the front. Try having someone else play through the amp, or better yet, use a CD as a signal source - full-range music is a much better diagnostic tool for subtle problems than guitar is, and you can concentrate fully on the amp's sound rather than your playing. Then, turn the volume on one amp up and down relative to the other, and listen for the 'swoosh' of a phaser.

Testing absolute phase response of an amp is hard without a scope, Dale. I guess it wouldn't be too hard to construct a box with six .1 uF caps and 2.2M resistors, and switches to switch them in and out, to put in your loop to test phase response using the above volume-control technique.

Does any of this help at all?

Ray

Replies:

dale ray,while you lost ... -- 4/24/2000 8:11 PM
dale ok, i tried the phase test. i sent ... -- 4/24/2000 10:01 PM
Trace q{Also - inverting polarity (revers... -- 4/25/2000 2:06 PM