Amp channel phase?

Discussion in 'Amps and Cabs' started by mtndog, Oct 16, 2005.


  1. mtndog

    mtndog Gold Supporting Member

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    Based on a reply in another thread which made mention of different channels in the same amp being out of phase, I am prompted to ask those who know - how can you determine this in your own amp?

    Hopefully its a simple matter, that most of us can handle. Thanks.
     
  2. Blue Strat

    Blue Strat Member

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    What amp are you thinking of?

    For instance, all Fender reverb amps with 2 channels have out of phase channels.
     
  3. mtndog

    mtndog Gold Supporting Member

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    What got me started down this path was a post regarding using an A/B/Y box to switch between channels on a non-channel switching amp. The guy didn't name the amp. One of the responses brought up the issue of phase.

    I have an Adam Stark X-Ray that I switch with a Lehle Dual, and I don't perceive any issues, but maybe I'm missing something. That's why I was curious as to what might be a way to know. Thanks for any insights.
     
  4. Blue Strat

    Blue Strat Member

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    The only time phase becomes an issue is when you have both channels on at the same time. Even then, if the channels and settings aren't identical, there are often interesting tones to be found.

    Adam Stark is likely the only person who can answer questions like this.
     
  5. VaughnC

    VaughnC Supporting Member

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    Not to open up another can of worms....but many claim they can hear a difference in the amp & speaker phase. So, mixing channels or not, it may be worth it to just try reversing the speaker cable leads to see if you can hear any difference.

    To get back to the original question though, if you have a schematic diagram of your amp and know how to interpet it, unless there's a cathode follower stage in the signal path (which doesn't invert the signal), each tube stage inverts the signal 180 degrees. The output transformer can also invert the signal if wired to do so. However, unless the manufacturer can supply you with the phase info on their amps, a signal generator and a dual channel oscilloscope would be a sure way to test the signal phase through an amp.
     
  6. Blue Strat

    Blue Strat Member

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    Here's another phenomenon I'd love to see demonstrated in multiple blind tests.

    Phase of the speaker with respect to what? String vibrations which can't be heard if the amp is turned on? :p
     
  7. VaughnC

    VaughnC Supporting Member

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    There are a couple of theories talked about. One is that if the speaker moves foreward on the initial string attack, the amp will sound better than if the initial speaker movement is backwards. The other theory is that, when an amp is overdriven, the clipping may not be linear on both sides of the waveform, so an amp will sound better in one phase than the other. Of course, all this is highly subjective ;) .
     
  8. Laroosco!

    Laroosco! Member

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  9. Blue Strat

    Blue Strat Member

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    Yeah, but phase is relative. Without a reference it doesn't appear to mean anything (eg, one amp or speaker out of phase with another).
     
  10. 908SSP

    908SSP Supporting Member

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    I built a pedal that changes the phase of the speaker line. I put this in line between my amp head and my cab. You can stand back and play your guitar and amp stop for a second hit the switch and play again with the phase of the cab reversed from what it was. My friends and I can hear a difference. One choice has a more responsive interaction between guitar and speaker, it feeds back easier and sustains longer has more harmonics. I don't know scientifically how to explain this but I do hear it. I do hear difference between tubes I don't hear a difference between speaker cables or power cables. ;)
     
  11. hasserl

    hasserl Member

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    I'm NOT getting into that speaker phase issue again! ;)

    But to the original question. Short of knowing whether the channels are in phase or not, if you want to try running the channels together, and are worried about whether they are in phase or not, just give it a try. If you like the results use it. If you don't like it, oh well. No harm done, and there's nothing wrong with trying it.
     
  12. jkr

    jkr Supporting Member

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    I'm glad you asked the question. When ever I've run my 64 Blackface Deluxe Reverb into the Vibrato channel at the same time as I am running my Germino or 4 input Marshall into it's Lead channel, the two amps are out of phase. If I switch to the normal channel on my Deluxe, the amps are in phase again.

    I'd like to run my Germino or Marshall in the Lead and channel and Fender Deluxe vibrato channel at the same time without a phase problem. Short of using a pedal to reverse the phase of one of the amps;..............

    Has anyone reversed the phase of the two channels on their Fender Deluxe Reverb or other type of Fender dual channel amp?If so, what did you do?

    Thanks!

    PS. I don't mean to hijack the thread but I think it is along the lines of the thread.
     
  13. Blue Strat

    Blue Strat Member

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    I can completely agree that "in phase" would result in more feedback. As for more harmonics exclusive of feedback, it just makes no sense. It's almost like saying that you can hear were a sine wave begins.....

    How about multiple BLIND (someone besides the player is doing the switching) tests (excluding feedback) where the same conclusion is reached 90+% of the time?
     
  14. Laroosco!

    Laroosco! Member

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    The Barber launch pad will correct the phase of reverb Fender's so both channels can be used at the same time.
     

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