Sound thins out when playing 2 amps...

Discussion in 'Amps and Cabs' started by cap'n'crunch, Dec 11, 2005.

  1. cap'n'crunch

    cap'n'crunch Member

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    Each amp on its own sounds fine but, when I plug both into a stereo 4x12 cab the sound thins out. Kinda like the sound you get when you wire pickups in reverse phase. I made a custom speaker cable to reverse the polarity of one amp hoping to solve this problem but, it still does it. Any ideas whats going on here? I know people use multiple amp rigs but, I've never heard anyone having this problem.

    Thanks
     
  2. axpro

    axpro Member

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    check your wiring on the custom cable, the only thing i can think of is phase cancelation!
     
  3. Dave Orban

    Dave Orban Gold Supporting Member

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    +1.
     
  4. LSchefman

    LSchefman Member

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    As I understand it, different amps can be out of absolute phase with one another. So the phase of the cables can be "correct" but the result can still be out of phase.

    You may have to resort to trial and error and do it by ear.
     
  5. bjm007

    bjm007 Member

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    If it's like most "stereo" 4x12's it doesn't have a baffle in-between the two sides of the cab. Even if it does, it's still one cab. The output resonances from two discrete amps get forced together inside the airspace of that single cab.

    IMO, you're never gonna get quite the same sound as two separate mono cabs creating a "true" strereo soundstage by using just a single 4x12 and feeding the outputs from two separate amps into it.

    The outputs from the heads are going to be different and therefore the resonances created by two separate cabs are going to be slightly different as well. They need their own separate airspace to vibrate and create a true stereo effect...
     
  6. Randy

    Randy Member

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    As I understand it, two amps that have different numbers of gain stages (one odd, one even) will be out of phase with each other. You need a splitter that has a phase reverse switch to correct this. I use the Axess BS2 which works great. I don't know if reversing the phase on your speaker cab will help.
     
  7. Swarty

    Swarty Member

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    If I understand correctly you have 2 heads, 2 speaker cables and a stereo cab w/ 2 inputs. If this is the scenario you just need to reverse the leads on end of one speaker cable.
     
  8. bluesdoc

    bluesdoc Gold Supporting Member Gold Supporting Member

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    He did that:

    I made a custom speaker cable to reverse the polarity of one amp hoping to solve this problem but, it still does it
     
  9. DANOCASTER

    DANOCASTER Supporting Member

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    Because I enjoy the multiamp thing - whenever I get a new amp, I make sure its in phase w/ my others. It's easy :

    Take a pedal w/ dual outs - a DD-20 , a stereo chorus, etc.. - keep the effect in bypass ( OFF ). Feed each of the outs ( one left and one right ) to the amps in question. If they seem to thin out, get really "wide" sounding , lose bottom , etc..they are OUT of phase. Reverse / resolder the +/- speaker wire in ONE of the amps /cabs. That should totally fix your problem !!

    REMEMBER : if one amp is a reverb combo w/ 2 channels ( like a blackface fender ) - ONLY the side you initially tested will now be in phase. The other side will STILL be out of phase ( in fact , the other side WAS initially IN PHASE but you changed it ) SO choose which channel you will want to use in a multiamp setup - you cant use both
     
  10. Dave Orban

    Dave Orban Gold Supporting Member

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    Are the AMPS out of phase, or the SPEAKERS?

    If it's the SPEAKERS, couldn't you just turn one of the cabs around...? (Presuming, of course, that it's an open-back combo...)
     
  11. RL in Fla

    RL in Fla Member

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    The reason flipping the polarity on one cable doesn't do it - if the stereo cab wiring is a simple setup (shorting jack and not a double pole switch) , the grounds (- terminals) to all the speakers in that cab are still tied together .
    If the jack plate is metal and the jacks aren't isolated, same deal , even if it's switched . Common grounds .
     
  12. aeolian

    aeolian Member

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    Phase isn't always an either or proposition. Tone circuits skew the phase of a signal all sorts of ways. As someone mentioned, both sets of speakers in one cab is going to be harder to mix signals that might be 90 degrees out of phase instead of 180. Remember phase shifters? You might be anywhere along that continum.

    And the common connections at the cab doesn't matter. If you reverse one wire, you'll be taking the amp output and putting it 180 out of phase with the other.
     
  13. Shea

    Shea Member

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    And shorting out the output from that amp. I think RL's right.

    Shea
     
  14. RL in Fla

    RL in Fla Member

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    Correct , but then you also just reversed polarity , unless that output isn't referenced to ground anywhere .
    I'm not arguing phase at all , just saying that as long as the 2 cabinet "halves" are tied together (common -)
    you can't change one amp output without effecting the other at the speakers .
    Then again , my argument may not have any teeth in it . :p
     
  15. cap'n'crunch

    cap'n'crunch Member

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    Thanks for all the replies. Here's the scoop: The 4x12 cab in question was a mono cab to start with and I modified it to be a stereo cab. I rewired it so that the 2 speakers on each side go to different input jacks. There's no switching or shorting jacks like on a Marshall mono/stereo cab. Just seperate jacks for each half of the cab. Like I said originally, I switched the polarity of one side to see if the phasing would go away but, it did not. Guess I'll have to get 2 seperate cabs huh? Thanks again for the replies
     
  16. Mark Robinson

    Mark Robinson Member

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    There is also the possibility of a latency induced time delay, creating a volume cancelling partial offset in the two amps phase. I have no idea of what you are running, but if it's a setup where one of the two amps has something like a G-major or similar time based effect device, jacked into the signal path in series, that could create the exact sound which you have described.
     
  17. cap'n'crunch

    cap'n'crunch Member

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    I hate to drag this out but, Actually what I'm doing is running a wet/dry rig. I'm plugging into a JCM800 2204 (dry) that has a preamp out. I run a signal from the JCM800 preamp out to the power amp in of a Laney AOR protube 50 via the Alesis Midiverb for the wet signal. The sound from both power amps goes into the stereo cab that I described in an earlier reply. Thanks
     
  18. Giga

    Giga Member

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    I've been following this thread because I was curious what could be going on. From your last post I understand that you're not using the entire Marshall signal, right ? You use the Marshall pre-amp signal to feed the wet part of the rig if I understand it correct. That could be the key here; when you play the Marshall by itself you're hearing pre-amp + poweramp -> fat tone. Try this: use a line-out from the Marshall (= line-level signal taken from the speaker signal), I use the H&K Redbox Pro for this but f.i. a hotplate has a line-out too, from there go to your fx and seperate poweramp(s).

    Good luck !

    Giga.
     

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