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Poweramp in Parallel mode = halved input level?

Discussion in 'Amps and Cabs [BG]' started by Petebass, Jul 29, 2005.


  1. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Here's a strange one! Out guitarist has a small-mid sized PA. He usually runs 2 monitor wedges (4 ohms total) from one side of his power amp. The other side does nothing.

    We're experimenting with 4 wedges, the plan being to run the power amp in parallel so that both sides see the same input signal.

    He tried it at home and claims that in parallel mode, the power amp halves the strength of the input signal. That doesn't sound right to me, but he swears the input to the poweramp gets halved.

    Is this normal. The amp is an old EV P3000.
     
  2. seansbrew

    seansbrew Supporting Member

    Oct 23, 2000
    Mesa AZ.
    I use this configuration from time to time on my QSC power amp and have never noticed any variation in the signal. I don't know that this means there is something wrong with your power amp. Maybe start from scratch and hook it all up again. Is there any way to boost the signal from the mixer? ( + or - mic line switch)Can you run two signals from the mixer ( stereo) ? How many power amps are you using and are you also running mains?
     
  3. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Yep that's what he did. He basically pulled the PA rack apart and put it back together, trying new things and looking for a better overall setup. This poweramp development caught us both by surprise.

    We're feeding the poweramp from the one remaining auxilary send. If's only a smallish mixer (Mackie), the other Aux send is used for the effects.

    I'll get him the check the other PA components, maybe the graphic EQ, for a mic-line switch to boost the signal.

    Only one power amp and it's job is to run the monitors. The mains are all self powered (Yorkville Elite's, good little PA actually. I think the processor helps a lot.)
     
  4. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Unless the signal source has a really high output impedance, paralleling the amp inputs should have no audible effect on the signal strength; they should be getting the same signal voltage either way.
     
  5. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Thanks Bob, that's what I thought. But I'm not sure what would constitute high impedance from the source?

    The source is Beta 58 mics, to a Mackie mixing Desk, to an Alesis EQ, then on to the power amp. That's pretty standard stuff.

    Should I be telling him to take the amp to a technician?
     
  6. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Most power amps have an input impedance in the range of about 10 to 50 kilohms. Most line-level pro audio gear have an output impedance of a few hundred ohms or less. Let's say, for an example, that the source's (the Alesis EQ) output Z is 100 ohms and it's feeding an amp input Z of 20K. If the source's output voltage with no load is 1 volt, then the voltage with the amp connected will drop almost negligibly, to 0.995 volt (a drop of 0.04 dB). Add a second input for the source to drive, by paralleling the inputs, and the voltage would drop to 0.990 volt (a drop of 0.086 dB from the original voltage).

    If the source's output Z were much higher, like 5 kilohms (that would be bizarrely high), then the voltage would drop to 0.8 volt (a drop of 1.9 dB) with one amp input connected and 0.667 volt (a drop of 3.5 dB) with the second one added in.
     
  7. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    What sort of things could he be doing to make the Alesis EQ generate a bizarrely high impedance?

    From memory, balanced signals have a higher impedance that unbalanced don't they? If he's running the entire PA balanced, could that be causeing the problem?
     
  8. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Not necessarily. But even a variation by a factor of ten would have little effect.

    My question would be how does he know the input voltage is halved? Via input meters on the power amp?
     
  9. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Yep, and his ears (which I trust) told him so. I guess if we measured it it wouldn't be exactly half, but if he says it was reduced considerably, I believe him. I don't believe everybody :)
     
  10. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    So the next question: how is he getting the signal into both channels. Y cord? Dual jacks on the EQ? Internal switching in the amp?
     
  11. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Internal switching on the amp. He goes into the input of one side of the amp, switch the amp to parallel mode, and in theory, the other side of the amp should see the same signal.
     
  12. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    When theory fails go to plan 'B'. Have him leave the rig in stereo mode and use a 'Y' splitter to get the input to both channels. If that works OK then there's something wrong with that switch.