Power amp without a 'parallel' setting...

Discussion in 'Amps and Cabs [BG]' started by Jemetch, May 5, 2005.

  1. Hi.

    I have just purchased a SoundTech PS1300 power amp. The amp can run in stereo or bridged modes, but has no 'parallel' setting. I want to drive both sides of the amp with the same signal, however my preamp only has one output jack (you would normally put the power amp in 'parallel' mode in this situation....)

    A search here revealed a previous thread where someone recommended using a simple 'y-cable' from the preamp out to both inputs on the power amp....

    Is this the only viable/technically sound option?

    I've been looking at a/b/y pedals etc but am concerned about how they would work in this application - since the signal from pre to power amp is higher than the 'instrument level' signals they are designed for...

    So yeah... any thoughts?... I guess specifically on:

    - Is a y-cable the best solution? Will there be any other detrimental factors involved with using one apart from a 3dB drop in volume. ??

    - Can a/b/y boxes be used in this application without clipping / other problems...??

    Any help greatly appreciated :D .

  2. Brendan


    Jun 18, 2000
    Austin, TX
    I'm not sure I understand the nature of your problem. You're wanting to run a stereo rig, but with a mono signal?

    Or, do you mean you want to run both cabs with as much juice as you can? In which case you hook both cabs up to the power amp in Bridged mode, for maximum draw (well, assuming they're both 8 ohm cabs).

    I can't seem to get a manual off the SoundTech website, so I can't help a great deal, not knowing the features (though, I probably could if you gave me a closeup shot of the rear panel).

    A Y cable of ABY box should be absolutely unnessicary. If for some reason they are not, you've purchased a crappy power amp.
  3. Joe P

    Joe P

    Jul 15, 2004
    Milwaukee, WI

    the general rule is: with like regular 'active', buffered stuff like that, it's perfectly fine to fan-out two or three from the outputs. What you don't want to do is 'Y' two OUTPUTS together into one input!

    ..What I mean by "active, buffered" is simply that this doesn't hold so true for mics or passive instruments or things like that, but any rack stuff or pedals or the like.

  4. Thanks for your reply.

    As I understand it, many power amps (QSC for e.g.) have 3 settings - stereo, parallel & bridged.

    In stereo, the amp acts at two separate amps, each with their own input signal & output.

    In parallel, the amp acts as two separate amps, however they have the same input signal.

    In bridged, the amp acts as one amp... with more power.

    My amp (being a bit old...) only has stereo & bridged modes.
    I do not want to run it bridged in some applications... but want to run it stereo, with a separate cab powered by each side.

    Therefore, I have to feed the signal (from my preamp) into the both the A & B inputs on the amp... Now normally, this is exactly the situation where the 'parallel' setting would be used. Then I would simply run a patch cable from the output of my pre to the (A) input of my power amp, flip the switch to 'parallel', and... the power amp would use the input on (A) as the input for both sides of the amp.

    However, I have to do this manually - take the signal from by preamp & split it so I can plug a cable into both the (A) and (B) inputs on the power amp...

    Any clearer?? It is not completely uncommon problem... (I found a number of references to it in past threads..)
  5. Thanks Joe.

    Exactly what I was hoping to confirm/deny.. :cool:
  6. Brendan


    Jun 18, 2000
    Austin, TX

    Ah, so you're looking to say, run the cabs at different volumes, or some other stereo type application (which would preclude just using the bridge mode, yes)? Otherwise, I'm not sure why you'd need two amp sections for two cabs.

    That said, I checked out Soundtech, and yeah, it does seem to be a bit older. It's not being made right now, anyway. This being the case, your amp just might be stupid (well, you know what I mean).

    Even that said, could you get a closeup picture of the rear panel? It seems rather odd that someone would manufacture a power amp without the ability to run both sides off one signal, and seeing the panel will either explain if it's possible, or if the situation will indeed require a Y cable or ABY box.
  7. Passinwind

    Passinwind I Know Nothing Supporting Member

    There are dozens (if not more) of very nice power amps like that. Not adding needless complexity is a substantial rationale for making amps that way. "Stupid" applies if it doesn't do what you want it to though, I reckon. ;)

    Ah, so you're looking to say, run the cabs at different volumes, or some other stereo type application (which would preclude just using the bridge mode, yes)? Otherwise, I'm not sure why you'd need two amp sections for two cabs.

    A classic reason would be to match impedances that bridging won't deal with. Say, 2 different four ohm cabs. Most amps will not do 2 ohms bridged, and dissimilar cabs in series are not too hip, generally. Dual mono is the preferred choice then, no?
  8. Brendan


    Jun 18, 2000
    Austin, TX
    Agreed on the definition of "stupid."

    As to the impedance matching; yeah, like I said earlier, "well, assuming they're both 8 ohm cabs."
  9. My Crown(CE 1000) is the same way,it doesn't have a parallel conection either.I just use a Y cable fron the output of my RBI preamp and it works fine,no lack of signal that I can tell.My cabinets are 4 ohm so when I use one cabinet I use the bridged mode,this is the lowest impedence it will do bridged.When I use two of my cabinets I run the amp in stereo with one cabinet at 4 ohms on each side,thus the need for the Y cable.I don't think Crown makes any crappy poweramps.It doesn't have all the bells and whistles some other amps have but it works great for me.Hope this helps with why to use a Y cable.
  10. Passinwind

    Passinwind I Know Nothing Supporting Member

    Heh... :cool:

    As to the impedance matching; yeah, like I said earlier, "well, assuming they're both 8 ohm cabs."

    Yep, but some of us don't like to do bridged mono at 4 ohms, or anything at 2 ohms. Modern stuff handles it better than the old "in my day, snowdrifts uphills both ways and we liked it" gear us geezers had to deal with, but it still scares and confuses me.

  11. 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
    Yes, for what you want to do, use a Y cable.

    The alternative is to open the amp up and electrically join the inputs together. That would be a PITA compared to the Y cable.
  12. Thanks for all the feedback.

    Brendan: I have the manual and I've seen the back of the amp... and it is a situation where a y-cable or similar is needed. Hence this thread... :D

    edit: But to prove that I am a reasonable man :p .. here is the back of the amp....


    13 - Bridge Mode Switch - Bridges inputs and outputs of channels A and B for high level, monophonic operation

    Yeah... as some of you suspected I have a 4ohm 4x10 and a 8ohm 15". So there are several reasons that I want to do... what ..er.. I want to do...

    #1 - The power amp doesn't like bridging into less than 4 ohms
    #2 - A parallel setup will let me balance the volume of the two cabs (as you can imagine a 4ohm 4x10 is a bit louder than a 8 ohm 15)

    This may not be the best solution... but it is the best I could come up with using the resources available (note my location...). My only other real alternative (in terms of power amps) was one manufactured by a company we wont mention... especially with Bob Lee's post standing so close to this one :ninja: ...
  13. jondog


    Mar 14, 2002
    NYC metro area
    I'm not sure, but many amps have the XLR and 1/4" inputs linked in such a way that you can use one as an input and the other as an output (still line level). So, if you went into the ch. 1 XLR in, you could try plugging the ch. 1 1/4" in into either ch. 2 input and it would probably be the same as using a Y cable.
  14. Hey... that sounds like it might just work...

    I'm sure a little multimetereringing will sort out what is connected to what... before I turn it on & am surprised by a pretty light & smoke show...

    Will let you know (when the amp arrives) if anyone is interested.
  15. Brendan


    Jun 18, 2000
    Austin, TX
    It's an idea, certainly. And, if it works, I am vindicated (needing neither a Y cable not an ABY box). Okay, so it'd only be a little vindication, but still, any vindication is good.
  16. True true.

    Nothing like some good ol'-fashioned vindication.

    Will definitely let you know.... :D
  17. Brendan


    Jun 18, 2000
    Austin, TX
    Only thing, though, is if it did do the parallel input thing (my Carvin does) there should be a switch on there that says something like "Parallel Input On/Off" on it. I don't see anything like that on there. Of course, it might be on the front. But, unless it's mentioned somewhere, I don't think they're parallel inputs.

    Of course, I'm batting .000 in this thread, so don't take my word for it.
  18. hmmm... I still don't think you're quite with us.. :D

    All the parallel switch on an amp does is connect the input to the (A) side to the (B) side of the amp as well... so it's just like it has a y-cable on the inside which can be switched on & off... nothing fancy.


    My amp works in stereo - Two power amps, feed by 2 different signals.

    This is just the same, except the two power amps are feed with the same signal.

    The issue is just how to get two versions of the signal from the preamp to the power amp. You can either use a cable which has one plug at the preamp end & two at the power amp end, or if your amp is smart :D , they have foreseen this situation and already wired in a 'y-cable' which you can just switch on...
  19. Brendan


    Jun 18, 2000
    Austin, TX
    No, no, I'm with you, I'm talking about jondog's idea about using an XLR to 1/4" instead of Y cable. He's saying that a lot of poweramps offer parallel inputs (so you can chain several together, like in a huge PA), and that if you looped the parallel out put back to the input, you could essentially do that.

    What I was saying, was that it didn't look like your amp offers the parallel input option, which differs, in that one input is an In the other is an Out in parallel input mode, not what you've been talking about, which is parallel amp operation on a single signal. Parallel inputs are different than the amps being parallel. (Christ, I think I'm right about that...)

    Which, I was saying it doesn't look like you can do, since you apparently don't have a parallel input option (like a through signal, not silmultaneous). So, a Y cable is probably the only solution, at this point. Yeah, I said it, since you lack parallel input and parallel amp operation options. Neither of which you mentioned until I made an ass of myself. Not that I helped, mind you, I make an ass out of myself on a regular basis.

    I'm still in the building, just in a different room.
  20. Dan1099

    Dan1099 Dumbing My Process Down

    Aug 7, 2004
    given 1/4" and XLR inputs on the same channel, Brendan, I would assume that they are in parallel by design. Therefore, running a cable from the unused jack on channel 1 into channel 2 SHOULD work. I dont think there would be a switch for such a feature.