Power amp clipping! Help needed please.

Discussion in 'Amps and Cabs [BG]' started by Classic5, Jan 18, 2003.

  1. Classic5


    Mar 31, 2002
    Finally, after about two years of saving, researching etc - I've put together my ideal rig.
    Demeter HBP-1 pre, 2 Wayne Jones 2x10's and currently a borrowed BSS power amp (1200W bridged 4 Ohms).

    One big problem though. The amps protection circuit keeps being activated, thereby switching the amp off. Ahhhh!

    I've tried to hook it up to a bag end s15d using bananna plugs - NO GOOD. I've tried to switch the Demeter from line level to moc level (less output apparently) - NO GOOD. Tried to play with the sensitivity knobs on the amp - NO GOOD. I've tried almost everything - NFG! Low volume too.

    What I want to do next is try the direct out of my GK400RB into the power amp, but I need to put a cable together. The amp has an XLR input so I'm using the XLR output from the Demeter. I need to put together a phono to male XLR so I can hook up the GK into the amp. This should also enable me to try the other line outs on the Demeter which are also 1/4" phono.

    My question therefore is how to hook up this cable - to go from preamp to poweramp (phono output to XLR input.)

    Any advise will be greatly appreciated as I am extremely frustrated at the moment.:(
  2. Petebass


    Dec 22, 2002
    QLD Australia
    I could be wrong but I think you're problem is the XLR output on the pre-amp. I suspect it's not really a pre-amp output but more of a DI. The preamp's job is to boost the voltage of the signal then send it to the power amp, something which a DI output won't do.

    I think a simple microhpne lead should solve your problem so long as it's phono jack one end, XLR at the other. These are available at every music shop.
  3. Classic5


    Mar 31, 2002
    Thanks for your suggestion Petebass, but the brief manual I have on the preamp does say that the 3 pin XLR is the output used for recording "and connecting to power amplifiers with XLR connections". So it should be ok. There are two other outputs from the pramp - both use phono jacks. One is an unbalanced line out and one is a balanced line out. I do have a standard mic lead but it has a 1/4" jack at one end and a female XLR at the other - the power amp needs a male XLR.

    I could wire one up but I'm not sure of the wiring configuration needed? Thats my present dilema.

    I'l try to attach a photo of the rear of the pre amp and power amp.:confused:
  4. Richard Lindsey

    Richard Lindsey

    Mar 25, 2000
    Metro NYC
    It's also possible that you have an impedance issue. I know the cabs are 8 ohms each, which should yield a total of 4, but you gotta remember that impedance fluctuates--that an 8 ohm cab isn't exactly 8 all the time. If these particular cabs dip below 8 much of the time, you could be giving your amp a load of < 4 ohms some of the time. Also, bridged into 4 ohms is right on the edge of what most amps are rated to do (being the equivalent of 2 ohms/channel) and not all amps handle such a load equally well, even when rated to do so. Have you tried hooking up one cab to each side of the amp? I know you'll get less power this way, but if you haven't tried this, it could be a good way of checking for amp stability. If your amp won't drive that load without shutting down, I'd guess there's a reall mechanical/electrical problem somewhere.
  5. The worst thing using the XLR would do is give you too low a signal to drive the power amp. Is the amp getting really hot before it shuts down? Are the clip lights coming on before it shuts down? Does it do it at really low volume, like speaking voice level? What are the impedences of the cabs and is the amp actually rated to run that load in bridge mode? The connection seems fine, I suspect it's a problem with the power amp. Try putting the amp back in stereo mode and run the preamp into one channel, then the other. See if the problem goes away. You could also disconnect the speakers from the amp and see what the amp does.
  6. Amp shutting down? It sure doesn't sound like a clipping problem to me, it sounds more like a low impedance problem, aka...a short circuit. Check the speaker cords and cabs for problems with an ohm meter.
  7. monkfill


    Jan 1, 2003
    Kansas City
    I would run one speaker on each side of the amp instead of bridging it. You'll get less power, but the amp will be much happier that way.

    I don't know if you're actually "clipping" your amp, but if you actually are clipping an amp that is bridged at 4 ohms, you're really taxing the amp. If you need to be louder, the solution may be to add a 4x10. Put the 4x10 on one side, and both 2x10s on the other side, assuming they are each 8 ohms so that combined they would be no less than 4 ohms.

    Generally, if you're heavily clipping the amp, you either need more speaker, more power, or both.
  8. Classic5


    Mar 31, 2002
    Thanks for your replies guys.

    I'm going to take my gear to a friends place tonight and mix and match our preamps and poweramps - maybe this will help to isolate the problem.

    I'ts definitely not the speakers - I've tried with one WJ cab (8ohms) - no good, one S15D bag end (8ohms) - no good. I've tries different speaker leads - still no good.

    I think there's definitely something wrong with either the protection circuit on the amp or there needs to be some sensitivity balancing done between the preamp and the poweramp.

    If I cant solve it tonight it's goint to have to go to the repair shop pro's. Oh Well:rolleyes:

    Thanks again for the replies, they were very much appreciated.