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Ampeg biamp/stereo questions - Ampeg Insider help??

Discussion in 'Amps and Cabs [BG]' started by asmith, Nov 19, 2004.

  1. asmith


    Nov 19, 2004
    Bear with me, this is way long, as I want to include all pertinent info! The main problem is a hum through my SVT-15E when I biamp, but not otherwise. I love the amp, it's loud as anything I've heard, and I can 'live' if this problem isn't fixed. But it's one of those things that I'd love to have answered... details follow!

    I'm running a Modulus 5-string through an Ampeg SVT-4PRO with two cabs, one is the 1x15, one is the 410HLF. I had been running the 410HLF mono-bridge and it works great and is loud, but wanted to add a little depth to the low B string, so I experimented at Guitar Center and tried adding the 1x15 cabinet. I liked the tone with it running stereo, but LOVED it running biamped. The manual shows a diagram running a shielded cable from "biamp low out" to "power amp A in (low)" which I did, trying several types of cables, all of which worked, but there's that semi-quiet, but definitely noticable, hum, only when biamping. When I run the two cabs in regular stereo mode, the hum goes away, and there's the usual super-quiet speaker hiss. I have tried various cables, and another interesting note is that it will do this regardless of whether the amp is muted, and even when the bass is not plugged in.

    It's not a big problem, because when I'm playing it's not noticable, but when it's sitting there it's kind of bothersome as one of my venues is a church, so when stuff is quiet, the amp isn't. I could just run stereo, but I lose the crossover point and balance feature, which when I run biamp gives it such a great, creamy tone, and still allows good slap/pop action with the pop not being overplayed by the 15.

    I have tried two different power conditioners (one cheapo, one by Monster which is supposed to be really good); I have tried cheap and expensive cables to bridge, and there's no difference whatsoever. One little interesting piece of info is that it buzzes immediately upon connecting to the "power amp A low" input on the back. Also, I tried a couple other duplicate heads at GC and they do the same, so it's not just my head.

    My other question is: is the Guitar Center guy right in saying that it's okay to biamp with the 15 cab on Power Amp A at 8 ohms and the 410HLF on B at 4 ohms? I know it wouldn't be so hot to mono-bridge them together (2.67, right?) so I'm not doing that, but they said it was 'no problem' to biamp or run these two cabs stereo. I do have time where I could return the 15 and get my cash back if they were wrong about the impedance - so far nothing has overheated and blown up, though.

    I would just return the 15 and mono-bridge the 410 (which is great) but it sounds SOOO good biamped except for the hum. I mean, people walking up in the store to see what combinations we're running and stuff.

    Any advice or experience with this?? Ampeg Insider any suggestions? Thanks in advance everyone! I'll be happy to supply any more info if needed!
  2. Bear with me because I have a SWR SM 900 which is similar and can bi-amp but I'm not sure of all the connections on your amp.

    Regarding question 1: are you using the shortest and best cables to connect from the low crossover into the power amp A? These cables should be sheilded, just like your guitar cord. The cables I use are 1 foot long, just like the cables you use to connect a bunch of FX foot pedals together.

    Regarding question 2: Yes this is ok to do because you have two completely seperate power amps so if you connect a 4ohm cab to amp A and a 8ohm cab to amp B they both see different loads and run different cabs. .... Sorry,... that wasn't a very good explanation. It is ok to run the different cabs because it's two different amps.
    :( Hope that helps
  3. asmith


    Nov 19, 2004
    Your explanation does help - it's kind of what I suspected but I couldn't find anything in the manual or anywhere that said for sure that the power amps see the cabs separately.

    For the cables I'm using the same kind you suggest - a one-footer that's an instrument-type cable (shielded), though it's not a really expensive one. I also play guitar (blasphemy, I know...), so I had a few available. I tried also a 3-foot gold-plated Monster cable for the biamp hookup and it did the same thing, and so did a longer Planet Waves cable. Do you think the guage of the cable would make a difference? I haven't tried anything really thick or thin. Or something really nice like the multi-shielded Monster studio series for like $40? I spent enough on the stuff so far that I'd be willing to try it if there's a decent chance it would work.

    Thanks again!

  4. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    As far as the amplifiers, when running in any mode OTHER than "bridged mono", the two amplifiers are completely separate.

    You can hook any load to either one that is within the ratings. They can have different loads on them, etc, and there isn't a problem.

    Just stay AT OR ABOVE the minimum rated impedance for the way it is set up.

    The individual amplifiers can drive 2 ohms, 4 ohms, 8 ohms, etc. In "bridge mono", 4 ohms, 8 ohms, etc, (but NOT 2 ohms).

    As far as the hum is concerned, I haven't run into that as an issue before. The connection is "unbalanced" (just signal and shield) so it may pick up hum from external magnetic fields. I suspect that if the existing cables don't take care of it, a really expensive one won't either.

    Sometimes the way the cables are laid makes a difference. I have found cases where making a loop in the cable and turning that a certain way has knocked it out almost completely. You might try that with your 3 foot cord. Just coil it into a loop (which you may have done anyhow) and try turning it different ways, flipping it over, etc.

    Might do something, might not. Hum issues are odd, because you can't see the cause, so you have to experiment a bit. If the unit itself does NOT hum, it is then much less likely that the issue is inside it.
  5. asmith


    Nov 19, 2004
    Thanks - I've got a gig tonight and I'll try the loop thing with the 3 footer. I didn't think of looping it, but I thought it might be something like a magnetic field. A guy at GC suggested a direct box along the line that had a filter in it, but I think that changes the signal type to balanced because when we tried it the 15 cab barely did anything. They let me try that on the store demo equipment :)

    As far as any of the units humming, I'm quite sure they're all fine because when I run the amp stereo without the biamp it's almost totally silent as far as noise goes, and also it's quiet when running mono bridged to the 410HLF. It's amazing how little noise it has even on pretty high output levels. Sometime I'll share the "capture the flag" story, but it's not 'on topic' so...

    If anything else comes to mind I'll give it a shot - thanks for all the info so far!

  6. asmith


    Nov 19, 2004
    Well, there was some change in the hum but it didn't go away. Anyone think it might help if I got a really nice shielded cable like the Monster Pro Studio 1.5-footer or something, or maybe the 3-footer and loop that one?