Power Amp Problem

Discussion in 'Amps and Cabs [BG]' started by Chace90, Apr 3, 2004.

  1. Chace90

    Chace90 Supporting Member

    Feb 1, 2002
    Denver, CO
    Ok, so this is the second time this has happened now. I just got a new rig that consists of an Acme Low B-4 Series II, a Stewart 1.6, and a Dememter VTBP-201s. The power is supplied but some cheap Furman power conditioner.

    So, my rig will last a couple of hours bridged no problem. But eventually I think it starts to run out of power. As far as I know, the Stewart continously draws power from the conditioner instead of charging up, so to speak. It doesn't really create a reserve. So, when I start really digging in, like slapping or something, one or both of the channels will turn off. When I try to run the cab with only one channel, it'll last longer, but any heavy playing will cause it to turn off again.

    My guess is that the power conditioner isnt allowing the amp to draw enough power over such a long span of time. Has anyone had this problem? Should I get another power conditioner or similar device? Any help here is greatly appreciated!!!

    It really sucks when I have to drop out of the mix because I have no power. Ask me any question you can think of that may help you understand this problem and help me work it out. Thanks a bunch in advance everyone!
  2. The power conditioner doesn't work that way. It doesn't "supply" or "limit" power. There's a circuit breaker which will trip if the current passing though the conditioner exceeds its rating, but this isn't your problem. The amp DRAWS power, as much power as its own internal power supply demands. Since the power conditioner's circuit breaker isn't tripping, it's not the power conditioner. The power conditioner will happily pass as much current through it as the amp is requesting, up until the circuit breaker trips. The power conditioner is not your problem.

    The problem lies in your amp, speakers, or the connection between them. Something is causing the amp's internal protection to shut down the amp. Usually this is caused by a load that is lower than allowed--trying to run two ohm loads, for example. Or something in the cab or cable is shorting out, that's very possible.

    Is there an internal cooling fan in the amp that's not functioning? Clogged air vents (pet hair is a common problem in my house)?

    But it's not the power conditioner.
  3. I agree with nashvillebill. It can't be the power conditioner.
    Check all your cables. If you're lucky that might just the problem. Otherwise, you might have to have the amp checked.

  4. jobu3

    jobu3 Artist formerly known as Big Joe Supporting Member

    Feb 17, 2002
    Mountain Top, PA
    How loud is the mix you are playing? What is your EQ like? If you scoop the mids, you will be less present in the mix. If you boost too much bass, you'll have less headroom. Check: Is your Stewie set correctly? For bridging on "most" power amps, you shouldn't have "both" channels on for one of them to quit on you. Are you using speakon cables to the cab? Is that wired correctly for bridging? If all these are in check and it is still happening then it sounds to me like you might actually need some more power for more headroom. I know that the 1.2's were very sensitive amps that couldn't be pushed too hard without running into thermal shut-down protection mode.
    Perhaps the same is true with the 1.6? Acme cabs do require quite a bit of "clean" power...
  5. Is the Acme cab broken in? I remember people having this problem bridging power amps into new Acme cabs. It seems when new and not yet broken in, the ohm load is greater. So bridging at 4 ohms causes the amp to shut down. It sounds crazy, but this may be the problem. Contact Andy and see what he says.
  6. Being the owner of a similar set-up I'll chime in. I have a 4 ohm Low B-4 being run with a bridged Peavey DPC 1400X. This amplifier is outstanding when driving bridged loads. It gets warm driving the Acme but the fan keeps things safe. I'd checked out Stewart first and nearly bought the 1.6 instead but the lack of a fan and thermal shut-down issues when being runned at 4 ohms bridged swayed my away from it (and the 1.2 too). It could be that you're pushing that amp too far. See if you can try another amp.
  7. Chace90

    Chace90 Supporting Member

    Feb 1, 2002
    Denver, CO
    Ok. Power conditioner is out as the cause of the problem. For the record (because I didn't mention it before), my conditioner is 15amps and I believe the Stewart manual calls for something that supplies 20amps.

    So, the cab is 4 ohms. I didn't mention that it has been clipping, but it doesn't usually clip too much when I run it bridged. The battery on board my bass is fine. There is a fan on the stewart, and as far as I know, it runs most of the time. There isn't anything clogging it such as pet hair. I realize that the Acme is a power hungry monster but it seems to me that running the amp bridged at 1600 watts should be more than enough. I do know that when the amp turns off, it's because I'm clipping the power amp.

    My cables are new. Just for my piece of mind, what sort of cable needs to run from the preamp to the power amp? I currently just have a cable (a very nice one) that's built for attaching effects pedals to eachother. It's fairly short and it's basically an instrument cable, but much shorter.

    I keep my EQ flat. If I boost or cut anything, it's on board the bass. The cabinet is definitely broken in. I bought it used from someone who was the second owner.

    The thing about the 4 ohm bridged load is that the amp will still shut down when I run it on only one channel. Obviously this channel isn't enough because it only puts out 850 watts at 4 ohms.

    I really have no ideas here. I think I'll contact both Andy at Acme and Stewart to see if they can help me out. After this additional input, anyone have some new ideas?
  8. 20 Amps? Most poweramps under 2800watts are 15amps or less, you rarely get any non-installed poweramps exceeding that kind of current. Then again, things might be different in the US.
  9. Chace90

    Chace90 Supporting Member

    Feb 1, 2002
    Denver, CO
    On the back of the manual it says for Power Requirements (Average operation at 4 ohms per channel) <4 amps, 120Vac (500 watts).

    Not sure exactly what that means. Somewhere it says that when the amp turns on, draw is only 6 amps.
  10. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Your preamp to poweramp cable is fine?

    Has this problem occurred at the same place twice or different places?

    Have you measured the mains voltage? If it's a lot below 120 Vac (like 108 Vac or less) then that could contribute to the problem.

    I presume that the Stewart can run bridged into 4 ohms. Doing this causes each channel of the amp to see 2 ohms, causing maximum heating of the output transistors and maximum loading on the internal power supply.

    Since this happens after several hours of happy playing, it sounds like a thermal condition in the amp. My guess is that the Acme's inefficient woofers are heating up, causing a loss in efficiency, causing you the crank up the amp further, causing additional heating in the woofers, causing a drop in their efficiency, causing you to crank the amp up more....see the cycle.

    Acme's desire to gobble bucket loads of power is a real problem if you're trying to play loud. You reach a point where the speaker simply can't get any louder despite the fact that you're willing to pour more power in...all you do is heat the voice coils and drive them toward thermal failure.

    If this is what's happening, you need to either roll off the low end a bit to reduce the power draw on the amp (lows eat most of the power), get the band to turn down or get a different cabinet.

    If this really only happens at one venue, perhaps that amp is just browning out and you won't have the same problem is you have an adequate power source at the next gig...
  11. My bad! Thought they were both one space fan-less amps. I have heard of thermal issues with both amps though so I evidently combined two nuggets of info on that one. I haven't noticed this with my DPC 1400X and it'a a less powerful amp. For the record I've driven it hard with no PA against a loud drummer for 3 hour gigs multiple times. Never had an issue. I think getting a hold of Stewart would be a good idea at this point as well. Let us know how this turns out!
  12. Chace90

    Chace90 Supporting Member

    Feb 1, 2002
    Denver, CO
    any new ideas?
  13. seanm

    seanm I'd kill for a Nobel Peace Prize! Supporting Member

    Feb 19, 2004
    Ottawa, Canada
    The US uses 110-120V rather than 230-240V, so for the same power it will take rougly twice the amperage.
  14. 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
    Sounds to me like a thermal problem. Either don't push the amp into clipping, get a more powerful amp, or assist the amp's cooling by arranging one or two fans to push air into its air intake.
  15. boogiebass


    Aug 16, 2000
    I used a Stew 1.6 for about a year, always mono-bridged into a 4 ohm load. I never had a problem and found the amp to be reliable and to work perfectly, often cranked at pretty loud levels.

    It does indeed sound like a thermal problem with the amp. I'd make sure the fan is working all the time. As I recall, the fan does not cycle on and off but should remain on continuously. Your plan to contact Stewart is a good one, I'd say. Good luck.
  16. Chaputa

    Chaputa Bass-Not just fo eatin anymore

    If the B4 is new, then I think that's your prob. I'm running a 4 ohm B2, and when it was new (even after what I thought was it being broken in) it would cause the poweramp to shut down after a certain amount of playing. The amp was rated at 540 watts at 4 ohms. I think the problem was that the speaker wasn't completely broken in yet, and caused the problem with the amp. Impedences for the speakers are not constant, and with the speakers not broken in, the ohm variances could be greater than normal I would assume.

    BTW...the problem only happened a 3 or 4 times, and haven't happened since.
  17. D.A.R.K.


    Aug 20, 2003
    i have run into this same problem over the years with many different types of power amps (for live sound as well as my oversized bass rig). unfortunately, it seems that technology has not come up with a better(affordable) alternative than large power transformers for high wattage applications imho. that statement is not based on manufacturer specs, but real performance experience over the years. your problem is also increased by your choice in speaker cabs...an amazing cab, extended mad low frequencies, you are obviously going for that feel-it (and hear it) tone...
    but a highly inefficient cab with it's extended travel in the voice coils.
    that's gotta tax the damping factor of that amp like mad.
    my suggestion(here comes the hard part) would be to take your rig to
    your dealer and try out some different amps, and POUND them..
    after trying out probably over a dozen amps for my bass rig, i settled with some stuff thats unfortunately heavier than i wanted, and seems overkill as far as headroom...but never lets me down.
    so, it seems like your rig needs more headroom(especially if you are clipping the amp input to get your volume up to expectation.)
    good luck
  18. 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
    The size of the power transformer doesn't have any bearing on an amp's thermal performance.

    Also, you can't "tax" a damping factor.
  19. D.A.R.K.


    Aug 20, 2003
    true, but a reserve of power keeps the amp from being pushed to it's limit 99% of the time, thus it works more efficiently, doesn't get clipped, and runs cooler(tending to not go thermal overload)
  20. 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
    Having headroom is a good thing, but the amp does not operate more efficiently that way.

    Headroom also is not a function of transformer size.