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Plugging speaker output of one amp into the speaker output of another amp...

Discussion in 'Amps [BG]' started by Tony G, Oct 24, 2007.


  1. Tony G

    Tony G

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    I was at my local music store talking to one of the guys there, and they were telling me about this kid that plugged the output of one amp into the output of another amp. Obviously, this is not good, but what actually happens? I guess the senario went as follows: kid plugs into amp 1, turns it on, then plugs the speaker output of amp 1 into a bass amp 2's speaker output while it is still on. Amp 2 shuts down immediately. I suggested they try changing the fuse, so they did, powered on the head, and it shuts off right away. They took out the fuse, and that was blown as well. They tried one more fuse and again the amp shut off and the fuse is blown. Interestingly enough, amp 1 is just fine, nothing seems to be wrong with it.

    Now, this is something I never thought anyone would ever do. It just seems like common sense not to do this. But I'm curious, what actually happened here? One amp is putting out power, and trying to force that into the output of the other amp. Why wouldn't they both shut down? Why is one still working and the other isn't? Why is "amp 2" blowing fuses as soon as it is turned on (no other cables are in it, only the power cable)?
     
  2. fdeck

    fdeck Supporting Member

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    That's pretty much it.

    Amps are protected against output faults, but only to a point. The degree of protection depends on the particular amp design. I would expect that any decent amp should tolerate a shorted output, but perhaps not the extreme of having current driven into it by the other amp.

    Amp 2 could have a blown power transformer. Even 2 amps of the same design, it might be like a tug of war where small variations in the two circuits leads to one getting killed by the other.
     
  3. Tony G

    Tony G

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    Thanks for commenting fdeck. What I don't understand is why "amp 1" is still working fine and "amp 2" is not. Technically, they were both on and plugged into each others speaker outputs. Why did one break and not the other? I would have expected both of them to fry.

    Also, your suggestion as to amp 2 having a blown power transformer. Is that a big part of the amp and expensive to fix? My guess is yes, but just wanted to hear what you think.
     
  4. fdeck

    fdeck Supporting Member

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    Some amps have an output relay and extensive protection circuitry. At the first sign of trouble, the relay opens up, sometimes until you shut down and restart the amp.

    Normally this is to protect the speaker from damage if one of the output transistors on the amplifier cuts loose. But it may have protected the amplifier itself in this case.

    The other possibility is that Amp 1 simply has a whole lot of oomph, and stood its ground while the other one fled the scene.
     
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  6. fdeck

    fdeck Supporting Member

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    Now I know how I am going to test amplifiers, next time I am in Guitar Center. Tug of war. Last amp standing goes home with me. :bassist:
     
  7. phat daddy

    phat daddy

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    +1 on everything said so far. And yes a transformer is a very big part of the amp-- it's what generates the power and is in most cases the largest single component. A very expensive replacement.
     
  8. Tony G

    Tony G

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    Actually, if I remember correctly, amp 1 was rated about 100watts less at 4 ohms than amp 2.
     
  9. Tony G

    Tony G

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    Ok, so a blown power transformer doesn't sound good. I've been reading some threads here, and it seems another possible problem could be a blown output transistor. How likely is this?

    The reason I'm asking is because the shop said they would give me a good deal on the amp if I was willing to buy it and fix it myself. :D
     
  10. '76SVT

    '76SVT

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    I once plugged the output from my Acoustic 370 head into the input.

    Instant noise. No smoke, fizzes, and it still works great to this day.



    I'd be more reliant on build quality than anything else, as to WHY one amp fried, and one didn't.
     
  11. Tony G

    Tony G

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    This would be interesting, because I would actually expect the build quality of amp 2 (the amp that blew) to be better than amp 1.
     
  12. Tony G

    Tony G

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    Haha, I wouldn't recommend it. :D
     
  13. fdeck

    fdeck Supporting Member

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    Blown output transistor seems less likely. It would not cause the fuse to blow, but would simply pull the output to one of the two power supply voltages.

    Having multiple blown output transistors would possibly blow the fuse.

    But even replacing power transistors is not 100% straightforward, because they can be rather embedded in the system, and must be correctly re-attached to the heat sink.

    What brand was Amp 2? Have the shop throw it in as a freebie with something else you want. Then sell it on eBay. I have seen blown gear fetch astronomical prices on eBay, I think due to the psychology of auctions.
     
  14. fdeck

    fdeck Supporting Member

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    One can always dream, and imagine the look on the manager's face when the salesman tells him about it. :eek:
     
  15. billfitzmaurice

    billfitzmaurice

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    I once bought a humidifier and a dehumidifer. Put them in the same room, let them fight it out.
     
  16. coolrunner989

    coolrunner989 Supporting Member

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    lmao, thats great.
     
  17. Trevorus

    Trevorus

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    So who won?
     
  18. billfitzmaurice

    billfitzmaurice

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    The power company. :mad:
     
  19. Tony G

    Tony G

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    Cute joke, but I was hoping you would be able to add more to the conversation.
     
  20. Rune Bivrin

    Rune Bivrin Supporting Member

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    Plugging the outputs of two different amps into each other is so unexpected you can't really predict what will happen. If they're really well desingned with regard to more common failure modes such as short circuit or too low impedance, they might be able to handle this situation too, as long as the power levels are within reason. But you could subject one of the output circuits to reverse voltages, and then blow some component.

    Once an output transistor breaks (and they tend to fail to short circuit), it will cascade and create a short across the power rails, and POP! goes the fuse.

    It's not likely to be the power transformer. They are generally quite tough, and will only blow after prolonged serious overload. That will very likely be preceded by alarming smell and smoke.
     
  21. Tony G

    Tony G

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    Interesting. Thanks for your comments. I certainly don't remember any alarming smells from the amp, and of course I was too late to see any smoke. I would only assume that if the power transformer did blow, I would still be able to smell it after the fact.
     

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