speaker impedance mismatching

Discussion in 'General Electronics Chat' started by leon23, Aug 4, 2013.

  1. leon23

    Thread Starter Member

    Oct 11, 2008
    21
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    What happens if you mismatch speaker impedances to a guitar amplifier? I have a bunch of old speakers and was thinking of putting them in a cabinet then plug in the amp. What about connecting them in series vs parallel?
     
  2. #12

    Expert

    Nov 30, 2010
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    The danger lies in putting too much impedance...

    Wait. What kind of guitar amplifier? Tubes or transistors for the output?
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    Tube amps have output transformers that are matched to the speaker.
    Most amps are designed for 8Ω speakers. Try and make the series/parallel combos match up to 8Ω.
     
  4. Shagas

    Active Member

    May 13, 2013
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    How so, isn't it the other way around?
    Won't I be safe putting a 16 ohm speaker to a 8ohm rated amp?
     
  5. cornishlad

    Member

    Jul 31, 2013
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    Most solid state amps these days state their rated output into 4 ohms. 8 ohms was common a few years ago for hiFi amps. There is no problem connecting up speakers with higher impedance it's just that you can't get the rated power.
    Example. A 4 ohm output amp withan 8 ohm speaker will accepts only half the power available. A 16 ohm speaker - quarter power. Same with valve amps.
    Speakers in parallel - normally no problem provided the combined impedance doesn't fall below the amps's impedance. solid state amps are more easily overloaded with speakers that are too low.
    Ideally only identical speakers work ok in series connection. In either case be sure to maintain the same phase.
     
  6. rogs

    Active Member

    Aug 28, 2009
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    If you want to connect a 'bunch' of old speakers you will probably have to use a combination of series and parallel connections, to get to a sensible final value.
    As suggested by Mr. Chips in post #3, aim for 8Ω as a typical starting value.

    Don't go below 4Ω, unless you know for certain that the amplifier can cope with very low impedance loads.

    Couple of other points you might like to consider.....

    1) An exact a value is not critical. A loudspeaker specified at 8Ω may well have an impedance as low as, say, 4Ω - or as high as 16Ω, depending on the frequency it is attempting to reproduce. It is not a fixed value, like resistor would be.

    2) Although, as cornishlad points out, you will only derive half the power from signal driven into a 16Ω load than you would from the same signal being driven into an 8Ω load, that is not necessarily as dramatic a loss as it may first seem....
    At 'half power' the volume only drops by 3dB.... so not a lot!...
     
  7. #12

    Expert

    Nov 30, 2010
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    What? You can answer "not a sentence" but you can't answer the question?
    I'm not going to spend my time telling you what the output transformer does to the output tubes if you won't even tell me whether you're using tubes.
     
  8. leon23

    Thread Starter Member

    Oct 11, 2008
    21
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    What would be the dif and why? I actually have a college degree in electronics engineering but it's been so long I've forgotten a lot.
     
  9. #12

    Expert

    Nov 30, 2010
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    Transistor amps can not be hurt by high impedance because they are obeying a voltage command. The output voltage will not exceed the specs even if there is no load. It's a short that will kill a transistor amp because the transistors will try to deliver enough current to keep the output voltage proper. Short the output and the transistors will die trying to deliver enough current.

    A vacuum tube amp has a current driven transformer, and it (the transformer) will try to increase output voltage enough to get rid of the energy that has been put into the primary side. A high impedance load will result in the output transformer delivering higher and higher voltages until arcs happen and the tube amp gets killed.

    So, a shorted load will kill a transistor amp and an open load will kill a tube amp. You want to play with the wrong load impedance? Just be aware of the amps limitations.
     
  10. Externet

    AAC Fanatic!

    Nov 29, 2005
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    Leon: a bunch means nothing. List how many of them with their individual impedances, and a decent matching can be sugested wiring them as an array, if you tell the amplifier output impedance too.
     
  11. Shagas

    Active Member

    May 13, 2013
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    74
    12#
    I'm not the OP

    I just asked a question....
     
    #12 likes this.
  12. MrChips

    Moderator

    Oct 2, 2009
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    Listen to #12. You can destroy a tube amp and a solid state amp by having the WRONG load impedance.
    First things first. Specify what kind of amp it is, tube amp or solid state amp.
     
  13. Shagas

    Active Member

    May 13, 2013
    802
    74
    NOT OP.

    But yeah , I didn't know that tube amps can be destroyed by giving a higher impedance load to them than they are rated for as you can do with transistor amps
     
  14. MrChips

    Moderator

    Oct 2, 2009
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    Never run a tube amp with no speaker connected.
     
  15. #12

    Expert

    Nov 30, 2010
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    I did lose track of who was the OP. My bad.:(
     
  16. leon23

    Thread Starter Member

    Oct 11, 2008
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    Well I don't know-I just have several lying around. I'm not asking for specifics. It did dawn on me to wire them up in such a way to get a comfortable load.:)
     
  17. leon23

    Thread Starter Member

    Oct 11, 2008
    21
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    Is there any harm in letting the tube amp run with no guitar plugged in?
     
  18. #12

    Expert

    Nov 30, 2010
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    No.
    .......
     
  19. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Leon23,

    What make/model is your amp ?
     
  20. leon23

    Thread Starter Member

    Oct 11, 2008
    21
    1
    Just a couple small practice a 75watt Fender Jam and a 25 watt Peavy.
     
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