Open Speaker

Discussion in 'General Electronics Chat' started by AppleWood, Mar 11, 2015.

  1. AppleWood

    Thread Starter New Member

    Mar 11, 2015
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    Hello All,
    I have a OEM car radio 4-speaker CD/Cassette system I'm working on.

    Just a general question about playing the radio with one of the four speakers disconnected. I have the 8 ohm speaker out while replacing a power window motor. Does this harm the audio output stage in any way if I play the radio while driving?

    Thanks in advance,
    Jon
     
    Last edited: Mar 11, 2015
  2. MrChips

    Moderator

    Oct 2, 2009
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    Tube amps do not like having no load connected to the output transformer.

    Solid state audio amps are ok.
     
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  3. alfacliff

    Well-Known Member

    Dec 13, 2013
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    I fixed a lot of car stereos that had blown output transistors and ic's due to open speakers. I would not do that.
     
  4. AppleWood

    Thread Starter New Member

    Mar 11, 2015
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    That's what I don't quite understand. Ohm's law says current decreases as resistance increases, yes? But infinite resistance doesn't equal zero current in the output?
     
  5. crutschow

    Expert

    Mar 14, 2008
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    Don't see how an open output can damage a solid-state amp with no output transformer.
     
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  6. alfacliff

    Well-Known Member

    Dec 13, 2013
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    it would not if output stages used transformers, but by opening the ground, the voltages get strange on the output stage. otl amps need a ground reference for the output.
     
  7. AppleWood

    Thread Starter New Member

    Mar 11, 2015
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    I mean, I played it a couple of times just listening to NPR, but got to thinking about blowing the output with only three of the four speakers connected. Waiting on a new window switch for the next couple days, so I'll probably just play it safe and leave it off.
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
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    Anything with an output transformer can generate back-emf when run open circuit - valve amplifiers can generate enough emf to break down the windings insulation.

    A DC coupled or IC amp doesn't have that exact problem, but I'd still put a temporary load resistor on the terminals just to be on the safe side.

    A pure resistance doesn't exactly match the impedance vector of a speaker, but its close enough for temporary if you don't run it flat out.
     
  9. DickCappels

    Moderator

    Aug 21, 2008
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    Death of a solid state output stage from driving a short I can believe (though it should rare with integrated circuit output stages designed after about 1990 CE) but from driving an open circuit? Why/how could that happen?
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    For DC coupled amplifiers, there's quite a few topologies - you'd have to consider each in turn.

    Suffice to say, if you run it otherwise than specified by the manufacturer - any subsequent mishap serves you right.
     
  11. crutschow

    Expert

    Mar 14, 2008
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    Well that's a interesting answer but not very satisfying as to why it could be a problem. :rolleyes:
     
  12. ian field

    Distinguished Member

    Oct 27, 2012
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    Only a guess - but most audio amplifiers are designed to drive a predominantly inductive load, an open load might possibly appear relatively more capacitive than it should be.

    Most amplifiers driving directly into "too much" capacitance can burst into oscillation.

    I've had experience of the output stages blowing up because they were amplifying oscillation at about 100kHz - but that was down to a fault in an earlier stage.

    As far as I can remember, I've never had an unloaded amplifier blow - but as I said, if you run it otherwise than specified by the manufacturer, if it blows up that's your problem.
     
  13. nigelwright7557

    Senior Member

    May 10, 2008
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    irs2092 based class d amps don't like having no speaker connected.
    The output sits at 17VDC or if it does oscillate it can blow up the output filter capacitor as the output inductor rings with no load.
     
  14. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    In a (badly-designed) DC coupled amp it's possible that the bias arrangement for class AB output transistors could depend on the normal speaker impedance being present to prevent the transistor collector current rising to a destructive level.
     
  15. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the common point between the pushpull transistors is grounded through the speaker. if not grounded, where is the reference for the bias?
     
  16. MCU88

    Member

    Mar 12, 2015
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    Yeah I don't either. I don't think you'll have any problems man with your car radio... not having any load connected. I blew up my sub-woofer in my Mitsubishi GTO twin turbo the other week though... BOOM BOOM whole car shaking.
     
  17. crutschow

    Expert

    Mar 14, 2008
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    A properly designed bias circuit should be set at zero volts and independent of the load. The reference is the circuit common (but not through the speaker) generally performed by DC negative feedback from the output. Otherwise you would have DC current through the speaker which is undesirable.
     
  18. AppleWood

    Thread Starter New Member

    Mar 11, 2015
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    Thanks all for feedback.

    Curiosity led me to check some readings with my Fluke DMM. Each speaker wire is sitting at 6.8VDC reference the metal in the door panel regardless of whether or not the radio is on. Basically zero volts (0.05) between the two speaker wires while turned on. Guess I don't understand the 6.8 with radio turned off.
     
  19. alfacliff

    Well-Known Member

    Dec 13, 2013
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    a lot of car stereos werfe not designed with "proper", they were designed to sell as cheap as possable. the output transformer less (otl) designs sometimes get a little inovative on saving money. I saw one radio a few years ago with all germanium transistors, all the same type number too that was direct coupled all the way from the 8 track head to the speaker output. the speaker opened up and it took out everything all the way back to the coil in the 8 track head. the other channel still worked.
     
  20. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    It's not the current that kills them in the open circuit case; it's punchthrough of the junctions due to excessive voltage. In addition, some power amps will oscillate with no load, which can do a lot of "interesting" things.
     
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