Marantz Receiver 'Thump'

Discussion in 'AV Receivers & Amplifiers' started by Bails, Jun 14, 2005.

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  1. Bails

    Bails
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    I'm hoping someone will have some advice on this. I have a Marantz SR-9300 receiver and I've recently added two CIAudio monoblocks to the front L and R channels, using the Marantz pre-outs.

    The problem now is that evertime I switch input on the Marantz (from CD to DVD to Tuner etc) I get a thump through the speakers. This thump does not appear to change with volume changes - it is a constant sound.

    I also get the thump whenever I change chapters on a DVD or even change television channels (using an external digital tuner box).

    I do not get the thumping when I use the Marantz's internal amps, that is, it only happens when using the pre-outs.

    I have tried the monoblocks with a different pre-amp and had no thumping problem. I've taken the Marantz to the repair shop thinking the muting circuit may have blown but they say there's no problem (though I'm not 100% convinced that this is not the problem).

    Does anyone have any ideas? Could it be the muting circuit even if it only happens with the pre-outs (and the repair shop says it isn't)? Could it be a compatibility issue between the Marantz and the monoblocks (different input/output impedences or some such thing)? Any other thoughts would be gratefully received.
     
  2. Cliff

    Cliff
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    Are you using an RF remote system to change inputs and chapters on your DVD player?
     
  3. Bails

    Bails
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    No. I get the noise regardless of whether I use the IR remote or just manually press the buttons.
     
  4. Cliff

    Cliff
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    If the thump occurs when you change channels on an external box or change chapters on a seperate DVD player then the Marantz would not be muting anyway- that would only occur when you select different inputs.

    You say you still get the thump with the volume down? Don't understand how that can happen.

    Is it just the case that the low frequency response is now much lower (DC coupling) and you can hear the changes ?
     
  5. Bails

    Bails
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    Thanks Cliff

    I get the pop whenever the signal changes. So, for example, changing channels on the tv causes the audio signal to stop, then re-start, causing the noise. I have noticed, however, that the noise is not present if I change channels quickly - I guess then the signal doesn't have time to fully drop out before coming back.

    Could you explain what you mean by DC coupling?

    Cheers
     
  6. Cliff

    Cliff
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    Well this is a bit of a stab in the dark - but is sounds like your system is passing a DC shift through to the speakers.

    Let me explain.
    It was the fashion a while back to design expensive amplifiers without coupling capacitors. (the advertising boys could then say the signal path was direct and the low frequencies would go down to zero.)

    From an engineering point of view this is nuts! Anyway rant aside.. it sounds to me that your main amplifiers and pre outs are coupled without a blocking capacitor.

    The solution is to put a 100micro farad capacitor in series with the left and right channels. Check with a meter which side is high so the capacitors are the correct way round.

    If you are not familiar with electronics then I'd get someone who is to try it.
     

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