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Old 9th October 2006, 02:13 PM   #1
diarav is offline diarav  India
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Default "Knocking" sound

Hi,

Problem:
I get a continous knocking sound on my speaker when connected.

History:
I built my LM3886 and connected my CD player source. Was working perfectly fine. When switching on initially, there used to be a knocking sound , and this died within a few seconds and the source played fine.
I later built a pre-amp using NE5532 and this boosted the output very well and I had no problems again except that this knocking sound on the 16 ohm speaker played for a few seconds initially and the source was in. I got some hum and the chip was also heating up. I switched off after about 3 to 4 minutes.

Next day I switch on the same setup (no reconnections ) and connected my source again. This time I dont hear anything except the continous knocking sound which occurs something like two thumps per sec. its something like |^^_|^^_|^^_| where ^ determines the knocking, one second duration between | and |.

The schematic is from ESP site's http://sound.westhost.com/project115.htm . R7 value was 10K.

Any inputs on what the basic problem could be? I am actually planning to rewire the whole thing, but any suggestions would help.

Thanks
Din
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Old 9th October 2006, 02:31 PM   #2
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Default Re: "Knocking" sound

Quote:
Originally posted by diarav


The schematic is from ESP site's http://sound.westhost.com/project115.htm . R7 value was 10K.

It look to me as oscillation in the output stage , the culprit must be R7 that can't be greater than 10 Ohms and you are using 10K Ohms...
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Old 9th October 2006, 02:56 PM   #3
diarav is offline diarav  India
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Oh I'm sorry I did not make it clear in my earlier post. The R7 I was referring to was the one in the PreAmp (R7LA). All other values are as per schematic.

Could I suspect the pot? I was having problems with this ? Can an 'open' pot cause this?

Thanks
Din
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Old 9th October 2006, 03:11 PM   #4
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Quote:
Originally posted by diarav


Could I suspect the pot? I was having problems with this ? Can an 'open' pot cause this?
I don't believe that the problem is in the pot.

Are you using in the power section, the R4-22K Ohms resistor?
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Old 9th October 2006, 03:18 PM   #5
diarav is offline diarav  India
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Hmm. Yes . the R4 22K is there.
The problem is, it was working initially and now it is not.

I'll try disconnecting the preamp and check individually both the amps.

Thanks
Din
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