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Old 11th November 2005, 01:28 PM   #1
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Unhappy Big noise when turning on gainclone

Quite a long scenario....... could anyone can point out what I had get wrong?

I had built a gainclone based on this circuit:
http://www.audioworkshop.com.hk/report/my_3886/3886.gif
The setup is:
A 270VA toroidal => Carlos snubberized PSU(unregulared) => 3886 board => speakers
I do not have pre-amp, and connect the gainclone to CDP, in between there's a 250K ALPS
It had been played for quite a period happily without problem.

Recently I had diy a new dacs. I pull out the toroidal that installed in the 3886 and connect to the dac for test, because I do not have spare transformer at that time. The dac is now all assembled OK and serving
my headphone amp happily . But the problem begin after i restored the toroidal into 3886...... When everything connected back and turn on the power, a very big 'saaaaaaaaaaa' noise (but not big 'pop' sound) come out from the speaker..... it had appeared 3 times and then gone quiet again.

I turn off the amp immediately for debug....... I disconnected everything, and:
- check the voltage of the toroidal unloaded, it still measured correctly 24-0-24VAC as before.
- Then I connected to the PSU, it come out 28-0-28VDC.
- I connect again to board to check the output of amp, without connecting speaker, I found that there's 0.2-0.3mV DC offset. I even had borrowed an analog MM to check if there's some starting large DC come out, but no DC found at startup.
And.....everything seems fine. I connect a cheapy speakers to it and pwr up, its still come out 3 big loud 'sa' noise and then go dead quiet. (At this point, the DMM still get only DC reading around 0.4mV) I try to connect it to sound source, and the sound still come out no error.

This situration happen everytime(and exactly the same 3 times 'sa' everytime) when I power up them. But I had try play some sound and it perform totally no abnormal outcome, after that big strange noise.......

I have totally no hint on how does this come.......Anyone can give some hints for debugging?
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Old 11th November 2005, 02:04 PM   #2
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How about the mute resistor? Pin 8! Check once more your grounding, input grounded via 22 kohms, is this resistor in contact with ground? Is the signal ground in contact with the speaker ground?
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Old 11th November 2005, 03:05 PM   #3
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Or you may try to post pictures,so your problem could be solved easily.
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Old 12th November 2005, 11:36 PM   #4
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The schematic shows pin 8 as not connected to anything????? I wouldn't think you would get any sound without the mute resistor... I don't think I'd trust that schematic

The other thing to check is the mute capacitor (if you have one) make sure you don't have a dry joint on either side... if this cap is effectively out of circuit you won't get any turn on mute.

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Old 13th November 2005, 02:14 PM   #5
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Thanks for reply and sorry for late follow up.

I just found that the schmetics at first post is different from my pcb.... the guy that sell me the pcb said he follow that schmetics on his pcb, but the sheet of schmetics he give me is different from it.... it should be the attached one.
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Old 13th November 2005, 10:36 PM   #6
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OK that looks a bit more like it (though there are still some problems with it, in that it shows v+ for both the positive and negative rails, and shows some earth connections as v- ). I'm not sure what the P means on the cap shown as 0.68P (polyester, polypropylene??) on pin 8, but I'd try changing it to a 100uF electrolytic, just make sure you mount it in the correct orientation, ie with the -ve pin connected to pin 8, as pin 4 is at -ve potential not +ve as shown in the schematic.

hopefully changing the cap will fix it, although it doesn't explain why it worked properly before

Tony.
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Old 23rd June 2006, 01:34 PM   #7
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Sorry for digging old thread (of mine). I had not yet fix the problem machine yet because I had been long time not at home since my last reply. Recently I back home and finally have time to check it. And found that the reason of big noise happen when the pot is turned fully anti-clockwise (i.e. mute). And I also found that when the pot at full anti-clockwise, the voltage of PSU dropped half (both +ve and -ve rail)

Anyone can give clue whats wrong with the amp?

Thanks!
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Old 24th June 2006, 12:03 AM   #8
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What's the deal with Pin 2 and Pin 4?

Also, the cap between 9 and 10?


I like this schematic a lot better.
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