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Old 29th December 2006, 05:10 PM   #11
Bengali is offline Bengali  United States
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Thanks for your help. Here is a pic of how things are connected. I have the Earth Gnd from the IEC Jack bolted to a screw along with the 2 (CH)assis points on the LM3886 pcb's.

The mogami cable, the blue wires are tied together for the positive and the white to gnd terminals of the rca jacks.

the snubber pcb power wires goes directly to the lm3886 pcb's.
speaker output goes directly to the banana jacks.

I measure about 46K between the rca jack's positive terminals.

From the feedback, I should connect the power,signal,speaker gnds all to the same point(?) If this is the case, then I would need to remove the isolation from the (-) terminals of the rca and speaker jack?

thx.
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Old 29th December 2006, 05:24 PM   #12
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As long as everything that has a ground (includeing the pcb ground points), is grounded separately and the + and - are very short wires(to cause less interference), i dont see too much of a problem. I don't have too much experience with chip amps though so i don't quite know their grounding procedure
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Old 30th December 2006, 06:19 AM   #13
Bengali is offline Bengali  United States
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here's the latest. one channel sound really distorted and the low volume remained the same even if I turn the preamp volume up.

I swapped out the chip, made no difference. I then swapped out the pwr and Ci caps. The audio is now normal. after testing it again, without the top chassis on, even with both rca connected to the inputs, No buzzing.

I then put the top cover back on, powered everything back up, heard audio for about 10 seconds, then it's back to the low volume distorted sound again.

I'm starting to wonder if my chassis is just too small, everything is crammed to tightly, and when the top cover is on, something bad is going on.

My next testing will be done without a preamp, all I have is a 100K pot, will that work or do I really need to get a 10K pot?

thanks.
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Old 30th December 2006, 08:58 AM   #14
AndrewT is offline AndrewT  Scotland
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Hi,
is the lid shorting (touching) out the bolt securing the transformer?
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Old 30th December 2006, 04:38 PM   #15
gootee is offline gootee  United States
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Quote:
Originally posted by Bengali
here's the latest. one channel sound really distorted and the low volume remained the same even if I turn the preamp volume up.

I swapped out the chip, made no difference. I then swapped out the pwr and Ci caps. The audio is now normal. after testing it again, without the top chassis on, even with both rca connected to the inputs, No buzzing.

I then put the top cover back on, powered everything back up, heard audio for about 10 seconds, then it's back to the low volume distorted sound again.

I'm starting to wonder if my chassis is just too small, everything is crammed to tightly, and when the top cover is on, something bad is going on.

My next testing will be done without a preamp, all I have is a 100K pot, will that work or do I really need to get a 10K pot?

thanks.
Unless something is shorting, or opening, or heating too much with the case on:

Can you check those caps more closely? Can you measure the DC and AC voltages across them, while operating? And then maybe replace them again and try to get the same measurements while they are operating correctly. Can you test the ones you removed to see if/how they failed? Do you have an ESR (Equivalent Series Resistance) Meter? (Or, if you have an oscilloscope and a square wave generator, you can use this, instead: http://www.fullnet.com/~tomg/esrscope.htm .)

- Tom Gootee
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Old 30th December 2006, 07:31 PM   #16
Bengali is offline Bengali  United States
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Thanks for the tips. I'll keep at it. No, the lid is not shorting, there is about 1/2" air gap between the top lid and the transformer.
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Old 10th January 2007, 08:22 PM   #17
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Quote:
Originally posted by AndrewT
Hi Ian,
seems like a similar, but slightly different problem.

Put a multimeter set to low ohms between the input RCA barrels.
does it read less than 1ohm?
If so, you have an internal half loop.
Connect an external half loop and you will get hum and/or buzz.

Andrew, I finally got around to doing your little test and there is practically zero resistance between the RCA barrels. Would it be possible to use one transformer winding per channel and remove the chassis ground connection, thereby, removing all physical connections between the amps (as if they were true mono blocks) and hopefully the internal loop?
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Old 11th January 2007, 10:25 PM   #18
AndrewT is offline AndrewT  Scotland
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Hi,
as a very temporary experiment, you could disconnect the chassis from the audio grounds on one channel. Then couple up and test with a stereo audio signal. If it is hum free then it comfirms that an earth loop is the problem.

BUT, you must reinstate the safety earth to audio ground connection BEFORE you put this amp to work.
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Old 14th January 2007, 04:24 PM   #19
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I'll try that, but in answer to my main question is it possible to use 1 primary per rectifier, i.e. split one primary so that it provides both the +ve and -ve feeds to the amplifier board?

I guess the equivalent would be trying to use a transformer with only one secondary winding to power a mono amp.

Cheers,

Ian
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Old 15th January 2007, 07:29 AM   #20
AndrewT is offline AndrewT  Scotland
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Hi,
using a bridge rectifier on a single secondary will give you a full wave rectified single polarity supply i.e. V+ & 0V.
Two half wave rectifiers can give dual polarity supplies but one should try to balance currents in the two halves, particularly with toroids.
You cannot get dual polarity supplies from full wave rectification (single bridge) and single secondary.

If you build two monoblocks inside a common housing then each monoblock needs it's own disconnecting network to safety earth to make the whole amplifier safe to use and isolate each half sufficiently to prevent a hum loop.
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