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Old 1st October 2006, 04:54 AM   #1
Stuey is offline Stuey  Australia
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Default DC on speaker output - LM3886

Hi there,

First up, I'm a total novice at this and have only scratch built speakers in the past. I've now built a BrianGT (Chipamp) stereo LM3886 Gainclone and have just finished.

The problem I have is that I have raw DC across the outputs - like 28V (the rectified DC is ~31V). This is without any input connected.

I'm stumped - I've checked and rechecked my wiring to no avail. Has anyone got any ideas? It's the standard Chipamp schematic with no variation.

I have a pot on the input - can this affect it? (Turning the pot makes no difference).

As a side issue, are all pots standard with respect to their terminals? I know the middle terminal on the dual gang Alpha I'm using is the output (wiper) terminal, but the others I'm not sure about.

Sorry about being such a dunce.

Cheers

Stuey
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Old 1st October 2006, 05:56 AM   #2
Stuey is offline Stuey  Australia
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...I forgot to say that this voltage was with the chassis grounds (CHG) from the two amp boards earthed to the actual chassis. Without this, the voltage at the output terminals is around 16.5V.

Does this give any hint?

I've just rechecked for shorts between the power rails and gnd to no avail.

Stumped...

Stuey
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Old 1st October 2006, 07:43 AM   #3
CJ900RR is offline CJ900RR  Sweden
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Hard to reply without pics or a schematic over how you really have connected everything. Please take quality picture and show us
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Old 1st October 2006, 10:41 AM   #4
Stuey is offline Stuey  Australia
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Thanks for the reply. In desperation I resoldered as many components and connections as I could, and I don't know what I did, but it's sorted. One channel has a DC offset of 15mV and the other 45mV.

Tested and playing beautifully, a tiny amount of hum at full 'volume' but nothing to worry about.

Nice sound; limited gain with a faked 50K ohm pot, but that's easily trial-and-errored out.

Cheers

Stuey
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Old 1st October 2006, 10:45 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
Quote:
a tiny amount of hum at full 'volume' but nothing to worry about
it may be coming from the source if it is volume control dependant.

With a shorting plug in the input RCA (for hum checking and for DC output offset checking) you should have no hum.
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Old 2nd October 2006, 12:45 AM   #6
Stuey is offline Stuey  Australia
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Thanks Andrew - I've read hundreds of your posts on this forum while building!

I know you can buy shorting plugs, but do they essentially short across each set of input terminals + to - ? (I've seen horseshoe shaped wire which is designed to bridge the two terminals + to + and wondered what this was for...).

I clearly didn't measure the offset properly; I should've remembered as I'd read where you described the methodology in another post only this week.

The source is a Marantz CD50SE. Not that it really matters, but if it still exists I might try and eliminate it.

Cheers

Stuey
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Old 2nd October 2006, 07:21 AM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
you can make your own shorting plug from an empty RCA plug.

Solder a shorting link into the plug from tip connection to ground connection. Put the cover back on and label it clearly.

Then use it whenever you are testing equipment.

You can buy them, they were popular for a while to short out unused digital outputs, supposedly to make the analogue output sound better.
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Old 2nd October 2006, 08:09 AM   #8
Stuey is offline Stuey  Australia
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Thanks again Andrew. I'll try that.
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