Dual-mono chipamp (3886) problem - possible grounding issue - diyAudio
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Old 26th February 2008, 12:48 AM   #1
phresh is offline phresh  United Kingdom
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Default Dual-mono chipamp (3886) problem - possible grounding issue

I've just been converting my stereo gainclone to a dual mono gainclone while building it into a new case and when turned on the transformer 'growls' (the best way I can describe it). I have to say i'm no expert on transformer noises so it could just be a (very) loud hum, although it does sound quite deep hence the growl.

The reason I suspect it is the grounding is because both channels work perfectly fine if the other is disconnected. I just want to make sure this is it before I do some kind of other damage or change things that are perfectly ok!

A few bits of info that may help;
-It's the chipamp.com 3886 kits
-240V primary, dual 22V secondaries. I think it is around 200VA
-All in a single case with single ground for chassis and two amplifier boards
-Mains passes through fuse (hot only) and dpst switch

Thanks in advance
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Old 26th February 2008, 01:23 AM   #2
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Old 26th February 2008, 02:32 AM   #3
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he didn't say the output was humming, he said the *transformer* was humming. And only with both channels connected. A toroid is only going to 'growl' if it's under serious stress like a current draw way in excess of its rating... does it 'growl' even when idle so long as both channels are hooked up?

So... dual mono, two rectifier boards, one tranny... got any pics? Sounds like the rectifier boards are not wired exactly the same way or something is grounded that *shouldn't* be...

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Old 26th February 2008, 08:34 AM   #4
phresh is offline phresh  United Kingdom
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Yes, it growls when idle too. Could adding another rectifier increase the load on the tranny enough to make it do that? It was a transformer recommended for the kit so i'm inclined to say that unless faulty the transformer is fit for purpose.

Won't be able to get any pics up until tomorrow night so i'll describe the wiring best I can. The black/red secondaries go to AC1 on each board (red in the front hole) and the orange/yellow secondaries go to AC2 on each board (yellow in front hole).

If something was grounded that shouldn't be then I would expect one of the channels to not work (or not work properly at least) and there was little-to-no audible hum (in the speakers) when listening to each half of the amp individually.

I think it's going to be a case of trying a different transformer and ruling that out.
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Old 26th February 2008, 09:28 AM   #5
Nuuk is offline Nuuk  United Kingdom
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Quote:
I think it's going to be a case of trying a different transformer and ruling that out.
If it sounds OK with one channel working then it is almost certain the traffo is fine!

Not to be pedantic but what you have is not dual-mono. But if you want to try a second traffo, you could then build a true dual-mono GC.

If you can show us exactly how this is wired up, I am sure somebody can show you how to fix the problem.
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Old 26th February 2008, 09:30 AM   #6
gfiandy is offline gfiandy  United Kingdom
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Since the amplifer works in a single configuration it is very unlikely to be a problem with the transformer. The idle load of two amps is not sufficient to cause it to hum.

I suspect you have one or more of the diodes the wrong way round in the bridge rectifier. This is working OK when the amp is connected on its own (probably half wave rectifiying but these amps have good PSU rejection so you may not hear it) but is causing a short accros the windings when you connect the two bridges together.

The other possibility is you don't have them wired to the same connections on both the PCBs so recheck the wiring again as well.

If neither of these turn up a problem then I would measure the diodes in the bridge to make sure they are all working correctly.

Regards,
Andrew
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Old 26th February 2008, 12:49 PM   #7
phresh is offline phresh  United Kingdom
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Nuuk -

I know it's not true dual mono but it seemed the easiest way to describe it in a topic title! As soon as money permits and other projects are finished then I do intend to put two smaller transformers in and make it true dual mono.

gfiandy-

Diodes are definitely in the right way and I tested them all for continuity to check they were working. Obvioulsy if there was a short somewhere then the continuity tests could be wrong. Wiring is identical on both boards, i've checked it enough times as that is the one bit that always makes me paranoid! I'm going to run with the idea of a short somewhere across the diodes and test that first.

Phil
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Old 26th February 2008, 01:02 PM   #8
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If each board works fine then the chances are that there are no shorts anywhere.
Do a "visual' check of each and every diode and compare against the other board to determine if its correct.

look at this thread :
Toroid question and LM3886 photos

The psu drawing is incorrect for the negative side of the supply as mentioned in the posts. The board is supposedly correct. BUT did you follow the ( incorrect ) circuit diagram ?
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Old 26th February 2008, 01:22 PM   #9
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Pictures please! The speculation is heading off into the twighlight zone.

Most of the suggested miswirings 'check this, check that' don't fit with the evidence you've presented. One tranny and two PSU boards, transformer 'growls' but only if both amps are connected (but either alone is fine) suggests you're shorting the transformer inadvertantly somehow... pics of the wiring please! Someone, quick, mail this man a digital camera! :-)

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Old 26th February 2008, 01:35 PM   #10
phresh is offline phresh  United Kingdom
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I'll get some pictures up tomorrow night, unfortunately I have to go and see the mrs tonight although she has a habit of growling too so won't matter which one I do
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