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Chip Amps Amplifiers based on integrated circuits

LM3886 on My_ref revc dead... but why?
LM3886 on My_ref revc dead... but why?
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Old 12th May 2020, 01:09 PM   #21
Joseph K is offline Joseph K  Hungary
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And, if C13 is in place, all this should NOT depend on potmeter position at all..
The only possible explanation remaining would be a high frequency oscillation, which then could depend on input AC impedance, could cross the C13 and /or C9 barriers, and could be rectified by observer multimeter or input/output junctions in the amp..
Also that can happen in real life..

Ciao, George
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Old 12th May 2020, 02:23 PM   #22
ilovepicles is offline ilovepicles  Brazil
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Found the source of the issue! Quite embarrasing: after George prompted me to look at C13 (which was fine btw) I saw that I had stupidly switched the wires for audio input and audio ground on both amps. Dunno what prompted me to do that... After swapping them DC offset is back to normal (6mv in one channel, 10mv in the other).

Now, if anyone can explain if/why this would have killed the LM3886, I'd be curious to know.

Thanks and sorry if I wasted anyone's time.
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Old 12th May 2020, 03:53 PM   #23
Panelhead is offline Panelhead
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Default We all learn as we go

I thought your initial issue sounded like RCA wired wrong. But do not think this killed the LM3886. Maybe probing around while powered zapped it.
I have used LM3886 amps for almost 20 years, and built some using poor techniques. But never lost a LM3886.
One “tweak” I have been using for a long time is local decoupling right on the power pins of the LM3886. The original Gaincard did this, along with many Gainclones. These were the 1000 ufd reservoir capacitors.
The My_Ref uses larger filter caps, and then a second set of 220 - 470 ufd closer to the chip. I solder two 22 ufd 50 volt FM caps to the pins. (+) pin 5, (groung) pin 7, (-) pin 4. With tight routing all four leads are maybe 1 cm in total length. Low inductance.
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Last edited by Panelhead; 12th May 2020 at 03:56 PM. Reason: Ommision
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Old 13th May 2020, 12:42 AM   #24
Joseph K is offline Joseph K  Hungary
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Join Date: Dec 2004
Location: gran sasso
Nice! So hm.. At least You have a nice amp with bold fresh blood in it's vains..
Yes, could have thought of it.. Though, usually when I do it, I get - - silence.. The ground on the input used to have this effect..
I think in your case it went into oscilllation.. And almost sure that all was OK, chips did not burn.. Just, after the change, oscillated differently..

Ciao, George
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Old 13th May 2020, 01:59 PM   #25
ilovepicles is offline ilovepicles  Brazil
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Quote:
Originally Posted by Panelhead View Post
I thought your initial issue sounded like RCA wired wrong. But do not think this killed the LM3886. Maybe probing around while powered zapped it.
Yeah, I think that's the only explanation I can think of at this point. Still seems somewhat strange; but I'll take what I can get.

Quote:
Originally Posted by Panelhead View Post
One “tweak” I have been using for a long time is local decoupling right on the power pins of the LM3886. The original Gaincard did this, along with many Gainclones. These were the 1000 ufd reservoir capacitors.
The My_Ref uses larger filter caps, and then a second set of 220 - 470 ufd closer to the chip. I solder two 22 ufd 50 volt FM caps to the pins. (+) pin 5, (groung) pin 7, (-) pin 4. With tight routing all four leads are maybe 1 cm in total length. Low inductance.
I see, that does make sense. I'm thinking of building a My_ref FE (have some boards lying around for many years now). I'll certainly try this out. Thanks for the tip!


Quote:
Originally Posted by Joseph K View Post
Yes, could have thought of it.. Though, usually when I do it, I get - - silence.. The ground on the input used to have this effect..
I think in your case it went into oscilllation..
Yeah, I suspect as much too. (I also disconnected the PGND from chassi as I suspected that might have been creating a ground loop, worsening the problem).


Quote:
Originally Posted by Joseph K View Post
Nice! So hm.. At least You have a nice amp with bold fresh blood in it's vains..
A silver lining, I guess

Here's a pic of the beast now on a very hipster table I made (I need to arrange the wiring a little bit better on the transformer side, I know).
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