LM3886 on My_ref revc dead... but why?

[Joseph K]

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

 

[ilovepicles]

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.

 

[Panelhead]

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.

 

[Joseph K]

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

 

[ilovepicles]

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.

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!

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).

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).