Audio question again. Suddenly quiet subwoofer

[FenderBender]

Hello everyone,

I decided to rewire the amp I built a year or two ago. It consists of 3 chip amps. One for left, right, and subwoofer. Before doing this, turning the subwoofer volume knob half way made the sub max so that it would sound like a motorcycle. Now I can't even get it that loud if I tried. Full volume now would be like 1/4volume before.

So I took some measurements. Somehow they don't seem right to me. But I'm not too sure what they should be either.

Between the Left + and the Right +, I measured 20kOhm.
Between the Left + and the Left -, I measured 38kOhm.
Between the Right + and the Right -, I measured 41kOhm.

First off, shouldn't Left + and Right + be open circuit?

But the amp works and the left and right channels sound very good (as does the sub; it's just quiet).

Any ideas?

[c4757p]

Have you connected the speaker to a known good amplifier to verify that it hasn't been damaged?

The resistances seem fine. They're close enough to an open circuit - the meter might be partially activating some junctions in the chip and getting a low reading, or they may really have resistances between them for whatever reason. What chip is it?

[FenderBender]

I have not done that...partially due to the fact that I don't have anything else to test it on. I can almost assuredly say it is not damaged since it was working correctly the last time I used it.

It works. It sounds quite good, but just noticeably quieter than previously. I'm not sure why that would occur based on the changes I've made.

[c4757p]

What is the chip?

[FenderBender]

I'm using two LM3886s for the left and right and an LM4780 configured in parallel mode for the subwoofer.

[FenderBender]

It's just very strange to me because I have not altered the sub amplifier at all.

[c4757p]

I just took a short glance at the datasheet - but it's basically a really beefy op amp? I don't have too many suggestions, but if you're not using it in unity gain, check the gain resistors. Maybe one is shorted or has drifted in value (piece of metal stuck underneath?) I'll sit on it for a while and see if I can think of anything else. Do you have a dummy load? Check that it can actually deliver the power - maybe the chip is burnt out.

[FenderBender]

I will check for said metal pieces. Perhaps it is, but that would just boggle my mind if it died, seeing it worked before.

Thanks.

[c4757p]

Could be slow, cumulative damage from heat - have you thoroughly went through the computations for the heat management you have set up?

[FenderBender]

Thouroughly. No. However, it is mounted on a CPU heatsink ( a big one with heatpipes and forced air fan cooling). I don't think heat is a major problem.

[FenderBender]

I think I've found the culprit. Thanks for putting out the idea of something being shorted out. There was a piece of solder splash lying between two of the pins on the 4780. Hardly discernible. I pulled it off with practically no effort...so I'm not convinced it was actually shorting...I can't crank the music too loud right now since it's bedtime for most here, but from the brief momentary test I did, it seems louder, much like it was before this whole nonsense.

Again I'm not 100% yet, but I THINK that's what happened.

Thank you for the advice. I'll report back in case it's not that.

[c4757p]

Thouroughly. No. However, it is mounted on a CPU heatsink ( a big one with heatpipes and forced air fan cooling). I don't think heat is a major problem.

Wow. Yeah, I don't think you have a heat problem.

There was a piece of solder splash lying between two of the pins on the 4780.

Since you didn't get a chance to fully test it - did you think to check which two pins so you can at least see if it could be the cause?

[FenderBender]

 Of course I didn't. 

I'd say somewhere in the pin 18-23 range.

Amateur mistake.

[c4757p]

Seems like a good candidate, anyway. Even if it doesn't look like it was shorting, it may have had thin splash-filaments extending from it. It's been a long time since I've found a solder splash in a running circuit that was not causing some problem.

I'd say somewhere in the pin 18-23 range.

That range covers one of every type of pin 

[FenderBender]

I really don't even know. All I know is that it's louder now. 

Very scientific approach I know...