Audio question again. Suddenly quiet subwoofer - Page 1
Forum section: electronics / beginners

author: FenderBender
author_rank: Super Contributor
timestamp: February 13, 2013, 01:39:01 am
content: 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?

author: c4757p
author_rank: Super Contributor
timestamp: February 13, 2013, 01:55:24 am
content: 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?

author: FenderBender
author_rank: Super Contributor
timestamp: February 13, 2013, 01:59:05 am
content: 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.

author: c4757p
author_rank: Super Contributor
timestamp: February 13, 2013, 02:00:49 am
content: What is the chip?

author: FenderBender
author_rank: Super Contributor
timestamp: February 13, 2013, 02:13:23 am
content: I'm using two LM3886s for the left and right and an LM4780 configured in parallel mode for the subwoofer.

author: FenderBender
author_rank: Super Contributor
timestamp: February 13, 2013, 02:19:25 am
content: It's just very strange to me because I have not altered the sub amplifier at all.

author: c4757p
author_rank: Super Contributor
timestamp: February 13, 2013, 02:19:40 am
content: 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.

author: FenderBender
author_rank: Super Contributor
timestamp: February 13, 2013, 02:24:47 am
content: 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.

author: c4757p
author_rank: Super Contributor
timestamp: February 13, 2013, 02:33:35 am
content: Could be slow, cumulative damage from heat - have youthoroughlywent through the computations for the heat management you have set up?

author: FenderBender
author_rank: Super Contributor
timestamp: February 13, 2013, 02:37:05 am
content: 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.

author: FenderBender
author_rank: Super Contributor
timestamp: February 13, 2013, 02:40:53 am
content: 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.

author: c4757p
author_rank: Super Contributor
timestamp: February 13, 2013, 02:42:20 am
content: Quote from: FenderBender on February 13, 2013, 02:37:05 amWow. Yeah, I don't think you have a heat problem.Quote from: FenderBender on February 13, 2013, 02:40:53 amSince 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?

author: FenderBender
author_rank: Super Contributor
timestamp: February 13, 2013, 02:46:43 am
content: Of course I didn't.I'd say somewhere in the pin 18-23 range.Amateur mistake.

author: c4757p
author_rank: Super Contributor
timestamp: February 13, 2013, 03:05:44 am
content: 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 wasnotcausing some problem.Quote from: FenderBender on February 13, 2013, 02:46:43 amThat range covers one of every type of pin

author: FenderBender
author_rank: Super Contributor
timestamp: February 13, 2013, 03:16:16 am
content: I really don't even know. All I know is that it's louder now.Very scientific approach I know...