Amplifier auto-start on sound input

[Dannyx]

Good day folks. This was originally meant for the "troubleshooting" category, but I decided it should be more of a generic discussion for now since I haven't actually started working on the device and I don't really want to ask for help before actually attempting something on my own.

This is about an active Jamo "SUB 260" subwoofer. The chap brought it in with no power at all after it's been fixed before by someone else (pretty sloppy job, but that's not important). He told me the thing only powers on when there's an input signal present, which I took with a pinch of salt at that time, but then I looked up the manual and indeed it supposedly turns on only when an input signal comes into the RCA terminals on the back. I tried it out for myself and of course I got nothing, no power at all.

The fuse was blown on the mains input despite none of the major components being shorted - I checked. Since I recently switched jobs, I don't have my trusty "safety lightbulb" assembly on hand at my new workplace (left it at my former shop) so I didn't want to plug it straight into the mains after simply replacing the fuse, so while I'm waiting to get my bulb back (sounds funny ) I began pondering how the circuit operates. I'm fearing I may have to do some digging in the control side if the power section appears to be fine and I still get no power.

I'll post some pics soon, but there aren't any microcontrollers in there - just some op-amps as far as I could tell.

The way I imagine it with my modest electronics knowledge is an op-amp in an amplifier configuration boosting the input signal from the RCA inputs, which is then fed to the NI input of another op-amp whose inverting input is tied to GND. Therefore any signal higher than "GND" will trigger it to turn on, driving whatever's required to turn on the supply...ok, so there's a lot more to it than this, because the way I "designed" it would probably mean the amp would either stay on all the time due to noise on the line (most likely) or shut off intermittently during quiet parts of a song (less likely)....but I hopefully got the basics down. Like I said, I fear it's THIS part of the circuit that's not operating and we'll have to troubleshoot multi-layer SMD boards....NOT very easy TBH...I'll post back with some pics tomorrow (haven't got them off the phone yet).

[budm]

Re: Amplifier auto-start on sound input

Search for Ps-10 thread, I provide the service manual that you can see how typical subwoofer turns itself on when audio input is present.

[Dannyx]

Re: Amplifier auto-start on sound input

Here's some pics of this thing. There's a smaller board at the bottom where the RCA inputs and pots are...not particularly great quality there, I admit...poor lighting over at this new shop, quite the eye-rape if you ask me....

[Dannyx]

Re: Amplifier auto-start on sound input

Ok, here's where we stand: I cleaned up some of the brown gunk and replaced a couple of caps, including one on the input board which seemed to have some orange stuff on the top and I attempted a power on. I plugged the supply in via a series lightbulb, the 15 and -15v rails went up and it seemed stable, with the bulb staying off all throughout.

I then tried it with sound: without the speaker connected, I turned the volume all the way down and hooked up some RCAs to the back. Upon increasing the volume of the laptop a bit, I could indeed hear the relay clicking on, so that part of the circuit is working. I took a cheapy little speaker I had lying around and tried hooking it up to the amp's output. I did the same routine and even though the volume of the sub was turned all the way down, it made a horrible buzzing sound through the speaker when the relay came on, forcing me to unplug it...could it be because I still had the lightbulb in series with the thing and it wasn't getting enough current ?

[Dannyx]

Re: Amplifier auto-start on sound input

More to come: I think I managed to identify the "ON" pin on that connector. The labeling was very vague so it wasn't clear what did what. V+ and V- and G were clear enough (even though I would've liked G to be GND but NVM) but then you have S and A.....like WTH :| Initially I thought maybe S stood for Sound, but then again it could very well stand for Standby so which one was it ? The way I managed to track it down was by having a look at the other board, the one with the pots and inputs: the power LED at the front has one of its pins connected to the A pin of the terminal on the board, so when the board decides it's time to turn on the amp, it sends a signal (presumably 5v, though I haven't checked) to that A pin, turning on the LED and also the power supply. I tracked the A pin to go somewhere around that optocoupler, so that's what provides startup (it looks like it's go no feedback though).

This would mean S stands for Sound and it's going to be a mono signal (combined L and R). To isolate the buzzing to the either one of the boards, I was thinking of lifting this S pin and trying it out: if it still buzzes, it's down to the AMP board, if it doesn't it's down to the preamp....just a thought. I'm not an audio guy and haven't got a scope, which is the most essential tool for audio gear. Supposedly this joint has a couple of scopes just lying somewhere so I'll try hunting one down sooner or later...

[budm]

Re: Amplifier auto-start on sound input

That is Class D power amplifier, there are so much bad brown glue on that board that may have caused the circuit to go into Oscillation. I would removed as much glue as you can.

[Dannyx]

Re: Amplifier auto-start on sound input

Yes, precisely what I was thinking. I started doing this around the two LM regulators today just for good measure and also replaced the two caps. I shall carry on doing this, focusing on the area around the other two large caps (the output ones)...and maybe replace them while I'm at it. It could be something really simple as I've heard that glue can become slightly conductive, enough to cause issues.

[Dannyx]

Re: Amplifier auto-start on sound input

I remember watching this video from Dave a while ago and I'm now rewatching it because it's fairly close to what I'm experiencing with my Jamo.

[Dannyx]

Jamo SUB 260 loud buzzing, blows fuse

For those of you still following along at home, I THINK I fixed it. The reason I say "think" is because I took the amp home with me to work on it in peace (this new shop is a clusterfuck...people running all over the place, screaming and yelling...no way you can do anything there) and I don't have the original speaker on hand to test and I didn't do individual tests after each operation I performed on the board: I finished everything that I could and then conducted one general test, so I'm not entirely sure WHAT fixed it.

So I've been patiently scraping away at the brown goop on the board, carefully removing components, scraping some more, then putting them back in, which also gave them a reflow, even replacing some of caps which seemed a bit beat up. I also replaced the two filter caps before the regulators for + and -15v and in doing so I somehow managed to break the trace between the input pin of the + regulator and the "source" circuit. I noticed this when I wasn't getting any 15v output on the connector but I WAS getting -15v. I think this was the defect all along: the output was being pulled down to the -15 rail all the time due to lack of....bias ? Is this the right term ? I fixed the trace and gave it another go. No buzz this time, despite the lightbulb still being in place. I even ran some sound tests with a center channel speaker I had lying around from an old home theater system and it worked: I got sound output, albeit it wasn't spectacular. The conditions were far from ideal: used a crappy FM radio as the source and this IS a subwoofer which cuts off at 200Hz, while I was trying to run a center speaker with a tweeter and capacitor filter on it....not a good combo AFAIK. Tomorrow I'll try it out with its actual speaker and fingers crossed it doesn't blow the fuse again. Cheers.