Static-like Sound Coming from Left Channel?

[MLA]

You can do both but do it in two steps; first, swap the output wires from the volume control so L+ and L- go to R+ and R- on the circuit board (and vice versa). If static moves to the other channel, the pot is the culprit.

If no change, swap the XLR input wires on the pot so the one previously going to left channel now goes to right (and vice versa). If static moves to other channel, it's the XLR input wire or input jack causing trouble. Not likely, but worth ruling out.  

[ThePhoenix]

57 minutes ago, MLA said:

You can do both but do it in two steps; first, swap the output wires from the volume control so L+ and L- go to R+ and R- on the circuit board (and vice versa). If static moves to the other channel, the pot is the culprit.

If no change, swap the XLR input wires on the pot so the one previously going to left channel now goes to right (and vice versa). If static moves to other channel, it's the XLR input wire or input jack causing trouble. Not likely, but worth ruling out.  

Awesome, thank you. I'll try it out when I have a chance and report back (crossing my fingers this is the whole issue!).

[ThePhoenix]

Okay, so I've played around with the wiring a bit. Naturally, the problem didn't immediately hold its hand up and reveal itself.

First, I swapped the L/R wires leading from the pot to the circuit board. The "pop"/static with the volume pot stayed with the left channel. The channels were definitely swapped, so not sure why that didn't change.

After playing around with it for a few minutes, I shut the amp back off and switched those first wires back around. Then, I swapped the wires coming in from the XLR inputs. Still no change as far as the turn-on noise (which isn't a big deal) and volume pot static go. Again, the L/R audio is definitely swapped.

I didn't notice any of the static I've been having in the few minutes I tested, so once I'm done writing this post, I'm going to just listen to silence and wait/see if that issue has swapped channels or not. I would guess from the results of the testing above, that it will not, but I'll report when I get some (damn these intermittent issues!).

I believe the channel imbalance remained on the left side. That said, I'm second-guessing everything I hear and I can't measure it, so maybe I'm just nuts (very possible). I've been listening to an old amp to compare/try and placate myself that I haven't damaged my headphones, and I do think there is an imbalance of some kind.

 

[ThePhoenix]

Well, for the two hours I tried, I got no static. Which is either insanely "lucky" OR the part finally failed OR the static was caused by a dodgy connection to the volume pot. So, I'm going to switch everything back to how it should be tomorrow morning and I guess keep waiting to see if any static comes across.

With the potential of it being a problem (if not THE problem) and there being the believed channel imbalance, I'm considering replacing the volume pot just to be safe. I know these vary wildly in price from "reasonable" to "are you kidding". Right now it looks like there's an Alpha brand pot in there; I know Alps and TKD are the names thrown around here most often; is there one I should look at/consider if I go this route for peace of mind?

[spritzer]

You need a 10K quad pot and they are quite rare.  TKD does make those in the 601 series but it is tiny so it would need a PCB to make wiring easier.  I'd stay away from the Alps RK27 and the RK50 is only available as 50K. 

[ThePhoenix]

That sounds like it's above my abilities. Hmm.... Well, I need to determine if the initial issue is resolved/gone or not and go from there. I'm switching the wires back this morning and do some A/B-ing between the BiPolar and my old amp to see if I am hearing a channel imbalance or not (I was very sure of it the other day, less sure yesterday). Obviously, if the static returns we know I just got absurdly unlucky last night (the story of my life with nice things).

Also, I know I've said it already, but thank you so much for sticking with me on this. I really appreciate it.

[ThePhoenix]

Okay, so I can report back with static updates (yay, what fun this is - not).

After doing some listening, I'm not wholly convinced the imbalance is outside of the norm or that it isn't something wrong with me physically. It seems to come and go, but that could also be the music, so I'm going to ignore this for the time being and focus on the static.

Anyways, while testing the above, I finally got some static. I quickly shut the amp off, swapped the L/R channels on the left side of the pot (that go to the circuit board), and turned the amp back on. The static remained with the left channel. [I think I've noted it before, but if I swap my headphone connections from L to R, the static does follow.)

With that discovery, I think it is safe to say that the issue is somewhere in the left channel circuitry (or even the XLR out). Is there anything else I can measure/test, or should I start packing it up to ship to an expert?

[I also have half a mind to sell it to an intrepid DIY'er who wants to deal with it. We'll see what happens.]

[MLA]

I think you've done all you can and done it well, so next step is examining the circuitry closer and possibly start replacing parts, or plug in a new left channel (which actually might be the faster and more straightforward option, but requires some soldering skills). Or sell, of course.

If sending to an expert, it's probably enough to send the circuitboard package rather than the whole amp; looks like you can take the whole thing out using a screwdriver if you loosen the headphone jack from the chassis; no need to desolder anything (unless I missed something in the pics).    

[ThePhoenix]

That's a good idea on taking it out of the enclosure. I'll look at it and do some soul-searching. I love the darned thing when it's quiet and performing correctly, but it sure is annoying to know something's not right and, of course, have one's music interrupted with a bunch of static. That said, maybe this is the universe's sign that now isn't the time to upgrade and I should just sell it, cut my losses, and move on.

Thanks again, everyone, with your suggestions and contributions! I'll report back what I decide to do and, later, if I get any resolution on the matter.

[ThePhoenix]

So, a small update. A very kind forum member offered to take a look at the amp and see if they can find the issue and rectify it. I'm very thankful to them and everyone else who's offered their advice and hope the problem can be eradicated. I will update this thread with anything they find.

Thanks again!

[ThePhoenix]

I've heard back from the awesome forum member who's checking out the amp. They found that three of the four MAT BJTs were bad, which is why I didn't notice a difference when I swapped them around. They've replaced them, adjusted the output, and have the unit on its way back to me as I type this out. So, thanks again to them and everyone else here! I can't wait to have it back and look forward to hearing it without concern of that awful static.

[ThePhoenix]

The amp has been back in my hands for a couple of weeks now and sounds great; there was a scare the static was returning one evening, but I think that was something else as I haven't heard it since (knock on wood).

That said, it was recommended to me by the forum member who worked on it to swap out the generic transistors they used with a pair of THAT340s once I could get my hands on a pair and, once I had them, to make sure I check the DC Offset and adjust as necessary. I'm assuming I measure off of the volume pot like I did when we first started troubleshooting; what is it, exactly, that I should be doing to adjust it?

[Pars]

You measure DC offset at the outputs (signal to ground) with the volume pot turned all the way down. DC V or mV on your meter. Ideally you would disable the DC servos when doing this, but I doubt this is possible on this amp.

[ThePhoenix]

1 hour ago, Pars said:

You measure DC offset at the outputs (signal to ground) with the volume pot turned all the way down. DC V or mV on your meter. Ideally you would disable the DC servos when doing this, but I doubt this is possible on this amp.

Thank you; I didn't know about needing to turn it all of the way down (good thing I haven't tried anything yet). Do you have any idea what I might be looking for to actually do the adjustment?

[Pars]

No, the inputs need to be shorted (i.e., pot all the way down). If it like an SS Dynalo, there will be two trimpots controlling offset (one per channel). You want the offset as close to 0V as you can get. <10mV for sure. Nothing to get anal about, 1mV or less is very good. Spritzer might weigh in if he wants to.

[ThePhoenix]

Sounds reasonably easy, thank you for your help!