Volume dropping intermittently on left channel

[gemini14]

I sent my Crack + Speedball back to Bottlehead for repair after wrangling with a maddening issue with the Speedball LEDs. That was fixed, but it seems another problem slipped through the cracks. I've narrowed it down to the amp (swapped headphones, listened with/without DAC, swapped cables, etc.) over a long period of time. The problem is that the left channel intermittently drops in volume -- sometimes significantly, sometimes just annoyingly enough to make you aware of it, at which point you can't stop un-hearing it.

Testing voltages is a huge pain, because of how difficult it is to catch the problem. As an example, I can sometimes go days without any trouble and then out of the blue have two incidents in a 2 hour time period. Many times I've tried to check voltages and by the time I started checking them, the problem disappeared partially or completely. However, each time I've been able to check the voltages, they've all been within the correct ranges, just as they were when I got the amp back from Bottlehead.

I'm not sure if this is just supposed to happen or not, but if the left channel has a reduced volume and I turn the amp off and then on again, the left and right volumes are usually equal again, at least for a short period of time. Also, during the few seconds when I can still hear music when shutting the amp off, the volumes of both channels seem to equalize.

In my ignorance, I'm guessing that this might be a bad solder joint somewhere on the path to the left channel, which, due to heat, is causing the left channel's volume to drop. Of course, I would have thought it would be a constant drop after it occurs the first time after switching the amp on. However, since it can come and go after switching the amp on and leaving it on, I'm really not sure what's going on. Is there a particular set of joints I should be looking at and reheating or is this caused by something else entirely? I'd rather not dive in and start soldering willy nilly after sending it back and screw something else up.

[Paul Birkeland]

Hello there Gemini,

I'm sorry to hear that you are still having some troubles.

There are a few simple tests that can be done, but the real challenge will be using the amp enough with each modification to narrow things down.

The absolute first thing you should to is to be sure to try an alternate set of tubes.  There are conditions (especially in a 12AU7) that could result in what you're describing.

Beyond that, you will want to start by soldering a jumper wire between the middle lug on each level of the volume pot, then run the amp until you observe the same issue.  This will sum the inputs to the Crack, and if your issue is with the input wiring, this will solve the issue.

If it doesn't solve the issue, let me know and we'll go onto the next test.

[rlyach]

I am sure the guys at BH know best, but I had a similar issue with a ASL EL34 amp that my son owns. One channel would get very quiet and even sometimes drop out all together. But it was intermittent. He tried different tubes and it went away for a while and came back. In the end I finally got it to misbehave on the bench and noticed that the bias point of one of the output tubes was fluctuating. The solution.... We cleaned and tightened the tube socket and the problem has not come back.

[gemini14]

Thanks for the fast replies guys. I've tried my only other tube, a Mullard 12AU7/ECC82, and swapped back and forth between it and the stock tube. I didn't keep track of the specific number of occurrences while using each tube, but the problem definitely occurred with both. I don't have any non-12AU7 input tubes or other power tubes, but I can pick up some if necessary for testing.

@rlyach, I thought of dirt being a reason, too, especially regarding the Mullard I picked up. However, both tubes' pins were clean as whistles and I couldn't spot anything on/in the tube sockets. Cleaning both unfortunately didn't do the trick.

@Paul I added the jumper as suggested and the audio is now centered properly and the volume is balanced. I've been listening for several hours throughout the day with both of my tubes and so far there's been no volume drop.

When you say sum the inputs, I assume you mean it takes the left and right channels and basically turns them into a mono stream, which is split by the headphones themselves, right? I ask because although the volume is balanced, I immediately noticed that stuff didn't pan any more (or show up in the correct channel).

[Paul Birkeland]

If the summing fixes the problem, this can only point to a limited number of things, the out likely of which was be a funky RCA jack.

You can sum the jack outputs by removing your current jumper and connecting it between the center posts of the RCA jack.

[gemini14]

Just to be clear, you mean solder the jumper wire to the spots on the posts where the red and white wires are currently coming out of the RCA jacks, right?

[Paul Birkeland]

Yes, that is correct.

[gemini14]

OK, summed the jack outputs earlier today and so far so good - no volume drops yet and the audio is still in the correct spatial position.

[Paul Birkeland]

Yeah, this sure sounds like a funky RCA jack.  Different cables may help confirm this, but luckily the fix is pretty simple.

[gemini14]

Sounds like a plan - I'll pick up some new RCA jacks later this week and run the amp through its paces for a few days before I confirm it's fixed.


Edit: I wanted to wait at least a couple of weeks before verifying this was fixed, but it looks like replacing both the jacks and the cables did the trick. Everything finally sounds great

I know for a fact that the cables were fine before, but after switching the jacks, I could only get balanced, clean sound by switching to some cheap RCA cables I had lying around (was using monoprice premium cables before). The cheap ones aren't shielded and occasionally let in some EM noise, but for now it's fine (I'll probably get some shielded cables in the future).