Low volume on one channel

[mpri]

Hi all,

I completed my build of the SSE about 9 months ago. I’m running it Triode, no feedback. It sounds great, and I love it.

However I do have one issue that I would like to fix. Volume is not balanced between the channels. One side puts out way more volume than the other. I can compensate on the input from my PC sound card, but I can’t balance the input from my CD player or turntable. I did buy a concentric pot to replace my single volume control, but I have not installed it yet as I would rather fix the problem rather than mask it.

I realize that with 5% and 10% parts there will be some difference, but the difference I’m getting seems a little extreme. I’ve done the easy checks, different input devices, and different tubes (EL34, KT66, 12AT7), with same results. I switched the speakers between posts and the low volume moved to the other speaker, so it must be something in the one channel. My question is there anything else obvious that I could check, or do I just have to bench it and take voltage (or scope) readings? Are there certain components I should measure first? Bias resistor, a cap or maybe a bum output tranny?

Thanks, any help would be much appreciated.

Mike

 

[SigloOne]

Have you tried swapping the tubes to see if the problem follows it to the other channel?

 

[mpri]

Yes I have tried switching the output tubes as well as different types of output tubes with the same issue.

 

[jrenkin]

I just had a similar problem with a tubelab SE done point to point. I found a broken volume control (Noble). First figure out which channel it is (easy) and then jumper from the input jack directly to the grid stopper on the driver. This will make the simple test to see if the problem is the volume pot or connections. If the jumped side still has low volume, carefully establish that all resistors are the correct value and not blown. This can be difficult when they are in circuit. Possible you have a value wrong somewhere, those tiny resistors with all the stripes...my old eyes can hardly sort them out.
Since you can push the volume by increasing the input signal on the down side, it sounds like something is in the way...like a bad or mis-wired volume pot. Assuming your other checkout values are correct.

 

[mpri]

Thanks for the tip on testing the pot. I gave it a try today and had the same results, low volume on the right channel. I also rechecked the wiring to the pot and it is correct. So at least that part is ruled out now. I have the resistors mounted on the top side of the pc board, which means I’ll have to drop it to recheck the values. I ran out of time today and need the amp running tonight for some music so I’ll have to do that another day.

I do have a scope, (but haven’t used it in quite a long, long time), so if resistor values appear to be ok I suppose I could use it to compare readings between the channels to see if there is a difference at one component or another.

Thanks again for the advice on checking the pot.

- Mike

 

[w5jag]

.... Volume is not balanced between the channels. One side puts out way more volume than the other.

Check the cathode bypass capacitor on the weak side power tube.

 

[mpri]

FYI more info.

1) I tested the volume control pot as jrenkin suggested. Still have low volume.
2) Check values of resistors as best I could, but I didn’t want to start unsoldering them on the board and risk lifting a pad. But all my measurements from left and right were close to spot on.
3) After the holidays I picked up a 50K concentric (stacked) volume control and installed it just so I could control the balance on the amp itself for different sources, turntable, CD, etc.

The result of switching the pot was a bit surprising to me. The difference in volume between the two channels is non-linear. That is, on lower volume the difference is way less than at higher volumes. At the low end the knobs are almost the same, but at higher volumes there is a big difference.

I do like the concentric pot better as now I can hone the controls just right to get the best balance. However I’m now not sure if the bad side is the lower side, or the higher side. Maybe some component is conducting (if that’s the right word) more than it should, or less than it should.

It’s the left side that is louder. My scope skills are minimal on audio. I was a computer tech way back when and only used one for checking for 1s and 0s. But I hooked a scope probe on one side of C21 and C11 where I could get a reading and it appeared that C21 had a higher amplitude that C11 using a/b probes together.

So now I’m not sure, but could it be the CCS conducting differently? Or something else? Any thoughts would be much appreciated and thanks in advance.

Mike

 

[rknize]

Well, that means the issue is likely on the input/driver side. Which side of the cap did you check? Since you are able, check the signal at R11/21, just to double-check that the issue is in the gain stage. Also measure the DC voltages across R10/20, R13/23, and R14/24.

 

[SigloOne]

You also might want to recheck your solder connections, could be a cold solder joint giving some resistance.

 

[mpri]

Thanks for the responses. For some reason I wasn’t able to get a signal at R11 or R21 on my scope. The voltage readings across the resistors are as follows.
R10/R20 = 1.96v/1.92v, R13/R23 = 2.99v/2.93v and R14/R24 = 88.4v/86.3v.
The picture shows where I was checking the signal on the MCaps. The one on the left has the higher signal when the volume control is the same for both channels. I’m not sure if that’s the plate side of the 12AT7 or not.
Thanks for any help.
Mike

An externally hosted image should be here but it was not working when we last tested it.

Scope points photo by tbobyt | Photobucket

 

[rknize]

From those voltages, the input/driver stage is operating correctly. From your picture, it seems that you measured the signal on the output tube side. Check the R15/25 to make sure they are the correct value. Also check R16/26 while you are there.

 

[mpri]

Thanks Russ for the help. I measured the resistors and R15/R25 were both 221K and R16/R26 were 100 and 99.8 ohms. I was hoping you were on to something. I also rechecked and reflowed some of the solder in the area with no change in the symptoms.

Could there be a problem with one of the coupling caps? I don't remember this as a problem right away. I think it came on slowly over a couple of months. But then I was so happy to hear it working that maybe I just didn't notice right away.

Thanks again all.

Mike

 

[rknize]

It would be worth swapping them out as the next thing to try. After that, getting the scope out again and probing each stage would be next.

 

[mpri]

Thanks again for the help.

Quick question: I have a pair of .22uf Auricaps on hand. Would attaching them in parallel with the other caps on board be a valid test to see if the originals are bad?

The Auricaps attached with alligator clip wire in parallel with the current .22uf MCaps would change the capacitance to .44uf, correct? I have read in another thread that George has said a coupling cap between .22 and .47 works the best.

My thought is maybe the parallel cap would either lessen or eliminate any difference between the caps to tell if that was the problem. In which case I would then replace them both. See I just don't want to unsolder components at will on the board and risk damage to it if I don't have to.

If not a valid test I just may do some more scoping first.

Although with the new concentric pot I am very happy with it now.

Thanks,
Mike

 

[rknize]

You can hook them in parallel with no ill effects. Just keep the leads relatively short or it may want to oscillate on you. If the cap is defective or was damaged by heat/etc, I would expect a LF roll off, rather than an overall attenuation. It's worth a shot, though.

 

[mpri]

Ok, thanks for the tip on the short leads. And yes, I guess I kinda would expect as you said a low frequency roll off if for instance a cap went lower in value. But I have read other threads on various sites where others have had low volume involving a coupling cap, or from a cold solder joint at a coupling cap, (which I have checked and re-soldered). So I hope it could be a good quick test to see if I should replace them before inflicting possible damage to the circuit board. If not then I'll have to break out the scope again and keep checking. I'll probably have time this weekend to check it. I'll post results either way just in case there are others that may have a similar issue, (I couldn't be the only one? Or am I?).

Thanks,
Mike

 

[Alan Hope]

Not trying to be funny, but you're sure it's not your speakers / speaker-cables / connectors?

 

[mpri]

Re:

you're sure it's not your speakers / speaker-cables / connectors?

Yes. When I test (scope, voltage, ohm readings) I move it to a bench and use other speakers and cables where it has the same issue.

Thanks for the suggestion, and all suggestions are wlecome.

Mike