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Old 13th November 2012, 08:45 PM   #1
mpri is offline mpri  United States
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Default Low volume on one channel

Hi all,

I completed my build of the SSE about 9 months ago. Iím 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 canít 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 Iím getting seems a little extreme. Iíve 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
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Old 13th November 2012, 09:53 PM   #2
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Have you tried swapping the tubes to see if the problem follows it to the other channel?
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Old 13th November 2012, 10:32 PM   #3
mpri is offline mpri  United States
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Yes I have tried switching the output tubes as well as different types of output tubes with the same issue.
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Old 21st November 2012, 07:35 PM   #4
jrenkin is offline jrenkin  United States
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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.
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Old 24th November 2012, 11:59 PM   #5
mpri is offline mpri  United States
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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 Iíll have to drop it to recheck the values. I ran out of time today and need the amp running tonight for some music so Iíll have to do that another day.

I do have a scope, (but havenít 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
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Old 28th November 2012, 04:51 AM   #6
w5jag is offline w5jag  United States
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Quote:
Originally Posted by mpri View Post
.... 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.
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Old 25th January 2013, 09:58 PM   #7
mpri is offline mpri  United States
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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 didnít 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 Iím now not sure if the bad side is the lower side, or the higher side. Maybe some component is conducting (if thatís the right word) more than it should, or less than it should.

Itís 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 Iím not sure, but could it be the CCS conducting differently? Or something else? Any thoughts would be much appreciated and thanks in advance.

Mike
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Old 26th January 2013, 03:10 AM   #8
rknize is offline rknize  United States
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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.
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Old 26th January 2013, 04:47 AM   #9
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You also might want to recheck your solder connections, could be a cold solder joint giving some resistance.
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Old 28th January 2013, 10:59 PM   #10
mpri is offline mpri  United States
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Thanks for the responses. For some reason I wasnít 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. Iím not sure if thatís the plate side of the 12AT7 or not.
Thanks for any help.
Mike
Click the image to open in full size.

Scope points photo by tbobyt | Photobucket
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