Scratching in left channel

[slomo85]

Hello everyone, I come to you in despair. I built the Crack standard amplifier a few weeks ago, it sounds rather good. Even purchased higher impedance cans for it that I am liking very much (DT880 600 ohms) because my main headphone was the AKG 702's.

I am having the hardest time getting over an erratic and inconsistant scratching/fuzzy sound in the left channel. I tried several headphones with no change. I know it's not my ears because I put the phones on backwards and the scratching appears in the other ear as expected.

Things I have tried:
Moving the amp to another room
Getting it far away from anything wireless
Reflowing pretty much all solder joints (and verified every joint has good conductivity, by touching probes off of the solder)
even bought new tubes
Poking with wood chopstick with amp energized all over every connection.

All voltage checks and resistance checks are good. I can take them again and post them up if it's necessary.
I grounded the scratchy left channel out as suggested in the T/S and the scratching is still there. It is unaffected by the volume pot.

It's really annoying when a silent or soft passage goes through and I can hear a scratchy sound in the left channel I really don't know what else to do, thus I am here. I have read many posts here that always seem to lead back to bad solder joints, but I have really really gone over everything and can't seem to find any. Maybe I don't know what else to check, maybe I'm here just for moral support. I don't know, I really just want this amp to stop being scratchy in the left channel .

Also, I'm not sure if it's worth mentioning but, since I'm reading that a lot of people are padding down their volume knobs with resistors because the amp goes to loud? On all my headphones I need at a minimum, 70-75% volume to get up to jamming listening levels.

[Paul Birkeland]

Is this scratchiness independent of the level of the volume pot?

A crackling like that is indeed generally a flaky solder joint, and occasionally can be tube related.  If you heard no scratchiness when poking with the chopstick and wearing some cheap headphones, and no improvements come from tube rolling, I would become curious about the potentiometer, though this is a long shot!

[slomo85]

Is this scratchiness independent of the level of the volume pot?

A crackling like that is indeed generally a flaky solder joint, and occasionally can be tube related.  If you heard no scratchiness when poking with the chopstick and wearing some cheap headphones, and no improvements come from tube rolling, I would become curious about the potentiometer, though this is a long shot!

Yes, totally independent of the volume pot. If it is all the way down, it comes and goes erratically. If it's all the way up, it's still comes and goes but, totally ruins the music. I have put a few hours on it and, when it's not scratchy in the left channel it's a very pleasant amp. I have been over (Re-soldered) and continuity checked literally every solder joint on the amp and I can't figure it out. I bought another set of tubes because the ones that came with the kit had corrosion (White powder on the 6080 and blackened pins on the input tube) with no improvement.

Also, when poking around the live amplifier I can not find a correlation to the sound and a specific area. Why a cheap set of headphones? I was using my regular headphones.

I'm gonna find the problem, it's just frustrating right now. Does anyone know of any specific places or things to check? I'm kind leaning on replacing resistors next.

[jjvornov]

Have you tried different sources and interconnects? Different headphone? I was convinced I had a bad channel on a pre amp that went in and out when I thumped it. It was the interconnect that had a flakey connection. Now I aways switch input channels and try another source to isolate a noise to the component in question.

James

[slomo85]

Have you tried different sources and interconnects? Different headphone? I was convinced I had a bad channel on a pre amp that went in and out when I thumped it. It was the interconnect that had a flakey connection. Now I aways switch input channels and try another source to isolate a noise to the component in question.

James

I shorted the inputs as suggested in the T/S FAQ's and the noise persisted

[Paul Birkeland]

Well, you've certainly taken a lot of efforts to get to the bottom of this. 

When you rock either of the tubes, can you trigger the scratching?  Since you have the chopstick, did you try poking each tube socket contact? 

Whenever I've had scratching noises, it's always been from a flaky connection (Which has sometimes included pins with too much oxidation, but generally just a solder joint).   It could really be any solder joint also, pay careful attention to those on the headphone jack and volume pot in particular.

[slomo85]

Well, you've certainly taken a lot of efforts to get to the bottom of this. 

When you rock either of the tubes, can you trigger the scratching?  Since you have the chopstick, did you try poking each tube socket contact? 

Whenever I've had scratching noises, it's always been from a flaky connection (Which has sometimes included pins with too much oxidation, but generally just a solder joint).   It could really be any solder joint also, pay careful attention to those on the headphone jack and volume pot in particular.

Yes, I have. I really want this scratching gone because otherwise this amp is sounding awesome. Listening to it right now actually. I will poke around it again this weekend. I poked and moved every connection on the amp and rocked the tubes back and fourth while listening live and couldn't find a correlation to the noise.

[slomo85]

    needed         actual
1  75-90            77.5
2  170               170.6
3  0                   0
4  170               170.4
5  75-90            78.3
6  0                   0
7  100               101.7
8  0                   0
9 100                 102.9
10 0                   0
11 0                   0
12 0                   0
13 170               169.5
14 0                   0
15 185               191.5
20 0                   0
21 206               212.3
A1 90                 78.6
A2 0                   0
A4 0                   0
A5 0                   0
A6 90                 77.6
A7 0                   0
A9 0                   0
B1 90                 77.5
B2 170               169.8
B3 100               101.7
B4 90                 78.3
B5 170               169.3
B6 100               101.9
B7 0                   0
B8 0                   0

I see what might be an issue. All voltages look good except for B1/B4 and A1/A6. They look a little low although TB1/TB5 give a range that these fall nicely into. I'm trying to look up tube schematics to see what these pins hook up to.

LED's are always on, with no flickering even when the scratching is present.

Xformer voltages read:
1  118.2 vac
2  0.220 vac
4  0 vac
5  5.79 vac
6  78.2 vac
7  81.2 vac
9  78.2 vac
10  81.2  vac

Does Transformer pin 5 look low? Don't the tube filaments need 6.3 vac?

[fullheadofnothing]

Your "low" voltages indicate a strong 12AU7 tube. There is no problem with any of your voltages.


To answer one of your previous unanswered questions: use cheap headphones when chopstick testing for two reasons: one is that it might reveal the problem is with your headphones/cable. The other is that you might damage the connected headphones in the process, therefore you should protect your quality gear from accidents.