50Hz hum from S.E.X. amp, only headphones and left channel.

[mwgrient]

Hi,
I have a 6 years old S.E.X. amp (v2.1) with C4S installed and with a 50Hz hum on the left channel. Does not increase if volume is up.
It is not notable on speakers, perhaps not sensitive enough? So only on my headphones... (Fostex TR-x00 EB) and not loud.

My question is where to look? I have tried to clean everything with alcohol.
cheers, Marco.

[oguinn]

Maybe start with the headphone jack? Or post some photos.

[mwgrient]

here are some pictures of the headphone jack.

[Paul Birkeland]

I would start by checking the DC voltages and swapping the tubes to see if the noise follows a tube.

[mwgrient]

I have some measurements

Terminal - Voltage
1 - 201
2 - 0
3 - 0
4 - 405
5 - 202
6,16 -  385 / 385
7,17 - 0 / 0
8,18 - 0 / 0
9,19 - 0 / 0
10,20 - 405 / 405
11,21 - 346 / 347
12,22 - 0 / 0
13,23 - 0 / 0
14/24 - 0 / 0
15,25 - 17 / 17
A1,B1 - 0 / 0
A2,B2 - 362 / 365
A3,B3 - 17 / 17
A4,B4 - 0 / 0
A5,B5 - 70 / 70
A6,B6 - 2 / 2
A7,B7 - 2 / 2
A8,B8 - 3 / 3
C1 - 2.4
C2 - 3.6
C3 - 0
C4 - 2.4
C5 - 3.8

C4S
Terminal
OA/OB - 66 / 66
10, 20 - 405 / 405

[Paul Birkeland]

Those look good, now I would try swapping tubes.

[mwgrient]

Thanks.
I noticed a weak connection because I was pulling the black wire from jack to terminal 23. Then it came of the jack. I have resolved that cable. The strong 50Hz signal is gone, but have now stereo light hum. Only on the headphones.
I swapped the tubes anyway. But no effect.

[Deke609]

Have you used the Fostex TR-X00 with the S.E.X. before without hum? If not, and this is your first time pairing those phones with the amp, I wonder whether the hum is a function of the phones' low impedance: 25 ohms. That's pretty low. IIRC, I heard increasing levels of hum through my LCD-4 headphones (200 ohms) with a Stereomour when increasing the nominal output impedance of the output transformers (i.e., 4 - 8 - 16). I cured that with DC filament heating - but I don't know if that is an option for the S.E.X.

If not, and if the hum is function of your phone's low impedance, you might consider adding impedance switches so that you can switch from speakers (e.g., 8 ohms) to headphones (e.g., 4 ohms).

Just a thought.

cheers, Derek

[mwgrient]

No, always hum/noise... the amp was a disaster when I bought/recieved it because of bad shipping. I bought it build.

the amp is wired for 4 ohms.

DC filament heating? Whats that? 

[Paul Birkeland]

Headphone sensitivity determines how important the noise floor of the amp is, not the impedance of the headphones themselves. 

All the SEX amps other than the original monoblocks of 30 years ago are DC heated. 

Can you post an overhead shot of the build? 

[mwgrient]

the overhead shot... thanks all!

[Paul Birkeland]

Everything looks to be about in its place. Is this noise present regardless of the volume pot position?  Do you have an earthed AC outlet?

[mwgrient]

"Is this noise present regardless of the volume pot position?" 
Yes.

"Do you have an earthed AC outlet?"
No. I Gould make or try this, did not think about it... thanks.

[mwgrient]

I have tested an AC earthed outlet with the amp. But there is no effect: de 'silent' hum is in stereo present.

[Paul Joppa]

Is the hum present with no input connected? You may have to turn the volume control all the way down to minimize radio-frequency interference. This is a test of ground current between source and amp, sometimes called a "ground loop". An equivalent test is to use a battery-powered source such as a mobile phone.