Hum varying with volume wheel

[[email protected]]

Hello everyone, I just completed my S.E.X 3.0 kit, and it sounds very good! But.. I am experiencing varying hum/ringing as I move the volume wheel (on hd600, and very present in lower impedance headphones). It is my first time doing any soldering and DIY electronics.. Could anyone help me out?

Symptoms: AMP set at 4ohms, with hum that varies with the potentiometer volume wheel. Figured it would go away after C4S. It has attenuated a bit, but far from gone.
The hum is there at the beginning (12), drops near 4, picks up and becomes louder after 6, occurs slightly differently on both channels (level of hum that is). Hum sounds high frequency, and is present without external input (from DAC). I have done resistance/voltage check, and everything is within normal range as stated in the manual.

Attached are photos, before C4S was installed. But I can upload specific areas if needed

[Paul Birkeland]

Hum sounds high frequency, and is present without external input (from DAC).

An open input will pick up noise.  If you don't have your source hooked up, this behavior is not abnormal.

[Paul Birkeland]

Also be sure the nut on the corner of the power transformer by power transformer terminal 12 isn't allowed to touch any neighboring terminals.

[[email protected]]

An open input will pick up noise.  If you don't have your source hooked up, this behavior is not abnormal.

Same humming persists after joining external DAC.

[Paul Birkeland]

If you listen to a 60Hz and 120Hz tone, which does it sound like? (or does it sound like neither)

[[email protected]]

Also be sure the nut on the corner of the power transformer by power transformer terminal 12 isn't allowed to touch any neighboring terminals.

Made sure to do that! After doing checks again, the resistance checks are fine, but the voltage on H2, H5 is 3.31, instead of 2.8~3.2. Might this be the issue?

[Paul Birkeland]

No, that would tend to suggest that your AC line voltage may be slightly high. 

Does your noise sound like a 60Hz or 120Hz tone? (or neither)

[[email protected]]

If you listen to a 60Hz and 120Hz tone, which does it sound like? (or does it sound like neither)

60 Hz is heard, but a ringing high pitched sound is also heard and more noticeable near the same areas of the volume dial (0~3, 6~12)

[Paul Birkeland]

I would start by giving this a shot:
https://forum.bottlehead.com/index.php?topic=11676.0

[[email protected]]

I would start by giving this a shot:
https://forum.bottlehead.com/index.php?topic=11676.0

Thank you for helping me out so far Mr. Birkeland! But I am sorry, as I don't think I have adequate electronics background for adding additional parts yet... I will try this if i must. I have a strong suspicion that there is some hidden fault with soldering/wiring, but am sure that the stock amp should have a black background if I just could find the defect.

Curiously I find that placing my hand on the metal chassis greatly reduces (not all of it) the high pitch ringing noise as well as 60 hz hum?! The ringing and hum is barely noticeable on hd600, but on lower impedance cans like focal clear and tago t3-01 it is immediately noticeable.

What should I check to remove the ringing? Where would I need to re-solder or check the circuit?

[Paul Birkeland]

Curiously I find that placing my hand on the metal chassis greatly reduces (not all of it) the high pitch ringing noise as well as 60 hz hum?!

I would tighten all your screws.  It's also possible that your outlet may not have a ground, which can cause some noise problems on its own.

[[email protected]]

I would tighten all your screws.  It's also possible that your outlet may not have a ground, which can cause some noise problems on its own.

I have tightened all the screws, went in and resoldered parts suspected of having cold joints or too much solder, checked the resistance and voltages but to no avail. BUT!!

Ladies and gentlemen, I have noticed that a dirty power source itself might be the problem. The amp had been connected to the same multiplug socket as my computer, but switching to another outlet, I immediately noticed black background on my senns. However, though reduced significantly, slight ringing still does occur on lower impedance headphones (kinda sucks a lot..). Now I get why audiophiles spend extra on cleaner power sources. Also, I suspect the cheap RCA cables may contribute to noise (as touching the metal part/head of the cable removes the high frequency noise).

So the route I'm taking is : get a pair of cables from a reputable brand, get a cleaner power source (maybe one from ifi?). Will update this after improvements. However, if anyone has suggestions to remove this noise, I am glad to read about it and try it out (within my ability!).

[Paul Birkeland]

I would recommend going back to the noisy outlet, then unplugging nearby devices until the noise disappears, then consider putting that device on an isolation device to keep it from polluting your powerline.

[[email protected]]

I would recommend going back to the noisy outlet, then unplugging nearby devices until the noise disappears, then consider putting that device on an isolation device to keep it from polluting your powerline.

I'm actually planning to just use the S.E.X. on another outlet, as it seems wise to use audio products apart from any other electronic. But I should check as you recommended what electronic is causing noise (probably pc) and isolate it.

[[email protected]]

I would recommend going back to the noisy outlet, then unplugging nearby devices until the noise disappears, then consider putting that device on an isolation device to keep it from polluting your powerline.

I got a product that checks whether an outlet has been grounded, which solved grounding (I think), and I have isolated the amp to a single outlet, and I am not hearing the high frequency noise anymore. So the high pitched noise probably had to do with improper grounding.
However, I still hear a faint 60Hz hum with my grados (sr325x), with additional noise up in the volume dial from 7. Is there a way to figure this out? In some internet literature I found out that powerline transformers may create DC current instead of fully AC current, which can be a cause of hum?. I am wondering if I should get a DC blocker, or is there a fault with the AMP somewhere?