Hum (60hz) only with Headphones

[EnsgnofIndustry]

S.E.X. 3.0 just built. Amp set up to run at 4ohms. Passed all tests as best as I can tell. Listening through 8ohm (15 watt) bookshelf speakers and the amp sounds insane! Dead silent in both channels! No pops, crackles, hum, or other aberrancies from 0-10 on the volume dial. 

My issue is with headphones. Regardless the headphone model or headphone cable, I get the same low level hum, which I believe to be 60hz.
A couple observations with headphones:
I can kind of drown the hum out when playing music loud.
I get the hum with nothing plugged in but the phones.
The hum is present with the volume knob at zero but changes in tone (kind of like a wah pedal) when I roll up the volume knob.
The hum remains present but very gradually goes away when I power down the amp.
As mentioned in a previous post, I had to buy a CAT5e cable online but the specs seem to match the one included with the kit.
Oh, and I slightly melted the sleeves of a brown wire and brown and white wire together near the sleeve and ring connection area of the headphone jack (side closest to the volume knob). I couldn't tell whether the copper actually touched and it's a pain to get to with the green wires obscuring them. Also, didn't want to investigate and make things worse, but if that's the likely culprit, I'll address it.

Thoughts? And as always, I appreciate the support.


 

[Paul Birkeland]

Are terminals 29 or 36 touching the #8 nuts next to them that hold the power transformer on? This kind of issue generally happens when terminal 36 is in contact with that nut, which creates a ground loop.

[EnsgnofIndustry]

Thanks Paul. I was worried about the potential T36 and PT nut contact when building. It's a tight fit. I just double checked T29 and T36 and they aren't touching the nuts. 36 is close but I put a piece of electrical tape between the nut and T36 and still get the hum with headphones. Again, speakers are whisper quiet. Pics attached and I'm happy to take more.

 

[Paul Birkeland]

You can also try running a jumper wire between the two lowest lugs on the headphone jack, which some have reported as helpful.   It could also help to know which headphones you're using, as some slight modifications may not be a bad idea depending on what you're running.

[EnsgnofIndustry]

I’ll give that a go. I’ve also read elsewhere that the speaker jack soldering in particular needs to be on point, so if the jumper thing doesn’t work, I may just resolver the speaker jack connections. As for headphones I’ve tried, I’ve used Meze 109 Pro, B&W I forget the model, Audio Technica ath50, and VModa something-or-others that are like 10 years old. All have different cables and the frequency and loudness of the hum doesn’t change regardless of the headphones. Does the quality of the hum changing as the volume knob is adjusted offer any additional insights?
Speaker jack soldering pic attached just in case something obvious sticks out.

[Doc B.]

The changing of the hum could mean that part of it is coming from ahead of the S.E.X. kit. For example cables could be picking up some noise or there could be a ground loop issue between your source and the S.E.X. amp. Try removing the cables from the input jacks of the S.E.X. amp and see if the hum reduces.

[EnsgnofIndustry]

Thanks Doc. Hum persists at the same volume when all inputs are removed and when I unplug the speaker cables. The tone of the hum does change as I roll the volume even when only the power cable and headphones are plugged into the amp. Hum equal in both ears. Again, there is 0 hum when running the system through my speakers. And despite the hum the audio sounds perfect through headphones. Just need to crank the amp past 9 o’clock to drown out the hum.

I’m happy to resolder and reevaluate the build; just wondering what I can rule out and what may need more attn. Does the hum occurring only with headphones (irrespective the type of phones or type of cable) and not with the speakers rule anything out?

Again, I’m happy to do further diagnostics and get you resistance or voltage values. Just say the word. I feel I’m so close to having the perfect headphone amp!

[Natural Sound]

Doc B. and P.B. If I may share an observation. It looks like the top plate has been anodized and there is no provision for grinding down to bare metal at the grounding points. That cant be helping.

[EnsgnofIndustry]

Thanks for the insight. Yeah, I did anodize the entire plate but I sanded the key underside grounding spots that PB highlighted in a previous post.  I hit them with sand paper until I hit bare metal but I still wonder whether I didn’t sand them enough. Also, if it were the anodizing causing the noise, wouldn’t I also get hum when running through speaker outs? Again, speakers a totally silent.

[EnsgnofIndustry]

I just double checked the sanding I did around the volume pot. It’s definitely down to bare metal.
Oh, and not sure it matters but I get zero hum from the headphone out when the tubes are removed.

[Paul Joppa]

Aha! The output transformers and/or plate chokes are picking up magnetic fields from something nearby. I've seen that happen, even loud enough to hear on speakers. It's kind of unnerving  :^)

[EnsgnofIndustry]

I'll bring the amp to work and see if it's less noisy.
PB, I soldered a jumper to the sleeves but no change. I saw in another post there is a mod to take down the noise floor involving resistors at the headphone jack. Is that the next step if I just have a noisy house?

[Jdjung]

I just bought the amp from a good builder and also got a constant noise that sounds like a ground loop pass around the 1 p.m. position, it's very very low but did increase in volume as I pass the 1 o'clock position (not very loud at all but still barely audibly present), where the top of the knob (around 50% position as you go up in volume is 12 o'clock position so noise came in at about 60% to 70% volume). I installed the C4S after testing everything and am very happy with the amp for my Micca RB42 (desktop setup) speakers in near field configuration sitting at about 1 to 1.5 meters.  (When I have time I'll run a REW and put in a convolution filter to my Roon App). No hum from my speakers, it's dead quiet.

Then I did some testing and this is what I found. When I turned my audio interface (Antelope Audio Zen Q) volume all the way down I got that very very slight hum with volume all the way up. Now the weird part, I paused the music. After I paused the music, I waited approximately five seconds or so and the hum disappeared. My system is as follows.

Desktop PC running Roon with headroom management enabled (this lowers the volume by -3db), to Antelope Audio Zen Q through USB C, then 1/4 inch unbalanced monitor out to RCA, to my Senheisser HD6XX and also tested with my Dan Clark Aeon 2 Noire (hum a bit louder on the Dan Clarks, but again when I pause the music and wait five seconds, no hum). My audio interface has an output voltage of 4 volts, and I believe the Bottlehead clips at 4.6 volts, according to the Youtube from Korea (who I watched and decided to get this amp, I'm Korean btw.)

Additionally, I get hum from the following, the Astell and Kern SR35, but the hum is from a cable that I got off Amazon, which is a 3.5 to RCA. The hum is noticeable at the 50% volume level but changes volume based on how close I am to interferance sources so I know it's not the amp, but the cable. FYI, always get cables that have good shielding! I was having a heck of a time with my record player with hum, till I switched cables.

You say you get hum even with no inputs, which I don't get using those headphones above.

Now for kicks, I decided to use my Crinacle Red IEM's which impedance and sensitivity are not going to match with this amp. I get hum as well even with music paused (though the primary hum goes away), I hear a very noticeable hum, buzz and even me touching the amp is audible. However, you're NOT supposed to use most IEM's with this amp. What headphones are you using?

[Paul Birkeland]

.When I turned my audio interface (Antelope Audio Zen Q) volume all the way down I g

I would try this mod:
https://forum.bottlehead.com/index.php?topic=11676.0

[Jdjung]

Thanks! I'll try it out, but honestly, it's really only audible at high levels with no sound coming through. I hardly have my headphones even close to the volume where it becomes barely audible.