Persistent low level hum on build

[aokman]

Hi All,

Great to be here, just finished my first SEX build and everything is working fine except... hum! It is a persistent low level background hum that doesn't change with volume. Both channels have a similar level of hum.

So I have done quite a bit troubleshooting and fiddling trying not to waste anyones time on here, but I seem to have tracked it down to the output transformers... I have run around re-soldering joints, checking voltages, nudging wires around etc before I arrived at them, I was showing a 50hz hum on the scope briefly at the OT outputs with no volume but it was hard to isolate and track consistently. I took the right channel output transformer completely out of circuit with just the outputs still connected to headphones and the hum still persists. Removed the OT completely from the chassis and tried rotating it near the power supply transformer and the hum can get louder or quieter depending on orientation / coupling. I confirmed a good ground connection to the cases of the OT's and power transformer when mounted, not sure if that could potentially cause problems (not an expert on this stuff). Hum is the same with the tubes removed also.

The OT's seem highly sensitive units for sure as even sitting on my work bench it picks up a solid hum from my soldering station at a decent distance. I have confirmed the winding wiring and also tried 8 ohm just for the hell of it with no luck.

A couple of points, Australian voltage is quite high these days (250v) so slightly above the rated spec in the build manual, the power transformer does run very warm, too warm to touch for more than a few seconds after hours of running, idle power consumption is about 49W. Also using Klipsch HP-3 headphones (25 ohm) which may be quite sensitive... Would you consider some hum on the SEX normal or am I simply expecting too much from this build? My only previous experience is the Darkvoice which doesn't have OT's but honestly sounds better in the current state unfortunately as I listen to a lot of quiet classical music where the background hum comes in and out of the mix at lower volumes...

Thanks all, any advice you can offer would be great or modifications to improve if possible

I have attached a build image also, PS ignore the black Mogami shielded wires going to the OT's as I suspected induced noise initially in those unshielded wires originally when I saw 50hz at the OT outputs but not at the tubes, the hum was there before changing the stock wiring...

[Paul Birkeland]

One thing I've run into is that the front two nuts on the power transformer could possibly touch the ground lugs on the 8 lug terminal strip in front of it, and if they do you'll get some hum that shouldn't be there.

The cabling you've used on terminals 5 and 10 of the output transformer could certainly also be causing issues.

[aokman]

One thing I've run into is that the front two nuts on the power transformer could possibly touch the ground lugs on the 8 lug terminal strip in front of it, and if they do you'll get some hum that shouldn't be there.

The cabling you've used on terminals 5 and 10 of the output transformer could certainly also be causing issues.

Thanks for the reply I will swap that wiring back to stock but the hum was present prior already was just trying some things to see if there was any improvement, trying to limiting how many times I solder and unsolder to minimise wear and tear on the tie points. Yes I was confused about the setup of the 2 "isolated" terminal strips being grounded in circuit anyway at terminal 36 and 23. 36 isn't touching currently but ill reseat it just incase.

Likewise I can take an OT out of circuit with no input wires connected and tack on a new set of output wires to a spare headphone jack and the hum is still there

[Paul Birkeland]

Likewise I can take an OT out of circuit with no input wires connected and tack on a new set of output wires to a spare headphone jack and the hum is still there

I would not remove transformers as part of the debugging process. Yes, you can make the output transformers pick up magnetic hum depending on where you locate them, that's not particularly abnormal.  Noise measurements of a properly working sex amp are about 12dB below what I would consider very quiet for a headphone amp.

It's completely possible that the hardware on the power transformer isn't all the way tight, and perhaps what you're hearing is the transformer itself vibrating a bit and it just needs to be tightened down.  With no tubes installed in the amp if you are still hearing hum, this is perhaps one of the only possibilities remaining. If the hardware on the power transformer isn't properly installed (specifically the black fiber washers and the nylon shoulder washers), that can cause some strange problems as well.

[Mucker]

Hello,

I just finished building my S3X and came across an issue when mounting the Chokes and the Output transformers. By following the instructions to the tee, I was still ending up with continuity between the plate and the chokes. IMO, the provided mounting washers could be a little thicker to separate the chokes from the plate. The transformers had just enough clearance, but the chokes were touching after properly mounting.

I ended up putting a piece of nylon zip tie (3/16 x 3/4 .... 4 pieces, 2 per choke) between the chokes and the plate and then tightening down. I then measured for continuity and made sure the chokes and output transformers were insulated from the plate. I didn't have any plastic washers and that is why I went the zip tie route. Anyway, it worked well.

It might be worth having a look at the clearance between the chokes / transformers and plate. It could be a big benefit to mention this in the manaul IMO.

I really focused on making sure there was no continuity at the places where all of those insulating washers were installed, including the RCA's. Terminal 36 was another problem point that I had to fix but found it before wiring started.

It must have paid off because I ended up with a dead quiet amplifier all the way through the volume control.

Hope this helps ...

[aokman]

Hi all,

Just an update post on how things are progressing, took a break after getting burnt out on this and frustrated. Came back to it today and spent a lot more time investigating so here's what I have found so far...

The hum must to be coming from the magnetic interaction of the PT and OT's, disconnecting all the wiring on the PT outputs results in the exact same background hum still. Removing the PT from the chassis completely and powering it separately / bringing it near or away from the OT's reduces or increases the hum accordingly. Probably not relevant but I also measured 50vAC between the outside transformer laminates and the mains input with the transformer sitting on the bench.

I decided to test the unit running on 120vAC isolated step down and rewiring the PT accordingly and the hum seemed to reduce but hard to do back to back comparisons for a definitive answer... Keeping in mind Australian mains voltages are pretty hot these days around 250v.

Finally, following another thread about hum recently, Paul suggested 8.2ohm and 2.2ohm resistors at the headphone jack for another user with sensitive headphones (32ohm). My headphones are 25ohm and I cannot say the 8.2ohm resistors helped that bypass the headphone switch however the 2.2ohm resistors in parallel with the OT outputs have made a massive difference for me, the hum is still there but very faintly in the background now where its fine. Not sure if it actually affects the frequency response / detail retrieval, it seems to but probably just from the resulting volume drop and requires more drive going back and forth testing.

Will continue testing and report back with final results but its great progress finally. Thanks to Paul for the suggestion as I was lost at this point and about to give up...

[Paul Joppa]

How are you wiring the resistors? From what you wrote, I think you may have misunderstood what PB was trying to say. It's frustratingly hard to explain a circuit in words!

The 2.2 ohms should be in parallel with the headphone driver, and the 8.2 in series between the transformer output and the headphone+2.2ohms.

[aokman]

Yes thanks for that I think I misunderstood initially about moving the wiring around but redid it and think its right now, I attached an image also. FYI the 8.2ohm is only a 1W carbon currently, cannot get anything quickly but will order some higher quality resistors from RS tomorrow for the final version but 1W should suffice for my headphones anyway.

Result is amazing, dead silent output now, noise floor and hum is excellent