Noisy sex

[Doc B.]

I'm sorry that I don't understand exactly what you think stinks. My point is simply that without hearing the amp ourselves we are limited in our ability to help by referring to the measurements that have been posted. If the amp was on my bench I could listen for myself with a familiar set of headphones, compare the noise floor to that of another S.E.X. amp as a point of reference, and possibly come up with some more ideas. As it is we are doing our best to help with something we can't experience first hand. Do try what PB is suggesting as that might give a cleaner picture of the noise floor in the hum frequency range.

[RW]

@PB
The only setting that I was able to find was bandwidth limiting, which only limits to 20Mhz. I couldn't find anything in the manual, or the google regarding HF reject, or reduction. The photo shows the measurement of the probe with the ground clip connected to the lead, and the BW limiting engaged.

@Doc
what I meant by this kinda stinks, is that I/we worked a lot on this issue and that I may be stuck with this hum. It wasn't meant to be in any way offensive. As I mentioned above, If all else fails, I would be more than happy to send you the amp and pay all fees associated with the service. Your team, and this forum have been very helpful to me in all of my BH builds, and I appreciate everyone and all of their efforts. I apologize if my statement came off in a negitive way

[Paul Joppa]

This is a long shot, but easy to check:

In the very first post, RW said "-tried adding 100 ohm resistors in C1 & C2, to C3". The purpose of this is to keep the DC heater supply centered at ground potential. It assumes that C3 is grounded to the chassis, by way of the adjacent power transformer mounting screw, through the end bell, to the mounting screw nearest the power inlet, and from there to the safety ground adjacent to the power inlet. You can check the resistance from C3 to the chassis, which should be essentially zero. While you are there, power transformer terminal 10 (the heater winding center tap) is grounded to terminal 3, bolted to the chassis. Check that one as well, it should also be zero resistance (within the ability of your meter of course).

It's possible these checks were made; I did not review the entire thread today to check.

[RW]

Hey PJ,
This may be the issue, or at least part of it. That terminal isn't grounded properly. I don't have any continuity from ground to the #3 lug. The only thing I can see that would be in the way is the fiber washers on either side of the plate, possibly? Because my plate is powder coated, I have the powder ground down to bare metal in all of the terminal, tube socket, and all screw locations. Should I remove the plastic washers and replace them with something else? Or, should I run a jumper from 3L to chassis ground?

[Paul Birkeland]

It grounds through the power transformer cover, which may also have powder coating on both sides? 

If so, you can crank down on that mounting hardware and that may help your cause.

[RW]

The bottom of the cover is ground down, but not the top. Should I pop it off and grind the top?

[Paul Joppa]

The most reliable thing IMHO would be delete the wire from terminal 3 to power transformer terminal 10, add a wire from power transformer terminal 10 to C3, and another wire from there to the safety ground adjacent to the power inlet.

Explanation: the ground through the power transformer end plate was not an intended design feature - nothing is connected to C3 in the stock configuration. If it were there would be a star washer between the C3 mounting screw head and the top of the end plate. To my mind that route is circuitous as well as sensitive to paint, thus unreliable. So I suggest (above) doing it with wire and solder.

Historical note: The SEX amp was originally designed to be a speaker amp. Before release it was decided to incorporate a headphone option (this was some 12 years ago) and we discovered it needed some work on hum. We added a power supply filter section for the driver stage, beefed up the main PSU capacitors, added a DC heater supply, and implemented the IHF standard 120 ohm source impedance resistors (since removed for unrelated reasons). IIRC we took the hum on low-impedance phones down around 20dB. This was our first foray into headphones.

[RW]

Hey PJ,
So I just got done rewiring. It definitely helped a bunch. I didn't have time to test with all of my headphones, valves, or take measurements to see where I'm at, yet. My Q701s which are my least sensitive, I can only hear the hum on the HI setting. My HD650s I can hear it on ML, MH, and Hi. The real sensitive headphones I can hear it on Low, but barely. It's still prominent on all other settings. It made an improvement, from initial testing. I'm still waiting on the 10r resistors to add to the headphone jack. I'll do some more in depth testing tomorrow after work, and report back.
Thanks Paul!

Btw, I totally dig the history of BH. I've read a bunch of old articles and interviews from magazines, And  while doing research, I still find BH posts on message boards and forums from the early 2000's. It's pretty nifty to see how BH has evolved over time!

[Doc B.]

Yup, I was the first commercial sponsor/forum host of Audio Asylum, in 1997 IIRC. And on the Joelist for a few years before that. There's archives of both around, I think. That was baby boomer joelist which you found by searching with Alta Vista. Google search #1 for Joe List is now a gen X comedian. Time flies.

[Paul Joppa]

Hey PJ,
So I just got done rewiring. It definitely helped a bunch. ...

That's great - it means that at least some of the hum was coming from the heater power. You are probably down around 0.3mVrms of hum now, which is normal - the amp is operating as designed.

There are more things that could be done, of course. However, when searching for noise solutions you must be aware of the iceberg effect - whenever you remove a cause of noise, you expose the next-most-significant one. (I spent a career in aircraft noise control...) There are no tested approved mods to further reduce the hum, so any such attempt becomes a research project, with likely more failures than successes - that's how research works. Sometimes it's a grand adventure, sometimes it's just frustrating and annoying.