Noisy sex
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Paul Joppa
Moderator
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.
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.
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!
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!
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
Moderator
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.RW said:
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.
