22 hours ago, kevin gilmore said:

the resistors are just there to make sure its stable with nothing plugged into it.

change the input resistors to as low as possible compatible with what is going to drive it.

10k is probably what i would do.

Thanks, Kevin.

At the moment I only had 50kΩ resistors on hand. After replacing the input resistors with 50kΩ, the background noise is almost completely gone. There's still a very slight hiss/current noise, so I may also try adding a small pF-range capacitor to ground for some additional RF filtering.

I also noticed something a bit strange with my layout. On my board, installing the 470pF capacitor causes the output voltage to rise very close to the raw DC rail, whereas using a 5pF capacitor doesn't cause that behavior. I'm not sure if it's layout-related or if I've made a mistake somewhere.

Has anyone else run into this issue with their build?

the 470pf cap is on the power supply right?

the reason for the 1 meg input caps is to give a minimum stable bias for the input stage. its expecting you to put in front of it a pot, or other much lower impedance resistor.

at 50k the remaining noise is far below any possible music at that level that does not significantly lower the input impedance.

Edited Saturday at 02:37 PM3 days by kevin gilmore

8 hours ago, kevin gilmore said:

the 470pf cap is on the power supply right?

Yes, it’s the C6 470pF capacitor in the power supply. I originally installed a 5pF capacitor instead of the specified 470pF.

After changing it to 470pF, the output voltage rises very close to the raw DC input. My raw DC is about 698V, while Vout increases to around 675V, instead of regulating to the expected 600V.

A small update on the Super Carbon debugging.

Today I tried shorting IN+, IN−, and GND together at the input. The background noise was reduced, but there is still a very slight residual hiss that I can't completely eliminate. It's extremely low that you can only hear it if you put your ear right up against the headphone. Because of that, I'm starting to think the input stage may not actually be the source of the remaining noise.

I'm wondering if it's possible that the compensation around the C3M0900170 devices is involved instead. Specifically, I'm referring to the 5pF capacitors C8 and C9 connected around Q10 and Q90. Could insufficient compensation there contribute to this kind of residual noise?

@kevin gilmore , I'd really appreciate your thoughts before I continue experimenting.

nothing with 60db of gain is going to be completely noise free. unless you want to move to a much colder climate. and even then maybe a couple of extra db of s/n. without a volume control in front of the input stage you have no idea how loud, loud is. that residual noise is undetectable when playing music.

you can try say 10pf instead of 5pf. anything higher and the frequency response will change.

you can also try a cap something in the range of .1 to 1 uf ceramic soldered to the switcher power supply outputs.