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 July 18Jul 18 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.

9 hours ago, kevin gilmore said:

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

Just to confirm, do you mean adding a .1 to 1 uF ceramic capacitor directly across the RKZE-1515 isolated supply output (+15V to -15V) that powers the CCS section?

yep exactly. on the back of the board as close to the output pins as possible.

On 7/18/2026 at 1:58 PM, Shawn said:

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?

Been a while since I built it, but I had the 470pF from the start and didn't notice any drift/rise in output voltage. Can have a look this weekend, was going to open it up and swap to a new trafo anyway.

On 7/23/2026 at 9:55 PM, MLA said:

Been a while since I built it, but I had the 470pF from the start and didn't notice any drift/rise in output voltage. Can have a look this weekend, was going to open it up and swap to a new trafo exactly. on the back of the board as close to the output pins as possible.

I ended up switching back to the 5pF capacitor. With 470pF, the nearby STN9360 starts behaving incorrectly, so something in my implementation clearly doesn't like that value.

On 7/22/2026 at 2:21 AM, kevin gilmore said:

yep exactly. on the back of the board as close to the output pins as possible.

I replaced the output capacitors on the switching supply with four 0.1µF/50V MLCCs, just as Kevin suggested. After doing that, the ripple on O+ and O− at around 120kHz dropped to roughly 96kHz.

At this point it seems like the tiny residual hiss may actually be related to the switching supply rather than the amplifier itself. What I'm not sure about is whether a 120kHz switching component could somehow be demodulated or otherwise make its way into the audible band.

there were no output caps on the switchers. there is a input cap, not labeled, 47 uf tantalum 30 volt is what goes in that spot. or anything else that fits.

13 hours ago, kevin gilmore said:

there were no output caps on the switchers. there is a input cap, not labeled, 47 uf tantalum 30 volt is what goes in that spot. or anything else that fits.

I checked the datasheet as well. Since I’m already feeding the converter from a GRLV, I don’t think the input side is the issue.

One interesting thing I noticed is that the datasheet for a compatible replacement part (with essentially identical specifications) recommends adding an LC output filter for noise-sensitive applications. The example value is around 0.47µF on the output.

I don’t have any 0.47µF MLCCs on hand at the moment, but I’ll probably place an order next week and give that a try to see if it helps reduce the remaining noise.

On 7/21/2026 at 9:52 AM, Shawn said:

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?

Did some testing last night on a second build I'm doing for a friend. Psus regulate and are stable with 470pf in place, at least for the 20 min I left it on (Kevins latest board with dual current sources).

Have no noise either (never had on a Carbon), but have a pot in front. Also, I'm using TTC004a rather than pzta42 as drivers for Q9/Q10.

2 hours ago, MLA said:

Did some testing last night on a second build I'm doing for a friend. Psus regulate and are stable with 470pf in place, at least for the 20 min I left it on (Kevins latest board with dual current sources).

Have no noise either (never had on a Carbon), but have a pot in front. Also, I'm using TTC004a rather than pzta42 as drivers for Q9/Q10.

Thanks for the information. It looks like 5pF is also stable on my build, although I’m using the SMD version of the power supply boards.

When you mentioned the “dual current sources”, what exactly did you mean? Are you referring to the two current source stages before and after the power supply on Kevin’s latest board?

I’m using PZTA42 for Q9/Q10 instead of TTC004A, so perhaps that’s something I should investigate next. I also have a pot at the input, just like your build.

If you have access to an oscilloscope, could you check the O+ and O− outputs? On my board there’s a roughly 120kHz sine wave (it changed to around 96kHz after I experimented with MLCCs on the RKZE outputs), and it seems to be related to the isolated DC/DC supply. I’d be interested to know whether your board shows the same behavior, even if it’s not audible.