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Help with hum in parafeed headphone amplifier

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I recently completed a build of a Parafeed headphone amplifier, and have some hum that I would like to try and resolve.
There is a fairly long thread where I explored various options for a 5842 based amps but ended up with a 6C45Π similar to the Bottlehead Mainline and/or Papa Rusa.

The circuit is pretty much the Papa Rusa, with the exception of using the DN2540N5 cascade CCS from SY's ImPasse/HMN designs, and paying some more attention to the AC heater wiring such as elevating to 50V above ground, using a 1nF ceramic cap to chassis and tying each node though a ~100ohm resistor.
The PSU is 220uF - 5H - 220uF. There is a 1kΩ + 1nF snubbers for the transformer secondary.

There is low (but audible) hum when there is no signal. Based on FFT, it is 60Hz and 180Hz (with lesser of 120Hz and 300Hz)
When there is signal, even fairly low amplitude, the 60Hz drops, so its not audible when music is playing, but I'd like to try and get it to a completely quiet background.
I also looked at it with a scope with a 10khz square wave and there doesn't seem to be any oscillation. The tilt of the sq. wave aligns with the FR measurement from ARTA (HF boost), but I am fairly novice when it comes to interpreting this.

The attachments are FFT at 0 output, 5mW and 100mW, and FR at 0 output and 5mW output.

Any ideas where to look?
I'm also curious to understand why the 60Hz component drops when there is signal.

Thanks!
 

Attachments

  • 6C45 - 300ohm - 0mW.png
    6C45 - 300ohm - 0mW.png
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  • 6C45 - 300ohm - 5mW.png
    6C45 - 300ohm - 5mW.png
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  • 6C45 - 300ohm - 100mW.png
    6C45 - 300ohm - 100mW.png
    18.2 KB · Views: 52
  • 6C45 - 300ohm - FR - 0mW.png
    6C45 - 300ohm - FR - 0mW.png
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  • 6C45 - 300ohm - FR - 5mW.png
    6C45 - 300ohm - FR - 5mW.png
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  • 2023-06-29 15.23.05 (edited).jpg
    2023-06-29 15.23.05 (edited).jpg
    575.5 KB · Views: 65
  • JX2gTExY.jpeg
    JX2gTExY.jpeg
    355.2 KB · Views: 74
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