I decided to pull my C4000 from the rack for some PM and to see what could be done to reduce a very low, but ever-present hum in the output.

I made a couple of modifications to the ground wiring in my C4000 to accomplish a 6dB or better reduction in spike noise from the rectifier switching current-induced eddy currents.

One step was to tie the grounds on the bank of electrolytic caps together, so that the + and - supply rails would both be referencing the ground at the same point.

Another modification was re-dressing all the AC wiring to/from the transformer. I twisted all the AC wiring and shortened the leads and re-soldered them to their connections.

I also shortened another pair of unshielded wires which, when touched, introduced HF noise into the outputs. I twisted this pair to give it some immunity to induced noise pickup.

The other, more substantial step, was to tied the power supply ground directly over to the input board ground at the point where the connection to the chassis ground thumbscrew is located, but through a few milliohms of resistance. Tying directly simply inverts the phase of the noise pulses. Since this required ultra-low resistance, I used an 2mH RF inductor as a source of resistance. It provided just the perfect amount of reversed phase pulse injection to cancel the rectifier switching pulses that appear on the output at about 2mV p-p. It is now down under 1mV p-p.

In the real world, the small hum, audible on my highly efficient pro audio loudspeaker array, is now reduced to inaudibility at the listening position. The hum from the overhead fluorescents is the loudest thing in the room now, when the system is idling.

One of the things that pushed me to make this modification was the fact that I am trying to clean up the noise floor on the inputs when the gain is way up to play my new Ultimate Fireworks Video, which has over 85dB of dynamic aperture. Ambient sound on the video was being competed with by hum and hiss from the preamp and it's power supply's ground loops. It's audibly quieter now, in that respect.