I got some shorting plugs for the input. The background hum drops from 120hz to 60hz with the shorting plugs and...the FM station is still present if you turn the power amp up to 50% (which is way higher than even loud listening volume). It's a good data point to know it's still an FM tuner w/ the inputs shorted.
The hum is quiet but present at listening volume. I ordered a ground loop breaker to try as well as the shielding internal wiring.
The hum is quiet but present at listening volume. I ordered a ground loop breaker to try as well as the shielding internal wiring.
You want to end up with two connections to chassis. The PE from the IEC connector needs its own dedicated (no other connections) chassis connection. The other would ideally include the single connection from signal "ground" to chassis, and termination of the various internal shields to chassis. All other connections to signal ground should be with twisted pairs (preferably within a shield), including B+ and its return.
Does that power supply use switched regulation? If so, it's also a significant source of noise and can contribute to RFI by sensitizing the active stages. If you can't get acceptable performance with optimum wiring and grounding practices, you may need to try to shield the power supply. But, first things first.
All good fortune,
Chris
Does that power supply use switched regulation? If so, it's also a significant source of noise and can contribute to RFI by sensitizing the active stages. If you can't get acceptable performance with optimum wiring and grounding practices, you may need to try to shield the power supply. But, first things first.
All good fortune,
Chris
Thanks for the continued advice Chris. I moved the star ground from the IEC to the chassis. At sane volumes the 120hz noise is pretty quiet now and radio seems gone at least with inputs connected. Hopefully shielding the internal wiring will be enough to be good enough.
Here's some more info on the power supply. It was designed to power stages like this EAR clone. It was made available as kits to people in the Lenco forum and eventually packaged as a commercial product. The website seems defunct now but the designer is still a regular on the Lenco group.
From her: If you have been following the epic ear thread, you may know that I have designed an improved PSU stabilising the B+ and supplying dual mono outputs, regulating and common mode isolating the 6.3V along with many component choices that are known to improve the sound like diode snubbers and polycarbonate caps."
dtnd5KW.jpg (1239×672) (imgur.com)
Here's some more info on the power supply. It was designed to power stages like this EAR clone. It was made available as kits to people in the Lenco forum and eventually packaged as a commercial product. The website seems defunct now but the designer is still a regular on the Lenco group.
From her: If you have been following the epic ear thread, you may know that I have designed an improved PSU stabilising the B+ and supplying dual mono outputs, regulating and common mode isolating the 6.3V along with many component choices that are known to improve the sound like diode snubbers and polycarbonate caps."
dtnd5KW.jpg (1239×672) (imgur.com)
Excellent! That's a conventional "linear" power supply, except better executed than usual. I was worried, seeing the ferrite core choke and fancy capacitors, that it might have switching regulators, which are fine, but can radiate more noise than conventional "linear" regulators (because at their higher frequency of operation they're better transmitting antennas).
B- can be brought to the PCB along with both the B+ 's, twisted into a three wire bundle. If you had a factory wire with three internal conductors plus shield, you could use that, but an unshielded twisted threesome will probably be fine. Do not also connect B- to chassis directly - ground loop.
All good fortune,
Chris
B- can be brought to the PCB along with both the B+ 's, twisted into a three wire bundle. If you had a factory wire with three internal conductors plus shield, you could use that, but an unshielded twisted threesome will probably be fine. Do not also connect B- to chassis directly - ground loop.
All good fortune,
Chris