It's encouraging that there's tolerable hum at minimum volume; that suggests that there's no objectional hum in the loop between attenuator ground, "wiper" tap and any following line stage.
Does shorting line inputs have any effect on hum? Is the hum same on all line inputs?
Try disconnecting input wire at the attenuator input so that there's no source-path wiring in play. If the hum show's no improvement, that would seem to point to that nearby rectifier circuit.
Remember to try to minimize "loop area" of your wiring--- i.e. try to twist wiring signal and ground when possible, which limits loop area. This applies to both AC wiring (transmit antenna) and signal path (receive antenna).
Good luck!
Shorting one of the 4 inputs at the RCA jack does NOT change things at all.
I'll try disconnecting the input at the attenuator and see what happens.
If I could narrow the problem down to the input wiring I'd be happy to replace all that wire with twisted pair.
I do have very large output caps for this circuit. I believe I used an Obliggato 4.7 film cap that has a metal case as that is what I had on my shelf.
Does anyone suspect the SMPS driving the attenuators?
I'll try disconnecting the input at the attenuator and see what happens.
If I could narrow the problem down to the input wiring I'd be happy to replace all that wire with twisted pair.
I do have very large output caps for this circuit. I believe I used an Obliggato 4.7 film cap that has a metal case as that is what I had on my shelf.
Does anyone suspect the SMPS driving the attenuators?
What is the DC resistance from the output of the attenuator to ground with it at low mid and high? What is the DC resistance between the signal input and the attenuator output at max?
What you are describing is usually capacitively coupled hum into a high impedance circuit . Since the hum is gone with the volume all the way down I would like to know the impedances involved. You may need shielded cables from attenuator to amp input.
Another common issue is the higher Z on the input of the amp couples with capacitive coupling to the output can cause oscillation which manifests as hum. Often just touching the input changes or stop the oscillation. Shielding/isolation and sometime just a small cap to ground fixes it.
What you are describing is usually capacitively coupled hum into a high impedance circuit . Since the hum is gone with the volume all the way down I would like to know the impedances involved. You may need shielded cables from attenuator to amp input.
Another common issue is the higher Z on the input of the amp couples with capacitive coupling to the output can cause oscillation which manifests as hum. Often just touching the input changes or stop the oscillation. Shielding/isolation and sometime just a small cap to ground fixes it.
OK. Disconnected the attenuator inputs as first test - no change. I cobbled together a test set-up. Latino PP ST120 amp that is quiet enough on its own and a pair of Altec 414 drivers in free air. Hum is easily heard and measured with REW spectrum analysis.
I BELIEVE I have found the problem. The Khozmo attenuator bodies need to be grounded separately from the audio signal it seems. There is NO indication on the attenuators that needs to be done. What stimulated me to try that is I replaced the brass volume shafts with Garolite ones thinking I might be creating a ground loop or similar as moving the volume knobs often changed the hum. Well that made the problem much worse. I then touched a grounded probe between the flexible coupler and the chassis - and lo and behold, dramatic decrease in volume!
I then moved the jumpers to a threaded hole (unmarked) on the attenuators and tried grounding to the chassis - good reduction and then to the audio bus and next to silence was the result!!!
I'm going to add those jumpers in permanently, rehook up the inputs and give it another spin.
Wish me luck 🤞
Pre and post below.
Thanks for all the help! You have been invaluable.
I BELIEVE I have found the problem. The Khozmo attenuator bodies need to be grounded separately from the audio signal it seems. There is NO indication on the attenuators that needs to be done. What stimulated me to try that is I replaced the brass volume shafts with Garolite ones thinking I might be creating a ground loop or similar as moving the volume knobs often changed the hum. Well that made the problem much worse. I then touched a grounded probe between the flexible coupler and the chassis - and lo and behold, dramatic decrease in volume!
I then moved the jumpers to a threaded hole (unmarked) on the attenuators and tried grounding to the chassis - good reduction and then to the audio bus and next to silence was the result!!!
I'm going to add those jumpers in permanently, rehook up the inputs and give it another spin.
Wish me luck 🤞
Pre and post below.
Thanks for all the help! You have been invaluable.
