120hz hum issue

[karl hungus]

Hello

I’ve been cutting more lately and have noticed a floor of 120hz hum
creeping into my cuts on a presto 8dg.

I had terrible dirty ground noise and I cleaned it up with an Equitech, dedicated outlet, and better cable management.

But now with the garbage out I can hear 120 mostly continuously, but it occasionally ebbs and returns. I don’t actually hear much 60hz hum in it, oddly.

Does anyone know what might be causing this and how to eliminate it?

Thanks!

[mratx]

Likely a rectified power supply issue or related to the ground plane on a power supply. What is your set-up in terms of interconnected powered items? Does your power amp or preamp have any 120 hum when they're not connected to anything else? If so, there's some issue with their power supply. If not, try hooking up one item at a time to your power amp (preamp, lathe, whatever else you're using) and see when it reappears.

Mark

[karl hungus]

Thanks for the suggestion, it made me realize that despite having my amps off, I didn't disconnected them from the speaker terminals on my lathe.

Regrettably the 120hz is still there with no audio device connected. It increases and decreases in volume sometimes which makes me think its something mechanical perhaps.

[discosdecorte]

Hi Karl,
Read this, could be useful: https://www.lathetrolls.com/viewtopic.php?t=8779

Best regards,
Fer

[mratx]

If it was mechanical, you should hear it if the amp volume is turned down (like if the transformer was humming.) Are there any electrical items nearby that could be causing an induced hum, like fluorescent lights, dimmers, etc? Since you have an equitech and dedicated outlet it shouldn't be coming through the power line. Do you have a different power amp that you could swap in and see if it also hums? Or move the power amp you're using to another location and see if it hums there? If it only hums in the one place (and especially if another power amp used in the same place hums) some other device is inducing the hum in the area (like a dimmer.) If it hums no matter where you move it and a swapped in amp doesn't hum I think it's the amp power supply.

Mark

[karl hungus]

Thanks for the suggestions Mark.
Unfortunately I don’t think it’s the amp as it still hums when the amps are powered off and disconnected from my head.

I cut a groove with the heater wires and audio leads of the head disconnected, and it was still present. The unheated stylus was noisier in surface noise, but looking at the spectrogram you can still see a hump at 120.

Also I found that the pitch changes if I cut 33.3rpm.
It’s starting to point to being a mechanical resonance of some sort.