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Hunting down studio monitor hum

Ok final sanity check, I took one monitor down to the kitchen and plugged it in, obviously without any input connections. Sadly, the hum was still there. So, unless shorting the RCA works, I think @Tony L is right, unfortunately.
 
You are getting mainly odd harmonics.
That is most likely to be rectified noise being picked up and amplified by the circuit. If shorting the rca cures it, I'll bet it'll be internal wire dressing as a quick google seems to show it's a common problem.
As @awkwardbydesign said, a quick and dirty test is to plug an rca lead into it and short the outer sleeve to centre pin at the opposite end. Make sure the screwdriver touches the sleeve first and slide the tip onto the pin. If you touch the pin first, you'll know about it...;)
 
f shorting the rca cures it, I'll bet it'll be internal wire dressing as a quick google seems to show it's a common problem.

Could you please share the Google search that you did so I can read some more? I must have been using the wrong queries.

In any case, thanks for all the advice everyone. I'll update here once I've had a chance to try shorting the RCA.
 
Thanks, that's from the "lesser" Alpha monitor range rather than the Shape range that I have, but it's close enough in description to suggest that they're related anyway.
 
Could you connect a chassis screw on the monitors to a chassis screw on your source/pre? I'm wondering if earth related?

I know active subs* sometimes hum if not earthed properly (*in particular Rel connected via hi-level).
 
Thanks to the generosity of @Neil P , I have borrowed some RCA shorting plugs.

I powered off monitors and monitor controller ("preamp"), unplugged the XLR interconnect from one, and installed the shorting plug. Upon powering up the monitor alone (I didn't bother with unplugging all other devices since we have established that it must be internal if anything). Unfortunately, the hum is still there.

Did I do that right? Is there anything else I can try with the shorting plugs, other (safe/reasonable) combinations?
 
Could you connect a chassis screw on the monitors to a chassis screw on your source/pre? I'm wondering if earth related?

I know active subs* sometimes hum if not earthed properly (*in particular Rel connected via hi-level).
I haven't tried anything like that yet, thanks.
 
'Unfortunately, the hum is still there.'
Bu99er. Sort of suggests it isn't signal input wiring picking up stray field from charging currents. Doesn't mean it isn't general internal wiring though. You may have to take the plate off and have a look. MAKE SURE THE IEC LEAD IS REMOVED BEFORE-HAND. Then take some internal pictures.
You can test @Neil P's theory if you have a voltmeter. Put it into AC mode and probe from the RCA earth to one of the case screws / backplate. If you get a few volts AC that could cause the issue. I also wonder how the design converts balanced to single-ended? Is there a service manual?
 
I think at this point since they are still under warranty (I think), my next step should be reaching out to the vendor for support before opening up the back panel. Although I'm loathe to send them off for repair since I use them every day!

If they're not under warranty there's certainly no harm in opening them up and having a look around. I'll report back when I do. I somehow don't have a voltmeter yet so I'll either borrow or buy one (I have a young son who's into electronics so it wouldn't be a bad investment by any means).

Thanks again for the advice everyone.
 


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