220V/50Hz to 110V/60Hz hum problem

Thread Starter

jdschaa

Joined Jan 3, 2019
4
I've been using US audio equipment in Singapore (220V/50Hz) using a 2000W step up/down transformer like this, and have not had any issues besides an occasional light shock when touching metal parts.

I recently added a turntable which requires 110V/60Hz to run at the correct speed, so i got this 200W travel converter which transforms frequency as well.

When i plug the turntable into the 2000W converter, there is only a barely audible hum, but it runs 15% slow. With the travel converter, the speed is correct, but there is a noticeable hum which shifts (seemingly spontaneously) between somewhat annoying and loud. It sounds like a ground loop hum but is still present even when i plug the power amplifier into the same travel converter as the turntable.

The turntable (an Elac Miracord 50H) has no ground for either the audio or power, but i did try running a wire to the ground terminal of the phono preamp (which is on the regular 220V circuit) and it didn't alleviate the hum.

Could this be a ground loop problem, or could the 60Hz output of the travel converter somehow be 'dirty'?

Thanks a lot for any help or insights.
 

DickCappels

Joined Aug 21, 2008
6,003
From you last comment, it sounds like you should concentrate on what is different between the wiring/shield/grounding of the two channels.

For example, what if the shield on the left channel from the turntable to the amplifier were not connected -that problem would be in the cable (can check with an ohmmeter) or the equipment at either end of the cable.

Certainly the frequency changer is part of the problem. Most likely it is putting out a"dirty" signal with a lot of harmonics that are easier to pick up than a clean 60 Hz sine wave. In that case getting a frequency converter that has a cleaner output would probably help.
 

Thread Starter

jdschaa

Joined Jan 3, 2019
4
Thanks, the culprit is indeed the dirty power from the frequency converter. Grounding checks out fine, but the audio leads connect to a muting switch close to the mains cable. The distance of the left channel's terminal (black) to the mains is about an inch, and the right (white) is an inch further away. I tried waving a brass plate between the terminal and the mains, and it reduced the hum by a good bit.

Any easy DIY solutions for shielding or cleaning up the power?
 

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DickCappels

Joined Aug 21, 2008
6,003
Congratulations! You've found the problem or at least a significant part of it. You might find braided shielding but you might find it easier to buy a cheap microphone or other shielded audio gadget and use a little bit of its shielded wire.

Be careful not to create a safety hazard by making it possible for un-insulated shielding to touch a power line conductor. Aluminum foil is a notoriously disappointing shield material for most uses since it oxidizes easily.

I hope somebody with more experience with hum can suggest some other DIY solutions.
 

Thread Starter

jdschaa

Joined Jan 3, 2019
4
I took the easy route and got a pure sine wave DC -> 110V 50/60Hz power inverter, which can be had for $50 or less these days. Problem solved!

I addition to the hum, it seems that dirty power may reduce a motor's lifespan, although i don't know if that would apply to the Papst hysteresis motor here.
 
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