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Old 12th June 2017, 09:36 PM   #1
russc is offline russc  England
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Default Interference from telephone base

I have spent quite a while puzzling over hum pick-up when listening to turntable.
Initially blamed turntable grounding.
After reading a post suggesting to check nearby equipment as a source of the hum I investigated.
Turns out my telephone base (landline with removable handset) was the source. Rotating the base in 3 axes causes the hum to come and go.
Moving my hand near the base likewise - moving a few cm goes from loud to silent. Moving base to opposite side of room made no difference.
Short of moving the base to another room, any suggestions? Base is powered by wall SMPS so not transformer hum. Unplugging base from power and the hum totally disappears.
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Old 12th June 2017, 10:40 PM   #2
JMFahey is online now JMFahey  Argentina
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That´s annoying.
*Just in case* noise is some SMPS emission, test your phone base powered from a conventional supply.

If it comes from the base, you will unfortunately have to move it farther away or maybe you can make some shield (kitchen aluminum paper glued to some wood and grounded).
Here experiment beats calculation so .... hands on and try different ideas

You might also try to shield the turntable itself, or at least its arm or pickup.
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Old 12th June 2017, 11:03 PM   #3
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Quote:
Originally Posted by JMFahey View Post
You might also try to shield the turntable itself, or at least its arm or pickup.
The turntable should be grounded too.
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Old 12th June 2017, 11:32 PM   #4
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some cordless phones are known to create interference.
panasonic are the main culprit i believe.
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Old 13th June 2017, 10:35 AM   #5
DF96 is offline DF96  England
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Maybe your phono input needs an RF filter. Alternatively, can you try a different PSU for the phone base?
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Old 13th June 2017, 07:01 PM   #6
russc is offline russc  England
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I'll experiment some more when I get the time.
Worst case I'll run an extension upstairs and swap the master/slave bases.
The slave base doesn't seem to interfere.
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Old 13th June 2017, 07:06 PM   #7
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Can't you put far away both telephone base and turntable? It seems to be the easiest way to avoid interference. EMI decreases with the distance squared.
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Old 13th June 2017, 08:05 PM   #8
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There was recently a similar discussion on a Dutch forum. Apparently DECT telephones and base stations emit bursts that repeat at a frequency that sounds like hum. The RF signal can get picked up by wiring and demodulated by any even-order non-linearity.

You can try shielding the phono amplifier and putting a ferrite ring around the incoming cable, as a common mode choke. The trick is to find a ferrite ring that works at DECT frequencies.
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Old 14th June 2017, 08:23 PM   #9
russc is offline russc  England
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I can get the interference when putting the base at the opposite end of the room 4 to 5 meters away. I have a 'scope. This could be my first real use of it - viewing the signal from the phono output. See the shape and frequency. Probably this weekend I'll take a look.
Funnily enough it's a Philips base.
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Old 14th June 2017, 08:29 PM   #10
DF96 is offline DF96  England
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I had a Philips DVD player which interfered with nearby AM receivers. It was worse when switched 'off' (but still plugged in to the mains). I took it back to the shop, described the symptoms and got a refund. I noticed that the shop assistant wrote "faulty" on the form as the reason for return. I bought a Sony and that was a lot better. Both items were made in China so I can only assume that Sony insist on better EMC performance than Philips.
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