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Old 12th Jan 2022, 11:02 pm   #1
Gabe001
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Default Hum on gram. Why?

I have come across this quite a bit.

Radio on 'gram' input, wander plugs in the pickup sockets, and the radio hums. Connect your device eg a Bluetooth receiver, turn the Bluetooth receiver on, and the hum stops immediately. Same when I plug in my phone.

What causes this phenomenon?
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Old 12th Jan 2022, 11:42 pm   #2
kalee20
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Default Re: Hum on gram. Why?

Hum pick-up with long wires, going to a high-impedance amplifier input?

And when power is switched on, for your Bluetooth adapter/phone/whatever, its output stage becomes active, goes low-impedance, and effectively shorts the hum pick-up out, at the remote end?
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Old 13th Jan 2022, 12:10 pm   #3
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Default Re: Hum on gram. Why?

I think you're right. It's definitely made worse by plugging in the wiring.
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Old 13th Jan 2022, 1:28 pm   #4
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Default Re: Hum on gram. Why?

perhaps a simple terminating resistor is all that's required between gram and earth sockets, something the bluetooth module is happy driving
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Old 13th Jan 2022, 1:35 pm   #5
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Default Re: Hum on gram. Why?

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Originally Posted by McMurdo View Post
perhaps a simple terminating resistor is all that's required between gram and earth sockets, something the bluetooth module is happy driving
Yes. If the PU input is separate from the radio, (ie switched) then try something like a 220R resistor across the socket. Kills most of the hum and the low impedance bluetooth unit will not even notice it.
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Old 13th Jan 2022, 7:42 pm   #6
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Default Re: Hum on gram. Why?

Across the socket meaning from signal to ground? Won't that end up in parallel with the grid leak resistor of the triode?
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Old 13th Jan 2022, 8:40 pm   #7
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Default Re: Hum on gram. Why?

Signal to ground yes, assuming of course that one part of the socket is grounded. There should be a capacitor in series with the signal path, so it shouldn't be in parallel with the grid. If there is a direct connection, then add such a capacitor. Anything between 10n and 100n will be fine.
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Old 13th Jan 2022, 9:23 pm   #8
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Default Re: Hum on gram. Why?

Depending on the GRAM switching arrangements, a low impedance load on the GRAM input could upset the radio detector signal, capacitor or not.
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Old 14th Jan 2022, 10:01 am   #9
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Default Re: Hum on gram. Why?

You mention that when wander plugs are connected you get a hum. Is the lead screened?
Is the lead just O/C at it's end, if not what have you got connected to it? Slightly confused. John.
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Old 14th Jan 2022, 5:44 pm   #10
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Default Re: Hum on gram. Why?

I wouldn't have thought a resistor would upset the input any more than plugging an old pickup into it.
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Old 14th Jan 2022, 6:23 pm   #11
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Default Re: Hum on gram. Why?

It wouldn't but leaving it plugged in when not in use, might, depending on whether the GRAM select switching isolates the PU input when radio is selected.
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Old 14th Jan 2022, 6:25 pm   #12
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Default Re: Hum on gram. Why?

I certainly haven't come across any issues, although it obviously could depend on the circuit. I would give it a try and see if things improve.
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Old 14th Jan 2022, 7:36 pm   #13
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Default Re: Hum on gram. Why?

Quote:
Originally Posted by Heatercathodeshort View Post
You mention that when wander plugs are connected you get a hum. Is the lead screened?
Is the lead just O/C at it's end, if not what have you got connected to it? Slightly confused. John.

For clarity:

Here a picture of the arrangement. The radio in this picture is a phonola (Italian Philips), just being used as an example. It's a little firecracker of a set with a pp el84 output stage and an ecc81 before that, and a seperate gram setting. From memory I think the gram input setting isolates the pickup from the rest of the radio

So the Bluetooth module is connected as per picture. With the Bluetooth module switched off, there's a lot of hum
Turning it on completely stops the hum. The project box by the way just contains 2 summing resistors to convert stereo to mono.

It's not a major problem as the pickup by definition need something connected to it and switched on to play through it, I was just curious about the mechanism. As others have said, it's probably just rubbish being picked up by the cabling which gets shorted out when the module is turned on. I'm sure the resistor mentioned will sort it too.

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Old 15th Jan 2022, 12:24 am   #14
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Default Re: Hum on gram. Why?

If there is a proper 'gram' position on a switch, it should be OK.

Quite a few radios had pick-up input sockets and no switching, the signal was fed crudely to the diode detector circuit so you had to tune to a quiet bit of the dial... and the detected AF appeared on the pick-up sockets when a station was received. Shorting the sockets, or putting a low-impedance load there, would have clobbered the radio signal.
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Old 15th Jan 2022, 10:07 am   #15
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Default Re: Hum on gram. Why?

The leads connected to the radio do not appear to be screened and this may well be the reason you are picking up hum etc. The pick up terminals have a relatively high impedance. You can test this by wrapping foil around the leads and that connector and earthing it to the radio chassis. John.
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