Power off pop even using speaker relay - diyAudio
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Old 20th March 2013, 08:37 AM   #1
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Default Power off pop even using speaker relay

I'm using a chip amp LM1875 with a protection relay to the speakers.

Relay wired to the amplifier in the NC terminals & ground in NO terminals, powered by a separate 12v DC wall wart through a switch on the front of case.

When I switch the amp off using the relays switch there is a slight 'POP'.
If I pull out the wall warts connector from the rear of the amp, no 'POP'.
So I'm thinking that the relay is sound, but the switch is causing the trouble.

I would appreciate any thoughts, I have seen reference to RC networks across the switch contacts, but I'm unsure how to calculate the values needed.
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Old 21st March 2013, 03:31 AM   #2
djoffe is offline djoffe  United States
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do you have a snubber diode across the relay?

If not, the act of opening the switch-relaycoil-powersupply circuit generates a very high voltage that radiates like crazy.

A reverse biased diode across the relay coil will stop that problem.


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Old 21st March 2013, 05:47 AM   #3
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Location: Dona paula, Goa
Looks like the wall wart switch is in the AC path and the filter cap V takes time to die out. Reduce the filter capacitor value in the wall wart to the minimum possible.

Gajanan Phadte
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Old 21st March 2013, 08:31 AM   #4
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Thanks for the advice djoffe!

I tried your suggestion this morning with an ultrafast recovery rectifier diode I had spare from a PSU project (200V, 1.5A rated) and it works an absolute treat

Absolutely no noise on relay power off and on, I really appreciate your advice, made even better by the fact I had something lying around
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Old 21st March 2013, 10:37 AM   #5
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
a 1n4001, or a 1n4148, or a 1n914 all work well in absorbing the back emf of low voltage relay coils (upto 24Vdc)
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Old 21st March 2013, 11:56 AM   #6
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That means the pops were due to back emf of the relay coil.

Gajanan Phadte
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