Power off pop even using speaker relay

[redal]

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.

 

[djoffe]

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.


Akitika GT-101

Update My Dynaco

 

[gmphadte]

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

 

[redal]

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

 

[AndrewT]

a 1n4001, or a 1n4148, or a 1n914 all work well in absorbing the back emf of low voltage relay coils (upto 24Vdc)

 

[gmphadte]

That means the pops were due to back emf of the relay coil.

Gajanan Phadte