Relay switching thump

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JAY X

Well-known member
Joined
Jan 9, 2009
Messages
683
Hi !

In my last audio project i have used  small relays TQ2-24V  (2880 ohms coil), for switching audio signals.

I use the typical snubber: one 4148 diode across  the coil, in parallel with a 1uf ceramic capacitor.

The relay is powered from the same PSU rail used to power the audio circuits (+17.5v) BUT, with a dedicated separated ground return to the psu gnd.

The switches I use to switch power to the relays are ALPS SPUJ DPDT.

With all these ingredients the transient THUMP is still quite noticeable.  In other projects the Thump was not as loud... ¿maybe because i used a 28v rail and a resistor to reduce the voltage within the margin of the 24v relay?hmmm....

Any comment is wellcome!

Thank you for your help!

Jay x


 
You might be looking in the wrong place. Are all switched terminals at the same potential or grounded?
 
What signals are you feeding into the relays?
Are they AC coupled? 
Have you tried shorting the inputs to the relays to ground, then switching?
I include a picture of what I think you should be doing
 

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Hi!

Thanks for your comments, i think i will post a schematic...

One relay is for mono summing L&R channels via 10k resistors. When not engaged, the contact is open, without a path to gnd.

Jay x
 
You really need a local charge source to prevent switching currents in the 0V line. At Neve we fed the relay supply via a 100 ohm series resistor and a 100uF capacitor to 0V.

Cheers

Ian
 
JAY X said:
Thanks for your comments, i think i will post a schematic...

One relay is for mono summing L&R channels via 10k resistors. When not engaged, the contact is open, without a path to gnd.
It's very likely that an accurate schematic would reveal the issue. The sort of thing that would cause a thump like that is a DC coupling cap that doesn't have a drain resistor so leakage causes it to charge up when the relay is open. When the relay is closed, that DC discharges into the downstream circuit (aka you get a "thump").
 

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