Big "plop" when switching with relay - need help !

Hello,

I've just racked two old siemens preamp, and i added 48v, phase reverse, pad with relay 24V signal relays, the psu is common for powering the preamps and the relays.

When i switch the relays it makes a "plop" that can be very loud and really annoying.

Anyone a suggestion for improvement ?

Thanx,

always have a diode across the relay coil. look at G9 for reference.

i added the diodes, now it works perfectly.
Thank you jacob

what effect do the diodes have?

Yes---after the relay pulls in, and reaches a steady state, there is a given current flowing, usually determined by the supply voltage and the winding resistance.

The electromagnet that is the coil has substantial inductance. A good deal of energy is stored in that inductance, namely (L* I^2)/2. When you attempt to de-energize the relay by opening the connection to the coil, the field energy has to go somewhere---and the end you are disconnecting flys to a high voltage (of a polarity opposite to what the steady-state voltage was), looking for another path for the current. A normally-reverse-biased diode provides that path.

Without the diode, you usually cause a sufficient overvoltage transient to induce arcing of a switch contact or breakdown of a solid-state switch---and the arcing especially has a bunch of high-frequency energy that sprays all over the place and causes lots of noise injection.