Passive A/B Switcher in relay form works but pops :-(

[simon111]

I have built the following in relay form..... (e.g. the 3pdt is replaced with a DPDT - as no lighting is needed)

http://www.fulltone.com/PDfFiles/AB_switcher.pdf

It works perfectly except for one thing. When switching it pops.

One channel e.g. B goes to a tuner and thus mutes the output of channel A this fine and causes very little to no pop.

But when switching back from the tuner to the main output e.g. Output B, it pops loudly. I have a rangemaster before it, and when bypassing this the pop isn't half as loud. I have added 1meg drop down resistors to the RM as I suspected that , but that doesn't seem to have helped much. ( i have no leds or lighting on this circuit so it's not that which is causing the noise)

I am thinking it's the relay, i.e. One side of the DPDT is switching faster than the other? could it be this?

Any ideas as to solve this problem?

Cheers in advance.
Simon111

[R.G.]

Relays have a built-in popping mechanism that is missing from stomp switches. The sudden change in voltage on the coil can couple to the signal contacts capacitively.

There are two ways around this.
(1) redesign the relay so the signal contacts are shielded from the coil. Some commercial relays are actually built this way.
(2) slow down the sudden voltage change on the coil so that it cannot couple effectively through the very small capacitances between the coil and the contacts.  See "Relays for audio switching" 28 December 2001 at GEO, http://www.geofex.com.

As a practical matter, you can either put a BFC (Big Freaking Capacitor) across the coil or otherwise slow down the voltage drive signals.

[simon111]

Thanks RG for your info.

I am using your 'slow down' transistor circuit for triggering the relay. So it's a fairly slow turn on and turn off.

I have just tried putting a 470uF cap across the coil. This doesn't seem to make much difference I'm afraid.

BTW not sure if this helps, but when I disconnect (break the track) Between one of the cross over wires (as show on the PDF above) it doesn't pop. This means that when I don't get any signal going to the tuner, it is pop free. I.e. When I press the switch and the relay diverst the signal from the tuner and to the amp it is fine. But when this track is connected (i.e goes to tuner) it pops. Could it be the tuner that is making matters worse?

Simon111

[R.G.]

Could it be the tuner that is making matters worse?

Yes. It is possible that there is a DC voltage coming out of your tuner. You could measure that with your digital multimeter... 

[simon111]

AHA! Perhaps getting closer?

When I disconnect the input to the tuner ......NO pops. 

Does this mean there is a DC voltage in the circuit?
What point do I measure with my multimeter?

Finally,  any work arounds for this. I tried putting a big cap across the input to the tuner, this stopped the pops ( I got all excited by that fact, then my excitement turned to annoyance when...) as you probably know it seemed to stop the signal getting to the tuner. So the tuner doesn't actually pick up the signal DOH!

[simon111]

Sorted! 

Thanks RG for getting me on the right path.

A bit of experimentation and a 330pF capacitor later and I now have a virtually silent tuner/mute switch.

Excellent. There's something good in finding a solution yourself. I added a 330pF inline with the signal to the tuner input. I started at 10uF and worked down, and found that the 330pF was as quiet as I could get. Below 330pF seemed to make little difference.

I don't quite know WHY this helped although I suspect the cap blocks DC voltages in this way. Can anyone verify this conclusion?

But thanks again RG. If you were UK based I am sure with all the people that you have helped on this very forum we could get your Knighthood or something.

Bow Sir RG!!

Cheers.
Simon111

[roknjohn]

Try placing a diode across the relay coil. The cathode or band on the diode should connect to the positive side of the coil.

May time switching the inrush to energize the coil puts a spike on the voltage supply. The diode can many time eliminate a large portion of that spike.

[roknjohn]

Ever have one of those days when the words just don't come out right. Yeah.....thats the way my typing is going today!!