Electronics

tempus fugit · Guest

Hey all;

I have a switching unit for my guitar effects that uses relays to switch the
signal in and out. When I first built this thing a couple years ago, the
switching was fairly quiet - the little pops were almost inaudible. Lately,
though, the pops have been getting louder and louder. Are my relay contacts
getting worn already? Is there some remedy for this problem (other than
replacing the relays)?

I had also thought of replacing the relay with some sort of solid state
switch. My only concern is that in bypass mode, the signal passes through 6
relays in total. With the higher on resistance of a SS switch, I'm worried
that the signal will suffer some serious degradation. Any suggestions here?
Are there any super low on resistance SS switches around?

Thanks

Jamie · Guest

look at something like a 4066 chip or equivalent.
just bridge them together via a latter network.

--
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

John Popelish · Guest

I doubt the relay contacts are the problem, but are probably just
reacting to the problem. I would be looking for a DC bias that is
being switched, with the audio riding on it. You might have a leaky
coupling capacitor. I think each contact should have a high value
grounding resistor on each side of the contact to precharge the
coupling capacitors to 0 volts DC long before any of them open or close.

tempus fugit · Guest

I'll check the ins and outs of the fx pedals for DC. The odd thing is that
it doesn't really matter which combination of fx I switch in or out, I still
get the pop, although I didn't do a comprehensive test of each available
combination. I'll check for DC first though before trying the resistor
trick.

Thanks

Bob Monsen · Guest

tempus fugit · Guest

Yes, that was kind of my concern about using the SS switches (although they
sure are cool). Unless there are some super R(on) models out there. I'd
still like to stay with the relays, as implementing the switches will take a
fair amount of altering the existing circuit.

Thanks

John - kd5yi · Guest

Signal switching may require gold-flashed relay contacts. Are yours
gold-flashed?

Cheers,
John

tempus fugit · Guest

no, don't think so. Are these much quieter than normal ones?

Thanks

John - kd5yi · Guest

As I understand it, some other contact alloys will oxidize unless there is
sufficient voltage and current to get through the oxidizing layer that
forms. Cadmium is a metal I remember, but I don't remember if it is
particularly troublesome. Maybe it is the "wiping action" on an oxidized
contact surface you are hearing. Maybe not.

The point is, they make gold-flashed relay contacts for a purpose. I was
told it was for very low signal levels. I have successfully used these
contacts in applications where the signal was in the
millivolts/microamperes.

Look at the detailed specs for your relay. You should see a voltage/current
*minimum* specified. If not, contact (no pun) the manufacturer and find
out.

Good luck.

John

tempus fugit · Guest

Hi John;

I checked the inputs and outputs of each effect, and had varying results.
Some gave a dead 0VDC, while others started at around 1 mV and dropped to 0,
or dropeed to a few 10ths of a mV. Still others just read in the 10ths of
mV. I had one that peaked at 24mV but then died down to 0.2 mV.

Are these readings close enough to 0V, or does it sound like a leaky cap in
there somewhere?

BTW, I tested each effect individually, with no signal applied.

Thanks

tempus fugit · Guest

I checked the DC levels of the relays themselves, and did indeed find a few
millivolts (sometimes more) on each one (this is from the signal contacts of
the relays). Where would this be coming from? There are no caps (except in
the effects pedals themselves), so could it be from the control voltage part
of the relay? Also, in this case, should I still drop a hi value resistor to
ground (from the signal contacts)?
Thanks

Guest · Posted: Sun Dec 04, 2005 9:35 am Post subject: Re: noisy relays

tempus fugit · Guest

Hi John

I didnt actually listen while I was testing, (I had removed each effect from
the chain to test) so I don't know. I know that some sound louder than
others in normal use, but the switching system may be turning off more than
one relay at a time, which would account for the louder pops.

Thanks again.

tempus fugit · Guest

Hi John;

I tried putting in the resistors as you suggested, but it made little or no
difference. I no longer measure any DC on the relays, but the pops are still
there. I connected the resistors about 18" from the relays (on the input and
output jacks going to each of the effects) because it was a little easier to
get to. I don't see how this would be any different than connecting them
right at the relays, but I thought I'd mention it just in case.

Thanks

John Popelish · Guest

That pretty well rules out my guess.

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