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noisy relays

Discussion in 'Electronic Basics' started by tempus fugit, Nov 24, 2005.

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  1. tempus fugit

    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
     
  2. 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.
     
  3. Jamie

    Jamie Guest

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

    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
     
  5. Bob Monsen

    Bob Monsen Guest

    These work well for lots of things, but have a large typical R(on) of
    250 ohms at room temperatures with Vdd=5V. For higher voltages, the
    R(on) is smaller, getting down to 80 ohms for a 15V Vdd. If he is
    stringing 5 of them together, he may have an issue with attenuation of the
    signal.

    (This is from the MC4066B datasheet from ON semi)

    --- Regards,
    Bob Monsen

    The question of the ultimate foundations and the ultimate meaning of
    mathematics remains open; we do not know in what direction it will find its
    final solution or even whether a final objective answer can be expected at
    all. "Mathematizing" may well be a creative activity of man, like language
    or music, of primary originality, whose historical decisions defy complete
    objective rationalization.
    - Hermann Weyl in 1944
     
  6. tempus fugit

    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


     
  7. John - kd5yi

    John - kd5yi Guest


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

    Cheers,
    John
     
  8. tempus fugit

    tempus fugit Guest

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

    Thanks
     
  9. John - kd5yi

    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
     
  10. tempus fugit

    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
     
  11. tempus fugit

    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
     
  12. Guest

    Did the pop sound the same as each contact opened and closed, or were
    some of them louder?
     
  13. tempus fugit

    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.
     
  14. tempus fugit

    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
     
  15. That pretty well rules out my guess.
     
  16. tempus fugit

    tempus fugit Guest

    Well, actually, maybe not. I did some testing again today, and there is a
    noticeable improvement. Sometimes the switching was dead quiet, and overall
    there were fewer pops, but some switching was pretty loud still.

    Perhaps the contacts are an issue, but I think this was definitely
    contributing to the overall noise.

    Thanks again for your help.
     
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