Noiseless momentary switch

Hi,
I want to build a momentary switch (normally off) box to go inbetween my vocal microphone and the board for live performances. I was experimenting with a spst switch and I put the switch on pin 2, while pin 1 and 3 go straight through the box, but this produced a nasty popping at the moment when the switch was engaged or dis-engaged. I expected this to happen, but I can't find any schematics anywhere that have a noiseless switching momentary circuit in them. Maybe a high value resistor on pin 2 to ground? I don't know...
-Mike

phantom volts ?

what if you were to just short pins 2 and 3 and kill the signal from the Mic
leave the mic channel open

the trouble is that ANY signal on the line will get amplified by the gain stage

does the desk have an insert point ?
a simple mute here may work better

You are rather up against it when switching really low-level signals---if there is d.c. present, especially so.

Kev's idea of shorting across pins 2 and 3 is a pretty good one, although there might be a tiny disturbance from the change in impedance, and if there is a differential voltage present. Also an active-circuit mic's output may be unhappy with a short across.

If you used a fairly big cap with a resistor in series between 2 and 3, and then switched across the resistor, it would work in some situations even when there was a phantom-power-developed potential difference between pins 2 and 3. The cap charges to the d.c. difference so when the swtich shorts it doesn't change the d.c. potentials (much). However, like the direct short across, some mics might have output stages that would become unstable (i.e., oscillate) with a cap across. A couple of small R's between the mic output and the switch would probably fix this. Thus we are left with a U pad with a switch across the middle R, which has a cap in series. We get a little attentuation in the bargain.

The problem with interrupting both output lines and terminating them is one of timing, as well as making the transition to a equivalent "dummy" circuit seamlessly.

Another thing to remember: if there is significant signal present when you are switching you will get some popping regardless, due to the abrupt change, what some call Fourier noise. This is reduced but not eliminated by switching at zero-crossings of the signal, hardly practical for a manual metal-to-metal switch.

We had to make some PTT (push-to-talk) switches for use by announcers at football games. The mics in question are low-Z dynamic headset microphones. We tried a NC (normally closed) pushbutton shorting together pins 2 and 3, which worked well except for the pop :roll:. A small cap across the switch solved the problem. We used .001uF, which has a high enough reactance at 20kHz to not cause an audible rolloff in a 150-200 ohm microphone circuit.

However, as bcarso says, shorting the output of a condenser mic is not a great idea. I agree that inserting a pad before the switch would be the way to go in that case (and just live with the attenuation). Interrupting the signal after the preamp would be even better; how about a remote controlled switch that you could plug into the channel insert on your board?

So the phantom power is the big culprit here? The fourier noise makes sense too...

I won't be using condenser microphones with the mute switch, just dynamic mics, but there will most likely be phantom power, so I'll have to try some resistors. The idea behind the box was so that I could use it anywhere regardless if the board has an insert or not, because I play some pretty trashy venues and I don't know what I'll be dealing with.

Of course, the idea of the insert mute switch is a good one... maybe i'll make one of those too, just incase its available.

The general idea here was to use an SM57 with the mute switch and distortion + eq that unfortunately induce feedback in addition to the vocal sound I want. If I'm only on when im singing, theres less feedback, but it still might not be enough control to stop the mic from feeding back even when im singing.

When I saw Sparklehorse a while back they had a really hi-gain distorted vocal mic sound, which they dealt with by having the soundman ride the fader the entire show. Unfortunately no soundman I encounter is going to know my songs, so I have to take matters into my own hands.

-Mike

http://www.epanorama.net/links/audiocircuits.html#micaccessories
this ought to do it try the top link, pg.2 has a schematic
it's basically like kev said, shorts 2-3 togeter through a resistor and a cap.

sleeper

A simple circuit that works extremely well - including with Phantom powered microphones - consists of a 47K resistor and a 220uF/16v Bipolar capacitor connected in series - across XLR pins 2 & 3.

To mute silently, completely click free - a normal switch contact is used to short circuit the 47K resistor only - leaving the capacitor effectively shorting the audio signal.

To increase the mute attenuation I have connected a 120 ohm resistor in each of the audio legs (before the mute arrangement). Match them accurately to preserve the best CMMR. This doesn't cause noticeable loss of the audio signal from the microphone.

If you use a DPDT switch (or footswitch for stage use perhaps) - the spare pole can be used to illuminate an LED.

If this isn't clear - PM me - and I will send you a drawing.

JG

Hey guys,
Thanks for all the help...
I changed out the parts, and it works a lot better now. Hopefully ill get a chance this weekend to see if i can actually handle using it...
-Mike

hi to all....is there a drawing of the switch mentioned further above by new york dave ?????