Microphone switcher assistance

Hi guys,

I've rigged up a microphone output switcher. One microphone input and and A/B output.

Because I didn't have a DPDT momentary switch at the time I made it I used a DPDT relay with a momentary switch to do the switching. It works fine but I get a definite click sound.

I was using an exernal phantom power supply on the microphone which should block all the DC on the output (but I haven't actually measured this).

So... now I have a momentary DPDT stomp switch that I'm going to put in instead of the relay to see if it is less "clicky." Outside of that working, any suggestions to reduce click? Do I need to use some caps to filter or some resistors in series with - and +?

Any help would be great.

CC

I would avoid phantom mics in this application, unless they are diposables.
I recall reading some argument that convinced me switching the load on a mic while it is powered up could cause a voltage spike or some other condition that might be fatal to the mic...though with blocking caps and a cheep condenser I would not be too concerned...but no u87s please.
also avoiding pops is a challenge because of the high gain involved. a multi-pole shrting switch may be helpful instead of a relay.

There are a number of ways to do this successfully - and clicklessly - even with Phantom power present, if this is necessary.

Actually switching the output is unlikely to work quietly, what you need to do is split the microphone signal into the two paths, either with a transformer with 2 secondaries with some low value resistors in each audio leg, or just using resistors on their own.

To switch each path off, as required - simply short the audio pair (XLR pins 2 & 3) on the mixer, or output side of the network. With careful choice of resistor value - the loss will be small and the interaction minimal.

You can still use your relay contact for this, or any other single pole switch on each path.

If this is not clear - send me a PM with a real e-mail address and I will try and send you a drawing.

Interesting idea about the split. What about maintaining the switch but grounding the unused output when it is not in use?

Is this essentially what you propose along with splitting the signal? Obviously if a split and grounding the unused side would require each signal to be isolated from one another. I have never split signals using low value resistors but I think I get it.

Do I have you correct?

I just noticed ProCo has a device called a "Panic Button". It does what I want to do but at twice the price of what I could likely make it for. Does anyone have one of these they can pry open?

It is interesting. The description says it can be phantom powered from both outputs and has clickless operation. It is 9v powered so it must be relay based. I wonder how it is working?

CC

Since my original message on this subject - I was asked to design and build a microphone switcher - which has now been completed, using the basic principle already described.

It is constructed in the form of a wedge shaped pedal, with a momentary footswitch to toggle between the A+B outputs. CMOS logic latches the momentary switch, drives the LED's and the switch elements.

It is all powered by 'sniffing off' a little of the Phantom power from the mixing desk.

The audio path is essentially passive and the switching essentially clickless.

Hi !!!
May I ask if you'd share your schematics...Or a link for schematics?
I also need a silent A/B mic switch...With a A/B headphone switch too...
By advance , thx.

John,

Could you share your circuit? I'm interested in this too.

Thanks,
Glenn

Regret that I can't reply to the PM from gltech - as your IN box is full!
Send me a 'proper' e-mail via my Web site:

www.orchid-electronics.co.uk

Sorry about the full PM box John. I only have 20 messages -- guess that's the limit. I sent you an email.

Here's a block diagram of specifically what I need. I welcome anyone's advice.



As specified, the "Program" path is super critical, and I'd prefer that the preamp/console it goes to would see the same thing it would if the mic were direct wired (load/impedance/etc.) The "Monitor" path can suffer somewhat sonically if necessary.

I found this schematic for Pr*o C*o's Cou*gh Dr*op, which claims to be a noiseless mute, but obviously it would mute both paths in my application.
http://www.freeinfosociety.com/electronics/schematics/audio/coughdropmicmuteswitch.pdf (schemo at end)

Thanks to all,
-Glenn

> As specified, the "Program" path is super critical
> see the same thing it would if the mic were direct wired

Put the monitor-out and program-mute after the mike amp.

I don't see a fault-free way to switch before the mike-amp and meet all your specs. Switching after preamplification gives you a lot more slack to work with.

The Coff Drop won't give a total mute. It "shorts" a ~~150r mike with a 100uFd cap. It will pass DC (needs to, to avoid thump), be -3dB at 11Hz, -20dB at 110Hz, -40dB at 1.1KHz, and probably level-out near -60dB at 11KHz. Yes, it will turn a HACK! into a mmmp, which if brief will pass unnoticed, which is what it intends to do.

Now that you have explained what you want to do - the advice above seems to be the way to go. So have a conventional high quality Mic amp - split the output into the 2 paths required, then use muting circuitry as found in mixing desks to perform the muting. Find the articles by Douglas Self to increase your understanding of signal muting:

http://www.dself.dsl.pipex.com/ampins/mixer/mixerdes.htm#7

There is further information available on muting by this former Soundcraft designer, including Wireless World articles. Search this forum and elsewhere.

To save R&D time - for your application I would use the SSM2402 or 2412 switching IC's - designed for this very purpose, which work very well. They include the ramping circuitry essential for 'soft' switching - this  ensures 'clickless' muting and with reasonable PCB layout - 'off' really is 'off'.......

Thanks PRR and John for the reponses. I completely understand your thinking. Unfortunately, in my application I cannot use a mike amp. This is for a high-end studio situation where the "program" output is sent to the engineer and he must have complete freedom and control over the mike amp and signal chain as if direct wired.

John mentioned the Analog Devices SSM24xx switching chips. Will they work at mic level? I'm guessing you'll say no. Plus, I'm sure that would change what the studio sees in some way in regard to impedance, etc.

I assume a quiet "fade" could be done with [quality] pots instead of switches? Any ideas there?

Thanks a lot!
-Glenn