PTT switch for Sennheiser EW wireless

[Simon Eves]

We are doing a production of NOISES OFF, and the play-within-a-play director character wanders the back of the room with a notional "wireless com headset" to put his voice in the house for the actors.

My current plan is for the actor to wear a real-but-not-plugged-into-anything single-sided Clear Com headset and also a wireless mic (Sennheiser EW100 G3 and a Countryman B3).

The tech crew for this show is minimal and not used to doing anything more than firing QLab cues, and the actor is adamant that he can be trusted to control his own muting, but not necessarily to find and correctly use the (admittedly fiddly) mute switch on the transmitter.

What might be involved in putting an XLR PTT switch in-line between the B3 and the Tx?

I'm up for the challenge of building the adapters, but what would be the connectivity required?

Any other ideas gratefully received. I will continue to argue it would be safer to just do it from the booth, even just with mutes on the very simple analog house mixer.

Simon Eves

[Bevan Archer]

If you're already using Qlab, I'm pretty sure there are a couple of different ways to have it control muting and unmuting a microphone.  I believe you could also build in "fader moves".  I'm not familiar with the show, but just as an example, maybe their mic is at -5 dB if he's just interrupting people speaking and there's no music, but if you need them to be above some running playback, the level would go to 0 dB.  You may or may not want to rely on it for "fine" level adjustments, but I'm almost positive it could handle your "gross" adjustments.

I'm sure there are 50 people on here with way more experience in exactly how to do this, but I'm 99.99% sure that it's possible.

Good luck!

Bevan Archer

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[page daniel]

Good morning Simon, etal

So you want a PTT? That of course means the microphone is only on when
the switch is actuated. Most wireless microphone switches preferred by
ministers and American football refs are latching or just a simple SPST
switch.

Sennheiser makes one with their miniture phone connectors (Sennheiser
RMS-1). They are reasonably priced. Of course Shure makes one for Ta4f
connectors. The standard for NFL football used to be an EV or Telex
bat handle toggle switch unit. They are simple, with a circuit to
remove the on and off clicks, but the prices are outrageous. I have
never seen a diagram of how to wire one for Sennheiser.

Turning a beltpack on and off is usually a pain, and there is a delay
before the receiver latches on and one gets audio.

page

[Paul Kraus]

In general, you do NOT want to interrupt the signal lead to mute a mic, but instead short it to common to mute. The reason is thumps going on and off. This all assumes the mic driving the line can tolerate it’s output being tied to common.

I would cheat. Assuming a 1k ohm input impedance on the mic input, put a 1k ohm resistor in series with the mic signal on the mic side of the switch, that will induce a 6dB loss of signal when on but not load the mic with a short.

mic —<>1kresistor<>———<tie point for switch>——— mic input to pack

switch is a simple spst connecting <tie point for switch> to the common lead. Switch ON == mic muted, switch OFF == mic ON.

This all assumes the power for the mic capsule is on a separate lead and not running on top of the audio signal. It also assumes the mic signal is centered on common and not biased on a DC potential.

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[page daniel]

paul,

I looked at Wiring to of B3 on Countryman site and at Shure's schematic
of a referee switch. Wiring for Sennheiser G series mics is two wired
with ring shorted to sleeve. So a transmitter's bias would be shorted to
ground unless a capacitor is placed on the output side of the switch.
So capacitor side value depends on input impedance of transmitter and
the desired cutoff frequency I suspect that the input impedance is
somewhere in the several 100K range so the value of the capacitor would
be in the microfarad the less than one microfarad range and low voltage.

A good example of an anti pop filter is in the shure schematic, but
Shure has the bias voltage separate from audio and notably passes audio
through an 0.12 microfarad Capacitor.

Yeah, I own referee switches for Shure and Lectrosonics, and still have
a telex that I need to convert.

page

[Paul Kraus]

Page,
Good catch, I missed that it was an EW100 with 2 conductor connector. Inline mute is more complex than I described.

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[page daniel]

Paul,

Don't known what model, but Countryman wiring for minature phone plugs
on Sennheiser are two wire, with ring on connector shorted to sleeve.
Had to look, because I do not own any Sennheiser wireless. Just their
MKE2 and MKE1 mics.

I would hate to experiment and find out shorting the bias to shield
would destroy a pack over time. Which it should not.
It is best to be kind to boxes with unknown input circuitry.

page

[Simon Eves]

Given the other live audio gags required for the show, my plan is already to use my Behringer XR12 controlled by OSC cues in QLab for the various speaker routing changes on the rotating set.

For the backstage Stage Manager character, I am arranging to borrow a table-top announcement mic with a PTT switch. That actor seems like she can be trusted to get that right, especially as it's a nice big physical switch.

The Director character is the problem, and I'm worried about timing issues and even outright failure (and a stuck-on mic) if his wireless mutes are controlled from QLab. Also, the lack of a fader to live-adjust either of their levels.

But maybe it'll be fine! :\

[Paul Kraus]

It’s Noises Off … what could possibly go wrong and *hurt* the show :-)

[page daniel]

good Evening Simon,

Looked in my box labeled Referee switch. found three units, one
lectrosonics, one shure, and one telex 4 connector lemo. Looked in
Lectrosonics..found nothing but a mute of audio input by shorting to
ground. Nice case, and yes I did not buy it used. It has connectors to
feed a Clearcom head set to mic and R1a receiver For single channel
wireless com. Made mostly of odds and ends that can be used for other
needs as well. Of course a Shure Referee switch for overnighting to
those have lost theirs and are desperate. And last but not least a
Telex yet to be rewired as needed. So I could wire it for just about
anything. The lectrosonics being so simple one could just use custom
adapters to connect to just about anything.

So if you need to borrow a switch, cost to you is shipping USPS small
box flat rate.

page

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[Russell Ault]

Late (perhaps too late) to the party, but given the challenges of muting an electret condenser mic where the bias voltage and audio are sharing the same conductor (thanks, Sennheiser!), I'd be tempted to explore using the Clear-Com headset's (presumably dynamic) microphone instead. It seems to me that adapting the mic signal out of the headset should be relatively straightforward, and adapting that mic signal into the SK 100 G3 should be relatively straightforward (just make sure that adapter includes a bias voltage blocking cap of some kind), and setting up a PTT button for a dynamic mic should be relatively straightforward, so doing all three should be...relative straightforward? Might be worth a shot (and it'd mean one less thing for the actor to forget to don pre-show).

-Russ

[page daniel]

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[page daniel]

Good evening all.

Yep Russ, I agree the dynamic mic found in old style intercom headsets
works fine, but one should probably put the normally closed switch
across the mic and feed the signal to the transmitter with a bias
blocking capacitor. Value for capacitor depends on the input impedance
of the transmitter. Try one and if you want more bass, use one larger,
or put two in parallel.

I am in the process of putting some items to work and in addition Opened
up some telex referee switches that I thought some day I would modify
for Shure with a handful of components and a schematic diagram that
shure still has posted on the web. Yeah, I do not think shorting the
bias to ground is a good idea, having no idea as to what circuitry is in
the transmitter. But with a reasonably sized capacitor in series with
the mute switch only the audio and no bias would go to ground. Keeping
bias on the microphone will probably result in much less noise if any
from occurring when switch is opened or closed. One can also modify the
shure circuit to add a few of the protections it offers.

In my possession is Lectrosonics first referee switch. It only worked
with a transmitter that had a circuit that sensed when one unused pin
was shorted to pin one. The present version by lectrosonics model
"MUTE" will work with a broad range of transmitters and has a opto
coupler in the switch that gets its power from the bias. To find out
more they provide some circuit diagrams for the "Mute" Of course
lectrosonics has a means of wiring the connector to select three
different bias voltages. Off topic but interesting lectrosonics use
bias voltage to put an FET amplifier in its guitar cable.

page

[Chet Leonard]

Suggestion #1.

never trust an actor. ( they mean well, but )

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[Simon Eves]

All good stuff, and I learned a lot, so thank you all, but in the end the director decided he didn't need the conceit of the director's disembodied voice coming from speakers, and the actor was loud enough, so we scrapped the wireless mic completely. Keep it simple.

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