microphone 'kill switch'

[Nottnick]

My son has a drum practice room (heavilly soundproofed thank goodness). As
part of the set-up for band rehearsal they use 2 microphones.
I want to install a 'kill switch' near his feet so he can cut them out if
they start to feed-back (he has blown several amps and speakers because of
it!!).

I shall have XLR inputs / outputs for both microphones + the switch and I
have no problems wiring this up, but I wonder if there is a common output
from the microphones so I can use just one switch for both? I know a 4 pole
2 way switch would be best, but the simple cheap foot switch I have is not
this sophisticated and is just right for the job in every other way.

Would it work with the shielding? Or will I have to use 2 switches (not as
efficient or reliable).

 

[Dan Hollands]

How about driving a small relay with the foot switch to get the number of
poles you need.

Dan


--
Dan Hollands
1120 S Creek Dr
Webster NY 14580
585-872-2606
[email protected]
www.QuickScoreRace.com

 

[Nottnick]

Hi Dan

I think I understand the theory - I'm sure it would solve the problem, but
could possibly give me a bit more in the way of specifics.

I do have a small relay, but how do I connect it all up?

What goes where?

If you can spare the time to educate - I'll get busy with the soldering
iron.

Nick

 

[ALBERT C. GOOD JR.]

Dear nottnick

I would also warn about the shield. Do not switch it or break it except to
insert the switch/relays.

I agree on the relay, but add that a compressor that starts to kick in
automatically when the feed back starts might be more convenient. Such a
device most likely would be found at a fair sized music store that caters to
rock musicians.

Other ways to get around this problem are:
1. Equalize the entire system.
2. Place the speakers in a way that the sound goes to the pretend audience
instead to the musicians.
3. Turn the gain down a little. As much sound as you are describing can be
ruinous to ones hearing and in some instances can even cause rupture of the
blood vessels in the eyes.
4. Make certain that the speakers of a sufficient rating and type to
withstand the power fed to them. Home entertainment speakers are not
usually up to the same abuse as given to musical instrument speakers.

Albert

 

[Nottnick]

---
Why not kill the speakers with relays? That way the high impedance
microphone wiring (and the shield) stays cherry and you only have to
interrupt one low-impedance wire per speaker to kill the feedback.

One footswitch to disconnect the speakers and another (or perhaps
better yet, a handswitch) to reconnect them.

Want a schematic?

Yes Please John - it would be a great help.

We have tried equalising etc. but the room is small and the gain needs to be
high so guitars / vocals can be heard over drums.

I have been advised that compressors etc aren't always up t the job and
actually - I do think a 'kill switch' to be the best solution.

Nick

Bye the way I have a relay (52 ohm) and I have a transformer giving me 5
volts. Would they be compatible? It would save on batteries. If not - what
do you suggest for a relay (to utilise my transformer).

Thanks

Nick

 

[Nottnick]

---
Why not kill the speakers with relays? That way the high impedance
microphone wiring (and the shield) stays cherry and you only have to
interrupt one low-impedance wire per speaker to kill the feedback.

One footswitch to disconnect the speakers and another (or perhaps
better yet, a handswitch) to reconnect them.

Want a schematic?

Oh, the transformer is rated at 500ma

 

[Nottnick]

Yes Please John - it would be a great help.

OK. View in Courier:

For a basic latch and if there's only one speaker involved you can
use the following if you have a double-pole single-throw
normally-open relay:

+-----------------+
| ___ | +--------<FROM AMP
+--O O----+--+ | |
MAINS>----+ | NO | | O | O--->TO SPKR
| | | O--> | O--> |
P||S [COIL]- - -|- - - -|
R||E |
I||C |
| | |
MAINS>----+ | ___ |
+--O O----+
NC

To operate it, the NO (normally open) switch is pressed momentarily,
causing current to flow through the coil of the relay and the NO
(normally open) switch, causing the relay contacts to close. When
that happens current will flow into the relay coil through one set
of the relay contacts as well as through the NO switch, with the
result that when the NO switch is released the relay will remain
energized. The other set of contacts is used to connect one of the
wires from the amp to the speaker, (the other wire being connected
normally) and as long as the relay remains latched the amp will be
able to send signal to the speaker. To disconnect the speaker, the
NC switch is pressed, breaking the circuit to the coil of the relay,
which will stop the flow of current in the coil and force the
contacts to disconnect.

If you have a relay with a 5 volt coil, I suspect that's 5 volts DC,
and you won't be able to use that with the AC coming from the
transformer, so you'll either have to get an AC relay or rectify,
smooth, and regulate the output from the transformer in order to
make DC to drive the relay.

Instead of doing that, however, what I'd do would be to start from
scratch with a design using components selected for the job at hand.

Assuming that there will be more than one speaker and more than one
amp, that the speaker impedances (regardless of how they're
connected) won't be less than four ohms, and that any amp won't be
required to put out more than 100 watts means that the relay
contacts can can be rated for a current of 5 amperes. (Or greater)

I'd use relays with coil voltages of 12VDC since they're pretty
cheap and the wall-warts to drive them with are, too. Make sure
that the wall-wart's rated to supply the current for all the relays
in parallel.

For contact configurations I'd go with DPSTNO because that's all you
need, but DPDT may be easier to find or, conceivably, cheaper.

Finally, for a multi-amp, multi-speaker rig, I'd do something like
this:

+-----+
MAINS>--|~ +12|---------+--------+--+
| | | | |
MAINS>--|~ -12|-+ | O | |
+-----+ | -->| NO | |
WALL | SET | O---+ | |
WART | | | | |
| +---|---|----|--|-------+
| | | | O | |
| | | O--> | O--> | |
| | [COIL]- - -|- - - -| |
| | | O |
| +---|----------------|--+
| | LATCH RELAY |
| |O |
| NC |<-- |
| |O RESET |
| | |
+-------+ +----------+
| |
| | +--------------<FROM AMP1
| | |
| | | +--------->TO SPKR1
| | | |
| | | | +------<FROM AMP2
| | | | |
| +-------+ | | | +->TO SPKR2
| | | | | | |
| | +---|---|----|--|----|--+
| | | | | O | O |
| | | | O--> | O--> | |
| | | [COIL]- - -|- - - -| |
| | | | |
| | +---|-------------------+
| | | SPEAKER RELAY 1
+-------------|-------+
. .
. .
. .
| | +-------------<FROM AMPn-1
| | |
| | | +-------->TO SPKRn-1
| | | |
| | | | +------<FROM AMPn
| | | | |
| +-------+ | | | +->TO SPKRn
| | | | | |
| +---|---|----|--|----|--+
| | | | O | O |
| | | O--> | O--> | |
| | [COIL]- - -|- - - -| |
| | | |
| +---|-------------------+
| | SPEAKER RELAY n
+---------------------+

John

That is fantastic.

I have just spent half an hour working it through with several printouts
showing the different stages - and I know why you are a circuit designer and
I am a primary school teacher!!

I can certainly make it - it's really neat.

Thanks again.

I'll post when it's up and running (a week or so I should think).

Finally - I've looked up my local store and they have 12volt 10amp DPDT
relays at a good price.

I also have a mains to12v power adapter - it is rated at 1.25 amp. If I
don't hear any more from you, I shall assume that that is powerful enough to
run 2 relays (1 latch, 1 switch).

Thanks again

NOTTNICK

 

[Nottnick]

---
You're welcome; my pleasure.
---



---
Unless your relays require substantially more coil current than
625mA each you should be fine. One caveat, though... If your
adapter's output is unregulated, its voltage may rise to higher than
the relays' maximum allowed coil voltage. Should that be the case,
then you might want to place enough resistance across the output of
the adapter to load it down to the point where its output voltage
will drop to below the relays' maximum coil voltage spec.

Sorry John, could you possibly give me a bit of info re. your caveat. I bet
you are starting to wish you never got into this now.... really sorry,
ignore this if you want, I'll understand.

How can I tell if the power supply is regulated or not?
I suspect as it is one left over from a piece of equipment I once owned (no
idea what) and is a mains plug/adapter that it is not regulated.
However, it does have one of those chunky black plastic cylinders on the
cable is this a regulator?
If not - how do I find out about the resistance needed.

PS I have also thought that I'll put a jack output in paralell with the push
to make switch.
I can then plug in an extension to use if necessary.

I guess this system would also work on the microphone inputs (that way he
could cut the mikes but keep on playing), do you think this would have a
diverse effect on the sound? A slight difference would be OK as it is just a
practice room. I'll bow to your wise counsell on this one, it was just a
thought.

If I can ever help with teaching tips......................................

Nick

Nottingham UK

 

[Nottnick]

---
Don't worry about it, it's not a problem.

The caveat is that relay coils generate heat, and if too much
voltage is put across them the current through them can overheat the
coils and damage the relays.
---


---
Measure its output voltage with nothing on its output. If it's a
regulated supply, the reading will be very close to 12V. Certainly
within 5%.
---


---
No, that's a ferrite ring used to reduce the EMI from the device,
which, since its there, almost surely means it's a switching supply,
and regulated.
---


---
1. Determine how much current the relay coils need. If you don't
have a data sheet, measure the resistance of the coils and solve:

E
I = ---
R

Where I will be the coil current, in amperes,
E will be the voltage impressed across the coil, (12V) and
R will be resistance you measured, in ohms.


2. Since the supply is rated to be at 12V when its output current is
at 1.25 amperes, the resistor needs to "suck up" the difference
between what the relays need and what the supply can deliver.

That resistance will be:

E
R = ---------
It - Ir

Where R will be the required resistance, in ohms,
E will be the desired supply voltage, 12V,
It will be the total supply current, and
Ir will be the current required by both relays.


For example, assume that your relay coils have resistances of 120
ohms each. That means that at 12V the current in them will be:

E 12V
I = --- = ------ = 0.1A
R 120R


And the shunt resistor will need to be:


E 12V
R = --------- = -------------- = 11.429 ohms
It - Ir 1.25A - 0.2A


That's an ungainly value, and 12 ohms would work just as well, but
we now have another problem in that a 12 ohm resistor connected
across a 12 volt supply will dissipate 12 watts. Not an ungodly
amount of power, but something that needs to be taken care of if
your power supply isn't regulated.

Anyway, this whole thing will be moot if you've got a regulated
supply, so if you measure its output voltage and post back with what
you find maybe we'll be in luck.

---


---
Good idea.
---


---
I'm always leery about switching inputs without knowing exactly what
they look like, since opening a mic input and letting it float with
the amp cranked might be worse than just leaving it alone. OTOH, if
you were to disconnect the mics and short the mic inputs that might
work nicely and allow you to use tiny relays. Or, with a bipolar
supply, semiconductor switches.

Have now made it and it works like a dream!!

Thanks so much.

One modification I might try to make is an output socket so I have the
option of attaching more relays in future should I so wish (in a separate
unit).

The extension kill switch is a useful extra too.

Nick

 

[Don Kelly]

----------------------------

Have now made it and it works like a dream!!

Thanks so much.

One modification I might try to make is an output socket so I have the
option of attaching more relays in future should I so wish (in a separate
unit).

The extension kill switch is a useful extra too.

Nick

----------------------------
I see that the intent is to limit the voltage applied, to the relay coil,
to the rated voltage in the case of an unregulated supply but, unless the
supply is really lousy , is there any net gain? The coil will be warmer but
by how much? This depends on the uncontrolled voltage regulation.
For example assume a no load voltage of 15V and a voltage of 12V at 1.25A.
The effective "line" resistance is 3/1.25 =2.4 ohms
The relay (120 ohms) will have a current of 15/122.4 =0.123 A giving a power
loss in the relay of 1.82 watts vs 1.2 watts at 12V. Call it double but
depending on enclosure (cooling) and duty cycle (on time and off time
between on times) , this may not be a problem. If a shunt resistor
dissipating 12 watts is in the same neat package -i.e. enclosure- as the
relay, then the heating problem may well be worse. The key to the choice
is - how bad is the regulation- i.e what is the no -load voltage?

My personal opinion, which is coloured heavily by power system experience,
is to try it without the shunt resistor (i.e such a scheme with deliberate
losses brings on involuntary shudders). It won't hurt performance of the
relay but will cause more heating as indicated.

 

[Don Kelly]

---
Looking at it again, it seems that if there is a problem, there's an
easy solution which we _both_ missed, that being a resistor in
series with the coil!

Assuming a 15V supply with a line resistance of 2.4 ohms means that
to drop 3V at 100 mA we need

E 3V
Rt = --- = ------ = 30 ohms,
I 0.1A

2.4 ohms of which is in the supply, so we need a 27.6 ohm resistor
to do the job. 27 ohms is a standard 5% value, so just for a feel
of how big it needs to be:


P = I²R = 0.1A² * 27 = 0.27 watts.

Beats the hell out of 12 watts, no?!

Just to make sure that everything's OK, the current through the
relay coil, the 27 ohm series resistor and the line resistance will
be:

E 15V
I = ---- = ------------------- = 0.1 ampere
Rt 2.4R + 27R + 120R

So, 0.27 watts for the resistor dissiption is OK, and using a 1/2
watter will be fine. Agreed?

I like that much better! Price is better as well!

Thanks,