Possible speaker damage

[[email protected]]

Can any PA/audio experts help with this please? I have recently
"inherited" the running if my church PA system but my knowledge is
pretty limited. We have a couple of radio microphones and I'm being
told that to turn them on and off at the transmitter, without turning
them off first at the mixer, will harm the speakers. Is this true?

There is no "thud" or other noise from the speakers when they are
turned on and off. To my un-trained mind that seems to suggest that
there is no signal of any great strength being sent to the speakers
and so no harm should be done. Is my assumption correct? I think
what *may* cause some damage is the occasional heavy-handed tapping of
the microphones "to see if they are on" or plugging/un-plugging the
leads from the transmitters which does cause load "thuds" through the
speakers.

If it makes any difference, we have two TOA radio microphones, via a
TOA WT-770 receiver, through a Yamaha EMX-5000E-20 powered mixer with
amplifier.

[Arny Krueger]

> wrote in message
oups.com

> Can any PA/audio experts help with this please? I have
> recently "inherited" the running if my church PA system
> but my knowledge is pretty limited. We have a couple of
> radio microphones and I'm being told that to turn them on
> and off at the transmitter, without turning them off
> first at the mixer, will harm the speakers. Is this
> true?

Anything can harm speakers if loud enough. Nothing can harm speakers if its
soft enough, and with a credible SR er PA system, that can still be pretty
loud.

> There is no "thud" or other noise from the speakers when
> they are turned on and off.

There might be some ultrasonics, or not.

> To my un-trained mind that
> seems to suggest that there is no signal of any great
> strength being sent to the speakers and so no harm should
> be done.

Depends on a lot of things. For example, do your receivers have a squelch
feature and if it is adjustable, how is it adjusted?

What happens when you power the wireless transmitter off? If you have good
squelching action in the receiver, then the sound will just go away, and
then you are good.

Any system that can be damaged by a wireless transmitter going down with the
fader up is inherently unstable. What does this mean if a wireless
transmitter battery runs down in the middle of a service - do you quick run
out and buy a new SR system or what? ;-)

> Is my assumption correct?

Probably more right than wrong, but not everything is obvious.

> I think what *may*
> cause some damage is the occasional heavy-handed tapping
> of the microphones "to see if they are on" or
> plugging/un-plugging the leads from the transmitters
> which does cause load "thuds" through the speakers.

If you have a good limiter in your system adjusted right, then none of the
above can hurt.

> If it makes any difference, we have two TOA radio
> microphones, via a TOA WT-770 receiver, through a Yamaha
> EMX-5000E-20 powered mixer with amplifier.

YOu didn't mention the speakers. If you have a high-powered amplifier, and
weak speakers, then you have a recipie for disaster. If you have speakers
that are a good match for availble amplfiier power, then you might even be
inherently safe.

BTW the EMX 5000 spec sheet mentions a power select switch. It might work
for you something like the limiter I mentioned above. You might try
experiementing with setting it as low as possible without it adding audible
distortion in normal use. Then, it will limit the amount of power that an
*accident* will cause to be applied to the speakers.

[[email protected]]

Many thanks Arny. You obviously know far more than I do about this
subject.

The power output selection is 500w or 300w or 100w and is already set
at the lowest - 100W. I'm told, by those who know more than I do,
that the speakers are adequate fro this but the 300w or more should
not be used with them.

There is a limiter in the desk. I can't see any way to adjust it. It
just seems to be red warning light. I will see if it lights when the
mics are switched on and off.

The receiver has squelch but it does not seem to be adjustable. The
manual says "Squelch Sensitivity Approx. 14 dBμV". Also the following
(apologies for the length)...

"1. Squelch Circuit
In a receiver employing only a noise or carrier squelch, the squelch
circuit is actuated and provides the output whenever the receiver
receives the same RF carrier as a receiving frequency. This causes
even a disturbing radio signal to be received provided its frequency
is the same as the receiving frequency.
As a result, it can happen that sound is suddenly heard from the
speaker due to disturbing radio signal even when the wireless
microphone's power switch is left OFF.
The squelch circuit of TOA's wireless systems consists of both the
tone and noise squelches, and is not actuated if only same RF carrier
as the receiving frequency is received. It is so designed as to be
actuated and output a signal only when the received RF carrier
contains a very exact pre-determined tone frequency component.
Therefore, disturbing radio signals are rejected and the speaker can
be kept completely quiet when the wireless microphone's power switch
is set to OFF, ensuring reliable use in every application."


So, it seems from what you say, the danger is pretty small. I'll
experiment with the switch on/off and watch the limiter. Otherwise, I
think you have set my mind at rest. Many thanks.