Hi


On Tue, 25 Apr 2006 23:06:39 GMT, biascomms
wrote:

James Thompson wrote:

Has anyone got a good but cheap circuit to eliminate or surpress feedback.
Im making my son some low power fm mikes to use in his church, but they
get feedback real easy. Is there a simple notch filter per say that I can
add
the the mike section of this. My pcb is only 1.5 by 2" right now.
Thanks.


We had the same problem in our local Church. The 'system' involved a
couple of bass guitar speakers hung on the wall, a guitar amp and a
not-so-cheap omnidirectional mic at the lectern.

The whole thing 'rang' like a bell at the slightest provovation.

The Church is in a smallish room - probably 30ft x 50ft - with a low
ceiling - and partly divided half-way by a couple of stub walls -
which made it very hard for people at the back of the Church to hear
wht was being said, unless the speakers at the front of the church
were turned up loud enough that you got feedback...

The solution was to replace the lot ! - but as a lot of it was
homebrewed it didn't cost a fortune.

The lectern mics were replaced by a pair of electret boundary mics
with a cardiod response (didn't pick up sound from the direciton of
the speakers. The giant bass speakers were replaced by two pairs of
decent 2nd-hand hi-fi speakers - one pair near the front of the Church
and the other pair at the stub walls, facing backwards.

The amp was replaced with a custom unit, containing three (mono)
40Watt IC amp modules - one for the front speakers, one for the back
speakers and the third one for future use to drive an inductive
hearing aid loop. Volume is independantly controllable between the
front and rear speakers.

This, of course, is a specific solution for our specific situation.
What you probably need to do is start with a simple plan of your
Church - showing the areas where mics are needed, and where you can
put speakers. If possible, you'll find that multiple,
independantly-controllable speakers will allow you to keep the sound
level high enough for intelligibilty without getting feedback - you
might find that you only need amplification 'at the back' of the
Church....

Look closely at mics - the closer and more directional the better -
may even be worthwhile using radio mics, if your people can be trained
to use them. The notch filter will work - but it's only a 'band-aid'
solution - get the accoustic design right and you won't need one !

Hope this helps
Adrian
Suffolk UK
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