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.

You need an equalizer in the amplifier line.

Regards
Gary K4FMX

On Sun, 23 Apr 2006 21:52:03 -0400, "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.

You can get DSP-based feedback eliminators that work well and are not
outrageously expensive. We use a Behringer DSP110 Shark unit, which
you can get for well under $100, and it works well for us. Google
"feedback eliminator" and you'll get lots of hits. I wouldn't even
think about trying to build one on my own, with such units available.
I'm sure there are better ones for more money, and others at similar
cost that you should also consider, but I don't personally have
experience with them.

Cheers,
Tom

Pay attention to the placement of speakers. For one, don't try to use mics
in front of speakers. The talker needs to have a zone and loud-speakers
should not "talk" to that zone. People near the talker don't need to have
loud-speakers to hear him/her, so if there are some, disconnect them. They
should also "point" where the sound is needed, not oriented because the
mounting is easy. You have to walk around and see where the loud-speakers
are needed and have them there, but not everywhere. I obviously don't know
if this is the case for you, but churches tend to have PA systems thrown at
them rather than designed.
You can "notch" the preferred frequency, for a few db of feedback margin,
with an equalizer. However, when you notch the primary frequency, the next
pops up a few dB down and you can keep chasing them until you have reduced
the gain all thougth the spectrum. Plus, just move a foot or two and the
path changes and the preferred frequency changes.

I am unfamiliar with DSP feedback eliminators, but the job should be easier
if you start at the source (of speaker / mic proximity)

73, Steve, K9DCI



"James Thompson" wrote in message
...
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.

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.

You can try an equaliser in the amplifier input feed, and try to notch out
the worst feedback frequencies. This will work, but is quite a limited
approach.

The best method is to use a frequency shifter that moves the whole audio
spectrum a few hertz up or down. Surrey Electronics here in England make
such a device, and it's transparent in use - you can't hear it working, but
feedback problems are almost entirely eliminated.
http://reflectionseurope.com/surreyelectronics.html

Chris

--
Everything gets easier with practice, except getting up in the morning!

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
======return email munged=================
take out the papers and the trash to reply

Hi again James

Should have read your original post more carefully - sorry !

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.

I guess that you could build a speicific notch into the mic pcb - but
that'll only work if the feedback is at one specific (resonant)
frequency. I think the best filtering systems are 'adaptive' - in
that they can spot feedback happening and can drop a tunable notch on
top of it in real time. Complicated though - and tends to be done at
the amp, rather than at the mic.

All the stuff in the other post applies though - look at the system
first......

Adrian
Suffolk UK
======return email munged=================
take out the papers and the trash to reply