I need a small, simple preamp to tailor the output of a standard
electret mic, to limit the bandwidth to approximately a telephone, say
300 - 3400 Hz and with fairly sharp cutoff.
Must be available quickly, I haven't time to design and build. If the
project works I will need several.
Most simple off-the-shelf preamps do not have tone controls, or are
large and unnecessarily complex for my purpose. Google hasn't been my
friend so far.
Any ideas?
Thanks.

"JohnTurd"

I need a small, simple preamp to tailor the output of a standard
electret mic, to limit the bandwidth to approximately a telephone, say
300 - 3400 Hz and with fairly sharp cutoff.
Must be available quickly, I haven't time to design and build. If the
project works I will need several.
Most simple off-the-shelf preamps do not have tone controls, or are
large and unnecessarily complex for my purpose. Google hasn't been my
friend so far.
Any ideas?


** Still waiting for the Tooth Fairy to arrive with your money ???

JESUS !!!

Where DO all these stinking ****HEADS come from ???? ??



..... Phil

This may or may not be a daft question here, but..
If the system is of restricted bandwidth, would not this fact do the job, or
do you get funny effects with wider bandwidth signals. Cannot say that
feeding nice quality down grotty phone lines sounds any less grotty if band
limited or not.

The nasty bodgit design inc solution is a high value capacitor in series
with the mic and a vvery ngative feedbacked op amp for the amp which will
maybe not be as noise free as it could be though.

Brian

--
Brian Gaff -
Note:- In order to reduce spam, any email without 'Brian Gaff'
in the display name may be lost.
Blind user, so no pictures please!
"JohnT" wrote in message
...
I need a small, simple preamp to tailor the output of a standard
electret mic, to limit the bandwidth to approximately a telephone, say
300 - 3400 Hz and with fairly sharp cutoff.
Must be available quickly, I haven't time to design and build. If the
project works I will need several.
Most simple off-the-shelf preamps do not have tone controls, or are
large and unnecessarily complex for my purpose. Google hasn't been my
friend so far.
Any ideas?
Thanks.

"Brian Gaff" wrote in message
om...
This may or may not be a daft question here, but..
If the system is of restricted bandwidth, would not this fact do the job,
or do you get funny effects with wider bandwidth signals. Cannot say that
feeding nice quality down grotty phone lines sounds any less grotty if
band limited or not.

The nasty bodgit design inc solution is a high value capacitor in series
with the mic and a vvery ngative feedbacked op amp for the amp which will
maybe not be as noise free as it could be though.

Brian


The problem is that John hasn't told us what the application is, so we don't
know what sort of performance he is looking for, nor do we know the reason
for asking for a telephone bandwidth. We don't know what sort of budget he
has either. He says he doesn't have the time to design and build, yet in
fact reserching for a commercial product to perform this could take him far
longer! Ordinary tone controls will not, of course, simulate a telephone
bandwidth with any kind of accuracy. Telephone band filters exist, they have
been used for many years, originally in front of modulators for FDM cable
systems and more recently in front of voice codecs for digital systems, but
these are industrial products not readily available in small quantities.

David.

On Mon, 6 Jul 2009 08:17:31 +0100, "David Looser"
wrote:

"Brian Gaff" wrote in message
. com...
This may or may not be a daft question here, but..
If the system is of restricted bandwidth, would not this fact do the job,
or do you get funny effects with wider bandwidth signals. Cannot say that
feeding nice quality down grotty phone lines sounds any less grotty if
band limited or not.

The nasty bodgit design inc solution is a high value capacitor in series
with the mic and a vvery ngative feedbacked op amp for the amp which will
maybe not be as noise free as it could be though.

Brian


The problem is that John hasn't told us what the application is, so we don't
know what sort of performance he is looking for, nor do we know the reason
for asking for a telephone bandwidth. We don't know what sort of budget he
has either. He says he doesn't have the time to design and build, yet in
fact reserching for a commercial product to perform this could take him far
longer! Ordinary tone controls will not, of course, simulate a telephone
bandwidth with any kind of accuracy. Telephone band filters exist, they have
been used for many years, originally in front of modulators for FDM cable
systems and more recently in front of voice codecs for digital systems, but
these are industrial products not readily available in small quantities.


Thanks for the comments.
What I am trying to do is to optimise a voice signal in the presence
of high background noise levels, so I need a fairly sharp bandpass
filter.
I have of course tried simple RC sections for HPF and LPF but this is
not sufficiently sharp for the purpose.

On Mon, 06 Jul 2009 10:39:03 +0100, JohnT wrote:

On Mon, 6 Jul 2009 08:17:31 +0100, "David Looser"
wrote:

"Brian Gaff" wrote in message
.com...
This may or may not be a daft question here, but..
If the system is of restricted bandwidth, would not this fact do the job,
or do you get funny effects with wider bandwidth signals. Cannot say that
feeding nice quality down grotty phone lines sounds any less grotty if
band limited or not.

The nasty bodgit design inc solution is a high value capacitor in series
with the mic and a vvery ngative feedbacked op amp for the amp which will
maybe not be as noise free as it could be though.

Brian


The problem is that John hasn't told us what the application is, so we don't
know what sort of performance he is looking for, nor do we know the reason
for asking for a telephone bandwidth. We don't know what sort of budget he
has either. He says he doesn't have the time to design and build, yet in
fact reserching for a commercial product to perform this could take him far
longer! Ordinary tone controls will not, of course, simulate a telephone
bandwidth with any kind of accuracy. Telephone band filters exist, they have
been used for many years, originally in front of modulators for FDM cable
systems and more recently in front of voice codecs for digital systems, but
these are industrial products not readily available in small quantities.


Thanks for the comments.
What I am trying to do is to optimise a voice signal in the presence
of high background noise levels, so I need a fairly sharp bandpass
filter.
I have of course tried simple RC sections for HPF and LPF but this is
not sufficiently sharp for the purpose.

The just make a multiple pole filter using op-amps. There are plenty
of cook book style recipes to do this. The usual way to maximise
speech intelligibility would be, as you say, a pass band of 300Hz to
3kHz (approx), but between those extremes, you should have a 6dB per
octave rise to accentuate the top end.

The Texas Instruments web site knowledge base section offers
interactive filter design pages that will do what you need. I'm sure
that every other op amp designer has similar application notes.

d

In article 4a53c6a2.2006077968@localhost, Don Pearce
wrote:
On Mon, 06 Jul 2009 10:39:03 +0100, JohnT wrote:


Thanks for the comments. What I am trying to do is to optimise a voice
signal in the presence of high background noise levels, so I need a
fairly sharp bandpass filter. I have of course tried simple RC sections
for HPF and LPF but this is not sufficiently sharp for the purpose.


The just make a multiple pole filter using op-amps. There are plenty of
cook book style recipes to do this. The usual way to maximise speech
intelligibility would be, as you say, a pass band of 300Hz to 3kHz
(approx), but between those extremes, you should have a 6dB per octave
rise to accentuate the top end.

That is so. I use cookbooks for such purposes. FWIW I've put some of the
simpler design rules on the web at

http://www.st-andrews.ac.uk/~www_pa/...rt3/page2.html

The snag is that we still don't know the signal levels, impedances, etc,
that would suit John's requirements.

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

In article ,
JohnT wrote:
Thanks for the comments.
What I am trying to do is to optimise a voice signal in the presence
of high background noise levels, so I need a fairly sharp bandpass
filter.
I have of course tried simple RC sections for HPF and LPF but this is
not sufficiently sharp for the purpose.

Better way might be to use a noise cancelling mic.

--
*I almost had a psychic girlfriend but she left me before we met *

Dave Plowman London SW
To e-mail, change noise into sound.

In article 4a53c6a2.2006077968@localhost,
Don Pearce wrote:
The just make a multiple pole filter using op-amps. There are plenty
of cook book style recipes to do this. The usual way to maximise
speech intelligibility would be, as you say, a pass band of 300Hz to
3kHz (approx), but between those extremes, you should have a 6dB per
octave rise to accentuate the top end.

I'd go up to approx 4.5Hz. Same as AM transmitters. The 'presence'
frequencies help with clarity.

--
*Young at heart -- slightly older in other places

Dave Plowman London SW
To e-mail, change noise into sound.

On Mon, 06 Jul 2009 11:32:27 +0100, Jim Lesurf
wrote:

That is so. I use cookbooks for such purposes. FWIW I've put some of the
simpler design rules on the web at

http://www.st-andrews.ac.uk/~www_pa/...rt3/page2.html

The snag is that we still don't know the signal levels, impedances, etc,
that would suit John's requirements.

Well, given the job it is to do, I doubt that levels are important
unless they are so high they cause overload - easily dealt with. And
for impedances, my normal way is to make the input high and the output
low; you seldom go far wrong that way.

d