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Randy Yates Randy Yates is offline
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Default Microphone Preamp Front-End Architecture

Is there ever a reason NOT to use a differential amplifier as
the first stage of a microphone preamp?
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Digital Signal Labs % sliding, it's magic."
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Arny Krueger Arny Krueger is offline
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Default Microphone Preamp Front-End Architecture

"Randy Yates" wrote in message


Is there ever a reason NOT to use a differential
amplifier as the first stage of a microphone preamp?


A transformer coupled input makes a differential input at least optional.


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Ian Bell[_2_] Ian Bell[_2_] is offline
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Default Microphone Preamp Front-End Architecture

Randy Yates wrote:
Is there ever a reason NOT to use a differential amplifier as
the first stage of a microphone preamp?



Transformer coupled inputs do not have problems with phantom power.

Cheers

ian
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Audio Empire Audio Empire is offline
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On Tue, 23 Mar 2010 08:40:00 -0700, Randy Yates wrote
(in article ):

Is there ever a reason NOT to use a differential amplifier as
the first stage of a microphone preamp?


I really can't think of any instance where NOT using a differential amplifier
as a microphone input stage would, in any way, be considered beneficial.
Differential amps have a high common-mode rejection ratio which is the raison
d'etre for balanced microphone cables in the first place. Common mode signals
are generally characterized as those unwanted signals such as induced hum and
RF interference, etc. that appear equally on both signal-carrying conductors
on a balanced microphone line. Transformers also can cancel common mode
signals, but not as well as a properly designed differential amplifier stage
(IOW, they do not as high a common mode rejection ratio). When transformers
are employed, it is not necessary to employ a differential stage following
the transformer. However, transformers have their own problems in that it is
more difficult (and therefore more expensive) to design and build
transformers with the very linear frequency response coupled with a minimum
of phase-shift that is desirable for coupling microphones to electronics.
But, transformers can add voltage gain to a circuit without the added noise
of a high-gain active stage, and are therefore probably necessary in tube
microphone preamps. They also make applying 48 volts DC of phantom power
easier as it is simply applied at the center tap of the primary and shows up
equally on both signal-carrying conductors of the balanced input line. The
transformer will not couple the DC across the transformer and thus it stays
out of the electronics where it can cause a voltage offset which will reduce
the signal handling properties of the electronics.

It is possible to design an effective phantom powering scheme for a
solid-state differential amplifier that is both cheap to build and which does
not couple through the electronics by simply making the differential amp work
down to DC. This way that +48 volts appears equally on both the inverting and
the non-inverting inputs to the differential amp and is "cancelled out
causing no DC offset. Capacitor coupling can also be used for this purpose.

To see how this works, go to:

http://sound.westhost.com/project96.htm

for a circuit diagram showing a simple phantom powering setup for a
differential amplifier type mic preamp.

Go he

http://sound.westhost.com/project66.htm

to see a decent design for a differential microphone preamplifier stage.

From the two schematics. it should be apparent how they work in concert to
provide phantom powering to the microphone and keep the +48 volts out of the
electronics.


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Ian Bell[_2_] Ian Bell[_2_] is offline
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Default Microphone Preamp Front-End Architecture

Audio Empire wrote:
On Tue, 23 Mar 2010 08:40:00 -0700, Randy Yates wrote
(in article ):

Is there ever a reason NOT to use a differential amplifier as
the first stage of a microphone preamp?


I really can't think of any instance where NOT using a differential amplifier
as a microphone input stage would, in any way, be considered beneficial.


1. They provide no galvanic isolation (transformers do)

2. If they have phantom power connected to them, user cock ups an
destroy the input stage (transformers don't)


Cheers

ian


Differential amps have a high common-mode rejection ratio which is the raison
d'etre for balanced microphone cables in the first place. Common mode signals
are generally characterized as those unwanted signals such as induced hum and
RF interference, etc. that appear equally on both signal-carrying conductors
on a balanced microphone line. Transformers also can cancel common mode
signals, but not as well as a properly designed differential amplifier stage
(IOW, they do not as high a common mode rejection ratio). When transformers
are employed, it is not necessary to employ a differential stage following
the transformer. However, transformers have their own problems in that it is
more difficult (and therefore more expensive) to design and build
transformers with the very linear frequency response coupled with a minimum
of phase-shift that is desirable for coupling microphones to electronics.
But, transformers can add voltage gain to a circuit without the added noise
of a high-gain active stage, and are therefore probably necessary in tube
microphone preamps. They also make applying 48 volts DC of phantom power
easier as it is simply applied at the center tap of the primary and shows up
equally on both signal-carrying conductors of the balanced input line. The
transformer will not couple the DC across the transformer and thus it stays
out of the electronics where it can cause a voltage offset which will reduce
the signal handling properties of the electronics.

It is possible to design an effective phantom powering scheme for a
solid-state differential amplifier that is both cheap to build and which does
not couple through the electronics by simply making the differential amp work
down to DC. This way that +48 volts appears equally on both the inverting and
the non-inverting inputs to the differential amp and is "cancelled out
causing no DC offset. Capacitor coupling can also be used for this purpose.

To see how this works, go to:

http://sound.westhost.com/project96.htm

for a circuit diagram showing a simple phantom powering setup for a
differential amplifier type mic preamp.

Go he

http://sound.westhost.com/project66.htm

to see a decent design for a differential microphone preamplifier stage.

From the two schematics. it should be apparent how they work in concert to
provide phantom powering to the microphone and keep the +48 volts out of the
electronics.




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Audio Empire Audio Empire is offline
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Default Microphone Preamp Front-End Architecture

On Tue, 23 Mar 2010 16:44:59 -0700, Ian Bell wrote
(in article ):

Audio Empire wrote:
On Tue, 23 Mar 2010 08:40:00 -0700, Randy Yates wrote
(in article ):

Is there ever a reason NOT to use a differential amplifier as
the first stage of a microphone preamp?


I really can't think of any instance where NOT using a differential
amplifier
as a microphone input stage would, in any way, be considered beneficial.


1. They provide no galvanic isolation (transformers do)


And this is important, when?

2. If they have phantom power connected to them, user cock ups an
destroy the input stage (transformers don't)


Most modern differential amplifiers are pretty well protected against that. I
still wouldn't consider that an overwhelming reason to use transformers and
still think that the disadvantages of transformers outweigh the advantages.


Cheers

ian


Differential amps have a high common-mode rejection ratio which is the
raison
d'etre for balanced microphone cables in the first place. Common mode
signals
are generally characterized as those unwanted signals such as induced hum
and
RF interference, etc. that appear equally on both signal-carrying
conductors
on a balanced microphone line. Transformers also can cancel common mode
signals, but not as well as a properly designed differential amplifier
stage
(IOW, they do not as high a common mode rejection ratio). When transformers
are employed, it is not necessary to employ a differential stage following
the transformer. However, transformers have their own problems in that it
is
more difficult (and therefore more expensive) to design and build
transformers with the very linear frequency response coupled with a minimum
of phase-shift that is desirable for coupling microphones to electronics.
But, transformers can add voltage gain to a circuit without the added noise
of a high-gain active stage, and are therefore probably necessary in tube
microphone preamps. They also make applying 48 volts DC of phantom power
easier as it is simply applied at the center tap of the primary and shows
up
equally on both signal-carrying conductors of the balanced input line. The
transformer will not couple the DC across the transformer and thus it stays
out of the electronics where it can cause a voltage offset which will
reduce
the signal handling properties of the electronics.

It is possible to design an effective phantom powering scheme for a
solid-state differential amplifier that is both cheap to build and which
does
not couple through the electronics by simply making the differential amp
work
down to DC. This way that +48 volts appears equally on both the inverting
and
the non-inverting inputs to the differential amp and is "cancelled out
causing no DC offset. Capacitor coupling can also be used for this purpose.

To see how this works, go to:

http://sound.westhost.com/project96.htm

for a circuit diagram showing a simple phantom powering setup for a
differential amplifier type mic preamp.

Go he

http://sound.westhost.com/project66.htm

to see a decent design for a differential microphone preamplifier stage.

From the two schematics. it should be apparent how they work in concert to
provide phantom powering to the microphone and keep the +48 volts out of
the
electronics.




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David Nebenzahl David Nebenzahl is offline
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Default Microphone Preamp Front-End Architecture

On 3/23/2010 5:40 PM Audio Empire spake thus:

On Tue, 23 Mar 2010 16:44:59 -0700, Ian Bell wrote
(in article ):

Audio Empire wrote:

On Tue, 23 Mar 2010 08:40:00 -0700, Randy Yates wrote
(in article ):

Is there ever a reason NOT to use a differential amplifier as
the first stage of a microphone preamp?

I really can't think of any instance where NOT using a
differential amplifier as a microphone input stage would, in any
way, be considered beneficial.


1. They provide no galvanic isolation (transformers do)


And this is important, when?

2. If they have phantom power connected to them, user cock ups an
destroy the input stage (transformers don't)


Most modern differential amplifiers are pretty well protected against that. I
still wouldn't consider that an overwhelming reason to use transformers and
still think that the disadvantages of transformers outweigh the advantages.


Why are we even discussing microphone transformers here? (I guess this
is the Usenet version of "telephone".) I mean, the OP didn't mention
them; neither did they mention phantom power, yet everyone seemed to
jump to the conclusion that this would be a crucial consideration.

I'd say that since a differential amp practically eliminates common-mode
noise, there's no reason *not* to use one (which, after all, was the
question the OP asked).

Sheesh.


--
You were wrong, and I'm man enough to admit it.

- a Usenet "apology"
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Kevin McMurtrie[_3_] Kevin McMurtrie[_3_] is offline
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Default Microphone Preamp Front-End Architecture

In article , Randy Yates
wrote:

Is there ever a reason NOT to use a differential amplifier as
the first stage of a microphone preamp?


Differencing doesn't have to be in the first stage. I just has to be
performed before common mode noise would be amplified too much.

In laboratory circuits, sometimes a high impedance voltage follower
buffers the input before the differential amp. The output of that
follower also drives a shield on the input wire. The result is a
virtually perfect cable with no capacitance and no insulation losses.
(It only works as long as the electrical wavelength of the input is much
longer than the wire, as with audio.)
--
I won't see Google Groups replies because I must filter them as spam
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Audio Empire Audio Empire is offline
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Default Microphone Preamp Front-End Architecture

On Tue, 23 Mar 2010 20:43:30 -0700, David Nebenzahl wrote
(in article ):

On 3/23/2010 5:40 PM Audio Empire spake thus:

On Tue, 23 Mar 2010 16:44:59 -0700, Ian Bell wrote
(in article ):

Audio Empire wrote:

On Tue, 23 Mar 2010 08:40:00 -0700, Randy Yates wrote
(in article ):

Is there ever a reason NOT to use a differential amplifier as
the first stage of a microphone preamp?

I really can't think of any instance where NOT using a
differential amplifier as a microphone input stage would, in any
way, be considered beneficial.

1. They provide no galvanic isolation (transformers do)


And this is important, when?

2. If they have phantom power connected to them, user cock ups an
destroy the input stage (transformers don't)


Most modern differential amplifiers are pretty well protected against that.
I
still wouldn't consider that an overwhelming reason to use transformers and
still think that the disadvantages of transformers outweigh the advantages.


Why are we even discussing microphone transformers here? (I guess this
is the Usenet version of "telephone".) I mean, the OP didn't mention
them; neither did they mention phantom power, yet everyone seemed to
jump to the conclusion that this would be a crucial consideration.

I'd say that since a differential amp practically eliminates common-mode
noise, there's no reason *not* to use one (which, after all, was the
question the OP asked).

Sheesh.




That's what I said. I only brought-up transformers as an alternative (after
all, they do replace diff amps in some designs) and to show that this older
technology was at once more expensive and less effective than are modern
differential amplifiers.

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Audio Empire Audio Empire is offline
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Default Microphone Preamp Front-End Architecture

On Tue, 23 Mar 2010 22:58:47 -0700, Kevin McMurtrie wrote
(in article ):

In article , Randy Yates
wrote:

Is there ever a reason NOT to use a differential amplifier as
the first stage of a microphone preamp?


Differencing doesn't have to be in the first stage. I just has to be
performed before common mode noise would be amplified too much.

In laboratory circuits, sometimes a high impedance voltage follower
buffers the input before the differential amp. The output of that
follower also drives a shield on the input wire. The result is a
virtually perfect cable with no capacitance and no insulation losses.
(It only works as long as the electrical wavelength of the input is much
longer than the wire, as with audio.)


Yep. But this technique is rare in most modern microphone preamps because
it's more expensive to implement.



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David Nebenzahl David Nebenzahl is offline
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Default Microphone Preamp Front-End Architecture

On 3/23/2010 10:41 PM Audio Empire spake thus:

On Tue, 23 Mar 2010 20:43:30 -0700, David Nebenzahl wrote
(in article ):

Why are we even discussing microphone transformers here? (I guess this
is the Usenet version of "telephone".) I mean, the OP didn't mention
them; neither did they mention phantom power, yet everyone seemed to
jump to the conclusion that this would be a crucial consideration.

I'd say that since a differential amp practically eliminates common-mode
noise, there's no reason *not* to use one (which, after all, was the
question the OP asked).

Sheesh.


That's what I said. I only brought-up transformers as an alternative (after
all, they do replace diff amps in some designs) and to show that this older
technology was at once more expensive and less effective than are modern
differential amplifiers.


Yes. My exasperation wasn't directed at you, but at some replies
up-thread of yours.


--
You were wrong, and I'm man enough to admit it.

- a Usenet "apology"
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Arny Krueger Arny Krueger is offline
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Default Microphone Preamp Front-End Architecture

"David Nebenzahl" wrote in message
.com

I'd say that since a differential amp practically
eliminates common-mode noise, there's no reason *not* to
use one (which, after all, was the question the OP asked).


Transformers are an effective means of elimination common-mode noise. They
are as a rule more effective than differential amplifiers. If you have an
input transformer, you don't need a differential input and you pick up some
inherent protection against EMI and casualty losses.

If you've ever had a grounding problem that a standard active differential
input can't handle, you know what I mean.

Trnasformers also provide galvanic isolation, or isolation of signal
ground. We would probably be a little less critical of the configuration of
safety grounds in audio if we were still in the days of transformers.

Transformers would probably rule the inputs of professional grade gear were
it not for costs. While it is hard to make transformers have as flat of
frequency response and as have ultimately low distortion as differential
inputs, it is possible to make them perform well enough to be sonically
innocious.

Transformers are still the preferred means for obtaining the best possible
freedom from grounding problems in areas where the cost issues are not so
severe, such as analog and digital communications.


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Randy Yates Randy Yates is offline
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Default Microphone Preamp Front-End Architecture

Hi Arny et alius,

Thank you all for your comments. They have been very helpful.

FYI, the reason for my question is that I am attempting to review the
design of the front-end of a heart rate monitoring device. This design
is based on the SSM2167 (e.g., from National). My thinking is that this
is the wrong part for the job since the sensor is fed to the preamp via
a run of cable perhaps 4 feet in length, and there could therefore be
lots of common-mode noise. Also, I believe the AGC action is, in this
stage, not appropriate.

This appears to be an electret type of microphone in that DC power is
fed to the device, so the poster who discussed how to feed phantom power
and simultaneously receive the input differentially was right on the
money.

Thanks again for your input and suggestions.

--Randy

"Arny Krueger" writes:

"David Nebenzahl" wrote in message
.com

I'd say that since a differential amp practically
eliminates common-mode noise, there's no reason *not* to
use one (which, after all, was the question the OP asked).


Transformers are an effective means of elimination common-mode noise. They
are as a rule more effective than differential amplifiers. If you have an
input transformer, you don't need a differential input and you pick up some
inherent protection against EMI and casualty losses.

If you've ever had a grounding problem that a standard active differential
input can't handle, you know what I mean.

Trnasformers also provide galvanic isolation, or isolation of signal
ground. We would probably be a little less critical of the configuration of
safety grounds in audio if we were still in the days of transformers.

Transformers would probably rule the inputs of professional grade gear were
it not for costs. While it is hard to make transformers have as flat of
frequency response and as have ultimately low distortion as differential
inputs, it is possible to make them perform well enough to be sonically
innocious.

Transformers are still the preferred means for obtaining the best possible
freedom from grounding problems in areas where the cost issues are not so
severe, such as analog and digital communications.



--
Randy Yates % "Maybe one day I'll feel her cold embrace,
Digital Signal Labs % and kiss her interface,
% til then, I'll leave her alone."
http://www.digitalsignallabs.com % 'Yours Truly, 2095', *Time*, ELO
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Ian Bell[_2_] Ian Bell[_2_] is offline
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Default Microphone Preamp Front-End Architecture

Audio Empire wrote:
On Tue, 23 Mar 2010 16:44:59 -0700, Ian Bell wrote
(in article ):

Audio Empire wrote:
On Tue, 23 Mar 2010 08:40:00 -0700, Randy Yates wrote
(in article ):

Is there ever a reason NOT to use a differential amplifier as
the first stage of a microphone preamp?

I really can't think of any instance where NOT using a differential
amplifier
as a microphone input stage would, in any way, be considered beneficial.

1. They provide no galvanic isolation (transformers do)


And this is important, when?


Not heard of hum loops then?

2. If they have phantom power connected to them, user cock ups an
destroy the input stage (transformers don't)


Most modern differential amplifiers are pretty well protected against that.


So the recent AES lectures on this very point were, er pointless then?

I
still wouldn't consider that an overwhelming reason to use transformers and
still think that the disadvantages of transformers outweigh the advantages.




I never said transformers were better, the OP just asked why you would
NOT use a diff. amp and I gave two examples.

Cheers

Ian

Cheers

ian


Differential amps have a high common-mode rejection ratio which is the
raison
d'etre for balanced microphone cables in the first place. Common mode
signals
are generally characterized as those unwanted signals such as induced hum
and
RF interference, etc. that appear equally on both signal-carrying
conductors
on a balanced microphone line. Transformers also can cancel common mode
signals, but not as well as a properly designed differential amplifier
stage
(IOW, they do not as high a common mode rejection ratio). When transformers
are employed, it is not necessary to employ a differential stage following
the transformer. However, transformers have their own problems in that it
is
more difficult (and therefore more expensive) to design and build
transformers with the very linear frequency response coupled with a minimum
of phase-shift that is desirable for coupling microphones to electronics.
But, transformers can add voltage gain to a circuit without the added noise
of a high-gain active stage, and are therefore probably necessary in tube
microphone preamps. They also make applying 48 volts DC of phantom power
easier as it is simply applied at the center tap of the primary and shows
up
equally on both signal-carrying conductors of the balanced input line. The
transformer will not couple the DC across the transformer and thus it stays
out of the electronics where it can cause a voltage offset which will
reduce
the signal handling properties of the electronics.

It is possible to design an effective phantom powering scheme for a
solid-state differential amplifier that is both cheap to build and which
does
not couple through the electronics by simply making the differential amp
work
down to DC. This way that +48 volts appears equally on both the inverting
and
the non-inverting inputs to the differential amp and is "cancelled out
causing no DC offset. Capacitor coupling can also be used for this purpose.

To see how this works, go to:

http://sound.westhost.com/project96.htm

for a circuit diagram showing a simple phantom powering setup for a
differential amplifier type mic preamp.

Go he

http://sound.westhost.com/project66.htm

to see a decent design for a differential microphone preamplifier stage.

From the two schematics. it should be apparent how they work in concert to
provide phantom powering to the microphone and keep the +48 volts out of
the
electronics.




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GregS[_3_] GregS[_3_] is offline
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Default Microphone Preamp Front-End Architecture

In article , Randy Yates wrote:
Hi Arny et alius,

Thank you all for your comments. They have been very helpful.

FYI, the reason for my question is that I am attempting to review the
design of the front-end of a heart rate monitoring device. This design
is based on the SSM2167 (e.g., from National). My thinking is that this
is the wrong part for the job since the sensor is fed to the preamp via
a run of cable perhaps 4 feet in length, and there could therefore be
lots of common-mode noise. Also, I believe the AGC action is, in this
stage, not appropriate.

This appears to be an electret type of microphone in that DC power is
fed to the device, so the poster who discussed how to feed phantom power
and simultaneously receive the input differentially was right on the
money.

Thanks again for your input and suggestions.

--Randy


Hope your using battery power.
Isolated preamps must be used when attached to the line.
They usually use switching and transformers.

greg



"Arny Krueger" writes:

"David Nebenzahl" wrote in message
.com

I'd say that since a differential amp practically
eliminates common-mode noise, there's no reason *not* to
use one (which, after all, was the question the OP asked).


Transformers are an effective means of elimination common-mode noise. They
are as a rule more effective than differential amplifiers. If you have an
input transformer, you don't need a differential input and you pick up some
inherent protection against EMI and casualty losses.

If you've ever had a grounding problem that a standard active differential
input can't handle, you know what I mean.

Trnasformers also provide galvanic isolation, or isolation of signal
ground. We would probably be a little less critical of the configuration of
safety grounds in audio if we were still in the days of transformers.

Transformers would probably rule the inputs of professional grade gear were
it not for costs. While it is hard to make transformers have as flat of
frequency response and as have ultimately low distortion as differential
inputs, it is possible to make them perform well enough to be sonically
innocious.

Transformers are still the preferred means for obtaining the best possible
freedom from grounding problems in areas where the cost issues are not so
severe, such as analog and digital communications.





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Randy Yates Randy Yates is offline
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Posts: 839
Default Microphone Preamp Front-End Architecture

Ian Bell writes:

Audio Empire wrote:
On Tue, 23 Mar 2010 16:44:59 -0700, Ian Bell wrote
(in article ):

Audio Empire wrote:
On Tue, 23 Mar 2010 08:40:00 -0700, Randy Yates wrote
(in article ):

Is there ever a reason NOT to use a differential amplifier as the
first stage of a microphone preamp?

I really can't think of any instance where NOT using a
differential amplifier as a microphone input stage would, in any
way, be considered beneficial.
1. They provide no galvanic isolation (transformers do)


And this is important, when?


Not heard of hum loops then?

2. If they have phantom power connected to them, user cock ups an
destroy the input stage (transformers don't)


Most modern differential amplifiers are pretty well protected
against that.


So the recent AES lectures on this very point were, er pointless then?

I still wouldn't consider that an overwhelming reason to use
transformers and still think that the disadvantages of transformers
outweigh the advantages.




I never said transformers were better, the OP just asked why you would
NOT use a diff. amp and I gave two examples.


Thanks for the information and examples, Ian. Actually what I should
have asked is, "Is there ever a reason NOT to use a differential
(balanced) configuration for mic inputs?" whether differential amplifier
or transformer. But I get you.

--Randy



Cheers

Ian

Cheers

ian


Differential amps have a high common-mode rejection ratio which is
the raison d'etre for balanced microphone cables in the first
place. Common mode signals are generally characterized as those
unwanted signals such as induced hum and RF interference, etc.
that appear equally on both signal-carrying conductors on a
balanced microphone line. Transformers also can cancel common
mode signals, but not as well as a properly designed differential
amplifier stage (IOW, they do not as high a common mode rejection
ratio). When transformers are employed, it is not necessary to
employ a differential stage following the transformer. However,
transformers have their own problems in that it is more difficult
(and therefore more expensive) to design and build transformers
with the very linear frequency response coupled with a minimum of
phase-shift that is desirable for coupling microphones to
electronics. But, transformers can add voltage gain to a circuit
without the added noise of a high-gain active stage, and are
therefore probably necessary in tube microphone preamps. They also
make applying 48 volts DC of phantom power easier as it is simply
applied at the center tap of the primary and shows up equally on
both signal-carrying conductors of the balanced input line. The
transformer will not couple the DC across the transformer and thus
it stays out of the electronics where it can cause a voltage
offset which will reduce the signal handling properties of the
electronics.

It is possible to design an effective phantom powering scheme for
a solid-state differential amplifier that is both cheap to build
and which does not couple through the electronics by simply making
the differential amp work down to DC. This way that +48 volts
appears equally on both the inverting and the non-inverting inputs
to the differential amp and is "cancelled out causing no DC
offset. Capacitor coupling can also be used for this purpose.

To see how this works, go to:

http://sound.westhost.com/project96.htm

for a circuit diagram showing a simple phantom powering setup for
a differential amplifier type mic preamp.

Go he

http://sound.westhost.com/project66.htm

to see a decent design for a differential microphone preamplifier
stage.

From the two schematics. it should be apparent how they work in
concert to provide phantom powering to the microphone and keep the
+48 volts out of the electronics.





--
Randy Yates % "Watching all the days go by...
Digital Signal Labs % Who are you and who am I?"
% 'Mission (A World Record)',
http://www.digitalsignallabs.com % *A New World Record*, ELO
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Ian Bell[_2_] Ian Bell[_2_] is offline
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Default Microphone Preamp Front-End Architecture

Randy Yates wrote:
Ian Bell writes:

Audio Empire wrote:
On Tue, 23 Mar 2010 16:44:59 -0700, Ian Bell wrote
(in article ):

Audio Empire wrote:
On Tue, 23 Mar 2010 08:40:00 -0700, Randy Yates wrote
(in article ):

Is there ever a reason NOT to use a differential amplifier as the
first stage of a microphone preamp?

I really can't think of any instance where NOT using a
differential amplifier as a microphone input stage would, in any
way, be considered beneficial.
1. They provide no galvanic isolation (transformers do)
And this is important, when?

Not heard of hum loops then?

2. If they have phantom power connected to them, user cock ups an
destroy the input stage (transformers don't)
Most modern differential amplifiers are pretty well protected
against that.

So the recent AES lectures on this very point were, er pointless then?

I still wouldn't consider that an overwhelming reason to use
transformers and still think that the disadvantages of transformers
outweigh the advantages.



I never said transformers were better, the OP just asked why you would
NOT use a diff. amp and I gave two examples.


Thanks for the information and examples, Ian. Actually what I should
have asked is, "Is there ever a reason NOT to use a differential
(balanced) configuration for mic inputs?" whether differential amplifier
or transformer. But I get you.

--Randy



No problem.

Cheers

Ian
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Richard Crowley Richard Crowley is offline
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"Randy Yates" wrote ...
FYI, the reason for my question is that I am attempting to review the
design of the front-end of a heart rate monitoring device. This design
is based on the SSM2167 (e.g., from National). My thinking is that this
is the wrong part for the job since the sensor is fed to the preamp via
a run of cable perhaps 4 feet in length, and there could therefore be
lots of common-mode noise.


It does sound like a less than optimal design. OTOH, if they are only
looking for cardiac sounds, they could make the case that everything
of interest is *below* the power mains frequency (50/60Hz) and can
be simply low-pass filtered. So the kinds of things we do for wideband
audio are maybe not necessary for that special case?

Also, I believe the AGC action is, in this stage, not appropriate.


I agree from an audio perspective. But maybe the full waveform is not
what they are looking for? Maybe only the cardiac rate is desired here
and preserving the exact waveform is not important? It would presumably
be different if they were running a full EKG.

This appears to be an electret type of microphone in that DC power is
fed to the device, so the poster who discussed how to feed phantom power
and simultaneously receive the input differentially was right on the
money.


There are some clever (and simple/elegant) circuits for using commodity
electret elements, feeding power, AND providing a fully balanced cable
run from the capsule to the preamp, etc.

When I went in for a sleep study, they taped a (disposable) sensor on
my finger with a SMD red LED over my fingernail, and what appears
to be a small photo-resistive element on the opposite side of my finger.
They said that they read both cardiac rate AND blood oxygenation
from the sensor. I kept it as a souvenir. It is possible that there is also
an IR LED and/or sensor in there, but I haven't put it under the micro-
scope yet.


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Audio Empire Audio Empire is offline
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On Wed, 24 Mar 2010 03:54:59 -0700, Arny Krueger wrote
(in article ) :

"David Nebenzahl" wrote in message
.com

I'd say that since a differential amp practically
eliminates common-mode noise, there's no reason *not* to
use one (which, after all, was the question the OP asked).


Transformers are an effective means of elimination common-mode noise. They
are as a rule more effective than differential amplifiers. If you have an
input transformer, you don't need a differential input and you pick up some
inherent protection against EMI and casualty losses.


Now that's interesting. My sources all say that transformers have a lower
common-mode rejection ratio than can differential amplifiers.

If you've ever had a grounding problem that a standard active differential
input can't handle, you know what I mean.


Yes, we all have in-line isolation transformers in our kit-bags for just such
a contingency.

snip

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Ian Bell[_2_] Ian Bell[_2_] is offline
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Audio Empire wrote:
On Wed, 24 Mar 2010 03:54:59 -0700, Arny Krueger wrote
(in article ) :

"David Nebenzahl" wrote in message
.com

I'd say that since a differential amp practically
eliminates common-mode noise, there's no reason *not* to
use one (which, after all, was the question the OP asked).

Transformers are an effective means of elimination common-mode noise. They
are as a rule more effective than differential amplifiers. If you have an
input transformer, you don't need a differential input and you pick up some
inherent protection against EMI and casualty losses.


Now that's interesting. My sources all say that transformers have a lower
common-mode rejection ratio than can differential amplifiers.


'Have' and 'can (have)' are not identical conditions.

Cheers

Ian

snip


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Audio Empire Audio Empire is offline
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On Wed, 24 Mar 2010 13:24:45 -0700, Ian Bell wrote
(in article ):

Audio Empire wrote:
On Wed, 24 Mar 2010 03:54:59 -0700, Arny Krueger wrote
(in article ) :

"David Nebenzahl" wrote in message
.com

I'd say that since a differential amp practically
eliminates common-mode noise, there's no reason *not* to
use one (which, after all, was the question the OP asked).
Transformers are an effective means of elimination common-mode noise. They
are as a rule more effective than differential amplifiers. If you have an
input transformer, you don't need a differential input and you pick up
some
inherent protection against EMI and casualty losses.


Now that's interesting. My sources all say that transformers have a lower
common-mode rejection ratio than can differential amplifiers.


'Have' and 'can (have)' are not identical conditions.

Cheers

Ian

snip


Well, of course, we're comparing best-case scenarios here. A transformer is a
transformer, and while there are a number of makers of microphone
transformers, they are all very similar in design and all have transformer
characteristics in the same proportion, both good and bad. Differential
amplifiers, OTOH, come in a myriad of configurations from cheap 709 style
op-amps to very good ones like the latest National Semiconductor LM 497XX
series to differential amps made with selected discrete components to potted
gain modules made specifically for the pro recording industry. A transformer
can be better than a old-school 709 or 741 era op-amp, but newer op-amps are
much better than that wrt common mode rejection ratios.

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Ian Bell[_2_] Ian Bell[_2_] is offline
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Audio Empire wrote:
On Wed, 24 Mar 2010 13:24:45 -0700, Ian Bell wrote
(in article ):

Audio Empire wrote:
On Wed, 24 Mar 2010 03:54:59 -0700, Arny Krueger wrote
(in article ) :

"David Nebenzahl" wrote in message
.com

I'd say that since a differential amp practically
eliminates common-mode noise, there's no reason *not* to
use one (which, after all, was the question the OP asked).
Transformers are an effective means of elimination common-mode noise. They
are as a rule more effective than differential amplifiers. If you have an
input transformer, you don't need a differential input and you pick up
some
inherent protection against EMI and casualty losses.
Now that's interesting. My sources all say that transformers have a lower
common-mode rejection ratio than can differential amplifiers.

'Have' and 'can (have)' are not identical conditions.

Cheers

Ian

snip


Well, of course, we're comparing best-case scenarios here. A transformer is a
transformer, and while there are a number of makers of microphone
transformers, they are all very similar in design and all have transformer
characteristics in the same proportion, both good and bad. Differential
amplifiers, OTOH, come in a myriad of configurations from cheap 709 style
op-amps to very good ones like the latest National Semiconductor LM 497XX
series to differential amps made with selected discrete components to potted
gain modules made specifically for the pro recording industry. A transformer
can be better than a old-school 709 or 741 era op-amp, but newer op-amps are
much better than that wrt common mode rejection ratios.


Actually, I just noticed you said 'differential amplifiers' rather than
operational amplifiers and to that extent I agree with you when you use
selected descrete components. However, when it comes to op amps, they
are not the prime determinant of achieved CMRR. It's all the other
components around it, and by the time you have included the dc blocking
caps and zener diodes, series inductors etc to prevent RFI and damage
from phantom power and so on necessary for a mic preamp, on a PCB then
actual measured CMMR across the audio band will be little better than a
good transformer with phantom power fed through a primary centre tap.

Cheers

Ian
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Arny Krueger Arny Krueger is offline
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"Ian Bell" wrote in message

Actually, I just noticed you said 'differential
amplifiers' rather than operational amplifiers and to
that extent I agree with you when you use selected
descrete components. However, when it comes to op amps,
they are not the prime determinant of achieved CMRR. It's
all the other components around it, and by the time you
have included the dc blocking caps and zener diodes,
series inductors etc to prevent RFI and damage from
phantom power and so on necessary for a mic preamp, on a
PCB then actual measured CMMR across the audio band will
be little better than a good transformer with phantom
power fed through a primary centre tap.


Good points.

Another hidden gotcha with specifying CMRR is dynamic range.

What is the CMRR of an op amp when the common mode voltage is 5 Kv?

120 volts?

Peak common mode voltage VCC which might only be 5 volts.

Some or all of these voltages are probable in the real world. Of course they
are inceasingly probable as the voltage goes down.

The answer can be very close to 0 dB. :-(


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GregS[_3_] GregS[_3_] is offline
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In article , (GregS) wrote:
In article , Randy Yates wrote:
Hi Arny et alius,

Thank you all for your comments. They have been very helpful.

FYI, the reason for my question is that I am attempting to review the
design of the front-end of a heart rate monitoring device. This design
is based on the SSM2167 (e.g., from National). My thinking is that this
is the wrong part for the job since the sensor is fed to the preamp via
a run of cable perhaps 4 feet in length, and there could therefore be
lots of common-mode noise. Also, I believe the AGC action is, in this
stage, not appropriate.

This appears to be an electret type of microphone in that DC power is
fed to the device, so the poster who discussed how to feed phantom power
and simultaneously receive the input differentially was right on the
money.

Thanks again for your input and suggestions.

--Randy


Hope your using battery power.
Isolated preamps must be used when attached to the line.
They usually use switching and transformers.

greg



Here is what I am familiar with in using chest sensors.............
http://www.analog.com/static/importe...eets/AD210.pdf

greg




"Arny Krueger" writes:

"David Nebenzahl" wrote in message
.com

I'd say that since a differential amp practically
eliminates common-mode noise, there's no reason *not* to
use one (which, after all, was the question the OP asked).

Transformers are an effective means of elimination common-mode noise. They
are as a rule more effective than differential amplifiers. If you have an
input transformer, you don't need a differential input and you pick up some
inherent protection against EMI and casualty losses.

If you've ever had a grounding problem that a standard active differential
input can't handle, you know what I mean.

Trnasformers also provide galvanic isolation, or isolation of signal
ground. We would probably be a little less critical of the configuration of


safety grounds in audio if we were still in the days of transformers.

Transformers would probably rule the inputs of professional grade gear were
it not for costs. While it is hard to make transformers have as flat of
frequency response and as have ultimately low distortion as differential
inputs, it is possible to make them perform well enough to be sonically
innocious.

Transformers are still the preferred means for obtaining the best possible
freedom from grounding problems in areas where the cost issues are not so
severe, such as analog and digital communications.



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Randy Yates Randy Yates is offline
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Kevin McMurtrie writes:

In article , Randy Yates
wrote:

Is there ever a reason NOT to use a differential amplifier as
the first stage of a microphone preamp?


Differencing doesn't have to be in the first stage. I just has to be
performed before common mode noise would be amplified too much.

In laboratory circuits, sometimes a high impedance voltage follower
buffers the input before the differential amp.


Hi Kevin,

Do you mean *two* "high impedance voltage follower buffers", one for
the "+" and one for the "-" of the differential input signal?

The output of that follower also drives a shield on the input wire.


"that" follower? Which one? Are you talking about shielded, twisted
pair cable?

The result is a virtually perfect cable with no capacitance and no
insulation losses. (It only works as long as the electrical
wavelength of the input is much longer than the wire, as with audio.)


Without the previous clarifications, it's hard for me to grok this
last statement, but it sounds like an interesting result.
--
Randy Yates % "Though you ride on the wheels of tomorrow,
Digital Signal Labs % you still wander the fields of your
% sorrow."
http://www.digitalsignallabs.com % '21st Century Man', *Time*, ELO


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"Randy Yates" wrote ...
Kevin McMurtrie writes:
The output of that follower also drives a shield on the input wire.


"that" follower? Which one? Are you talking about shielded, twisted
pair cable?

The result is a virtually perfect cable with no capacitance and no
insulation losses. (It only works as long as the electrical
wavelength of the input is much longer than the wire, as with audio.)


Without the previous clarifications, it's hard for me to grok this
last statement, but it sounds like an interesting result.


He appears to be talking about the technique of using a "driven guard"
http://ieeexplore.ieee.org/stamp/sta...umber=00706021
I hold a patent for using that technique to probe very low currents
on semiconductor wafers.


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In article ,
"Richard Crowley" wrote:

"Randy Yates" wrote ...
Kevin McMurtrie writes:
The output of that follower also drives a shield on the input wire.


"that" follower? Which one? Are you talking about shielded, twisted
pair cable?

The result is a virtually perfect cable with no capacitance and no
insulation losses. (It only works as long as the electrical
wavelength of the input is much longer than the wire, as with audio.)


Without the previous clarifications, it's hard for me to grok this
last statement, but it sounds like an interesting result.


He appears to be talking about the technique of using a "driven guard"
http://ieeexplore.ieee.org/stamp/sta...umber=00706021
I hold a patent for using that technique to probe very low currents
on semiconductor wafers.


I figured out where my copy of "The Art of Electronics" was hiding.
"Driven guard" or "bootstrapped guard" is it.

It's funny that a patent can be granted for the application of that to
silicon wafers sensors.
--
I won't see Google Groups replies because I must filter them as spam
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Richard Crowley Richard Crowley is offline
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"Kevin McMurtrie" wrote ...
I figured out where my copy of "The Art of Electronics" was hiding.
"Driven guard" or "bootstrapped guard" is it.

It's funny that a patent can be granted for the application of that to
silicon wafers sensors.


Well, of course, *I* think it is a valid application of patent protection.
:-)
OTOH, there is the claim that someone successfully obtained a patent
on a ham sandwich (appropriately disguised in legaleese.)


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