[Randy Yates]

Is there ever a reason NOT to use a differential amplifier as
the first stage of a microphone preamp?
--
Randy Yates % "Rollin' and riding and slippin' and
Digital Signal Labs % sliding, it's magic."
%
http://www.digitalsignallabs.com % 'Living' Thing', *A New World Record*, ELO

[Arny Krueger]

"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.

[Ian Bell[_2_]]

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

[Audio Empire]

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.

[Ian Bell[_2_]]

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.

[Audio Empire]

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.

[David Nebenzahl]

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"

[Audio Empire]

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.

[Arny Krueger]

"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.

[Ian Bell[_2_]]

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.

[Randy Yates]

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

[Kevin McMurtrie[_3_]]

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

[Audio Empire]

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.

[Randy Yates]

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

[Richard Crowley]

"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.

[Kevin McMurtrie[_3_]]

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

[Richard Crowley]

"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.)