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