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.