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.