MICROPHONE
PREAMP WITH S/PDIF AND USB OUTPUTS

This was my
first stab at a digital-out balanced-input microphone preamp using a very low
noise input stage op-amp, the AD797.  When I started out, the aim was first
to have a preamp with an analog output suitable for feeding to a sound card,
with a DC/DC converter that would run from 5V and provide rail voltages for the
op amps as well as phantom power.  Building my own, I could design it with
no electrolytic capacitors in the signal path. Then I would move on to providing
an A/D section and eliminating the sound card.

The
interesting part of the preamp board is the second stage, which uses an op-amp
in inverting configuration with a fed-back negative resistance provided by
another op-amp to provide pseudo-logarithmic gain control using a linear
pot.  This circuit has the additional advantage of delivering a balanced
output to drive the balanced inputs of the A/D converter.

I then added an
A/D converter board that used two now-obsolete Crystal/CIrrus parts, the CS5360
24-bit A/D converter and the CS8402A S/PDIF transmitter, with the aim of using
the preamp with the optical (TOSLINK) input of a sound card.

Eventually I
realized that USB would be more convenient for portability than S/PDIF, and I
found TI's PC2906B 16-bit S/PDIF-to-USB converter (that's not what they call it,
but that's what I use it for).  This of course limited the output to 16
bits rather than 24, and introduced the possibility of noise coming in through
the USB connection.

Noise coming in on the USB power line has not turned
out to be a problem.  I've made comparisons between the preamp set up with
USB output, and then set up with optical S/PDIF output with an independent,
isolated power supply, and found no serious difference (neglecting harmonics of
60 Hz coming from the independent power supply!).

Preamp
stage:

A/D and S/PDIF
transmitter:

Power.
Note that the converter will free-run until it is connected to and syncs with
the master oscillator on the A/D board:

USB board -
nothing much to see here:

Metering board
to display left and right peak level meters:

Noise spectrum
with 0 gain - as the input to the A/D is essentially shorted, this is the noise
floor of the CS5360 A/D:

Noise spectrum
at full gain. This is the gain of the input stage, 29 dB, plus the gain of the
A/D driver stage, 55 dB, for a total of 84 dB.

In this last
plot, the noise from the AD797 input stage is visible.  The AD797 claims
typical noise of .9 nV/root Hz from about 1kHz on up.  Amplified by 84 dB,
this is 14 uV/root Hz going into the A/D converter, which has a full scale input
of 2 V rms.  The spectrogram above shows data analyzed at a bandwidth of
5.86 Hz, and multiplying the 14 uV/root Hz by the square root of 5.86 Hz/1 Hz
yields 35 uV in 5.86 Hz.  This is 95 dB below the 2 V rms full scale input
of the A/D, and that is what's shown in the plot above.

With a
condenser microphone, the gain is likely to be set some 40 dB below this
maximum.  This puts the noise from the input stage below the noise shown in
the first plot. The reason for such a high gain to begin with was to accomodate
a very low output dynamic microphone.  But even with a dynamic microphone,
the gain will never be set to full.