[LAU] turning a consumer soundcard into "prosumer" w/ quasi-balanced outs

[Niels Mayer]

even though I found a workaround and won't need this approach, I just
wanted to share something I found out, for "hot rodding outputs" of
consumer equipment, aka "doing it like M-audio's balanced outs."

Executive summary: Use this configuration:
http://www.dself.dsl.pipex.com/ampins/balanced/balfig4b.gif
from http://www.dself.dsl.pipex.com/ampins/balanced/balanced.htm

Details: see part about ''"hot-rod" the output stage'' from
http://www.jensen-transformers.com/an/an003.pdf

.........................
2 - UNBALANCED to BALANCED INTERFACES

The interfaces on the following page do NOT provide the 12 dB gain
necessary to raise the nominal -10 dBV (316 mV RMS) "consumer"
reference level to the nominal +4 dBu (1.23 V RMS) "pro" reference
level. If the pro equipment doesn't have enough gain "reach", an
active interface may be necessary. A step-up transformer, even an
ideal lossless one, is not a viable source of gain in this application.
Reflected impedances cause excess level losses and compromise
both low frequency response and distortion.

                   Adding a Balanced Output

A simple modification to equipment with unbalanced outputs can
convert it to have true balanced outputs. Get (or trace the circuit to
make) a schematic of the equipment's output circuitry. Depending
on available panel space, the new 3-conductor output connector can
be added or used to replace the existing connector. This
modification uses the existing unbalanced output as the + output
and adds an impedance matched passive network to ground for the
- output. In most cases, it is as simple as shown above.

The output impedance of the existing output is defined by the
network between the op-amp output (whose closed loop output
impedance is negligible) and the output connector. An identical
network to ground is then added as shown.

This is also a good opportunity to "hot-rod" the output stage, by
lowering and tightly matching its output impedances. Lowering RS
to 100 S, ±1% and increasing CC to 220 μF, ±20%, works well with
any popular op-amp known to the author, except for the TL06x,
TL07x, or TL08x series (their high open loop output impedance
makes them unstable with capacitive loads such as cables). For op-
amps operating from symmetrical supplies up to ±18 volts, we
recommend Panasonic 16 volt bi-polar electrolytics, part number
ECE-A1CN221S, available from Digi-Key or other Panasonic
distributors. These parts have the lowest distortion characteristic of
any we've tested. The modified output will have balance as good or
better than most current pro gear and, with the exception of the
possible "gain reach" problem mentioned earlier, will produce
excellent results in a professional environment. If the unbalanced
output is retained, do not use (or connect cables to) both outputs at
the same time.
..............
http://search.digikey.com/scripts/DkSearch/dksus.dll?vendor=0&keywords=ECE-A1CN221S
[[ comment: not available any more as they contain Lead and not ROHS
compliant, any suggested replacements?? How about those "10,000hr
Japanese solid capacitor" types like they advertise for premium
motherboards?? also is a 220 μF cap overdoing it, or is that what it
takes do de-anemic-the-bass on a consumer card? ]]

For reference: http://en.wikipedia.org/wiki/Balanced_audio#Differential_signalling
..............
Signals are often transmitted over balanced connections using the
differential mode, meaning the wires carry signals of opposite
polarity to each other (for instance, in an XLR connector, pin 2
carries the signal with normal polarity, and pin 3 carries an inverted
version of the same signal).
Despite popular belief, this is not necessary for noise rejection. As
long as the impedances are balanced, noise will couple equally into
the two wires (and be rejected by a differential amplifier),
regardless of the signal that is present on them.[1][2] A simple
method of driving a balanced line is to inject the signal into the
"hot" wire through a known source impedance, and connect the "cold"
wire to ground through an identical impedance. Due to common
misconceptions about differential signalling, this is often referred
to as a quasi-balanced or impedance-balanced output, though it is, in
fact, fully balanced and will reject common-mode interference.
..............

Niels
http://nielsmayer.com