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

[Niels Mayer]

FYI, I just got and installed the Terratec DMX6Fire -- definitely a
better alternative for Linux as you can use "envy24control" to control
the card, instead of the confusing and error-inducing alsamixer. Plus
you get an extra MIDI I/O, spdif and toslink I/O, etc. Overall, an
excellent $20.00 card, perhaps even preferable to the equivalent
M-Audio Delta 66.

The card's performance seems in line with it's specs (
http://www.soundonsound.com/sos/Apr02/articles/terratecdmx6.asp )
"A-D converters: 24-bit 64x oversampling (part of AK4524 chip).
 Input dynamic range: 100dB (converters only)."

With the waveblaster connected to the db50xg, but no output coming
from the synth-card, here's what the noise looks like:
http://nielsmayer.com/npm/jaaa-DMX6Fire-L-fullband-db50xg.png
http://nielsmayer.com/npm/jaaa-DMX6Fire-L-lowfreq-db50xg.png
http://nielsmayer.com/npm/jaaa-DMX6Fire-R-fullband-db50xg.png
http://nielsmayer.com/npm/jaaa-DMX6Fire-R-lowfreq-db50xg.png

(above tests set with input amplification for the A/D set at 126/163
in envy24control, which seems to handle the full-output of the loudest
of my XG-midi test files without clipping... you can make the noise
disappear entirely (or at least go below -160db) by turning the A/D
amplification below 60/163.)

Subjectively, however, the sound is not better!  Although cleaner and
more noise free (the other dx-sc51 would light up the lowest level
-60db output metering LED's on my external DAC ... this one doesn't)
this card doesn't actually sound as "punchy" as the same synth playing
through the crap Dynex dx-sc51. Especially interesting is that the
card sounded particularly anemic&shrill when I first plugged it in,
but as it ran overnight it now sounds much better (electrolytic
capacitors might have been in storage for a while on this used card).
(For all my listening tests, I'm using TOSLINK outputs into a higher
quality external D/A, so the only change is the sound of the A/D and
analog inputs, not the D/A or outputs).

By lacking "punch" i mean that the bass-transients sound somewhat
"round" and "soft" . Bass drum doesn't have a forceful "kick" to it.
And likewise a there's little "snap" to the attack portion of the
bass. On the other hand, the high-end sounds a little brighter and
less "veiled" but sometimes shrill. To improve the bass-response, I'm
considering changing some of the input decoupling electrolytics to
larger values. The JRC4580 op-amps are surface mounted and therefore
difficult to replace. (Nonetheless, people have replaced the opamps
and caps on this board and claim/measure an improvement:
http://forum.rightmark.org/topic.cgi?id=4:122 )

Perhaps the input op-amps on the Dmx6fire are "slew limiting" or
rounding large-signal transients. The cheap Dynex card didn't have
*any* input opamps and  connects inputs directly to the Codec via a
coupling capacitor and resistor. The input opamps on the Dmx6fire, on
the other hand, allow for adjusting input levels. On the dx-sc51 the
input level could not be adjusted and would easily clip if the
main-volume control of the synth-card wasn't turned down.

Of course, with the external breakout box on the dmx6fire, it'll be
easier to perform a simple mod and get back to hearing the excellent
sound of this synth -- bypass the soundcard entirely (other than using
it to house and midi-drive the db50xg) and solder 1/4" TRS jacks
driven by a RDL STA-1 active transformer pair....

But first, I'm going to let this card "burn in" for a week (even
though it's 7 years old) as that seems to be improving the sound
without my doing any work, or spending any money :-).

-- Niels
http://nielsmayer.com