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Re: Apple IIGS Stereo Sound Card - Interest?
Drew wrote:
Hi Drew,
Nice looking card! The fabricator has done a great job on the matt
green.
Thanks :-), shame it didn't work properly first go :(, but all part of
the fun :-)
As for noise, I usually find power is a key contributor for noise.
Some low noise linear regulators (10uV/RMS) with high PSRR can really
make a difference, especially if you have stage gains around 20db and
above. Its amazing how much supply noise gets amplified up into the
output. Killing it at the source is easier than removing it from the
output
Amen.
Do you add the linear regs to the board itself ?
Right. You can regulate +12 down to, say, +9 (and -12 down to -9
if you need a negative supply, too).
I fully agree with Steve. For the AppleCrate II audio mixer, I
actually double-regulated +12 down to +5 for the audio stages,
and it eliminated the rampant digital "hash" on the Apple power.
Do you know for sure its white noise? (hiss) - is there any sign of
digital noise? When the floppy drive operates, do you notice any
change in the noise level for example?
Tonight I am going to connect a floppy drive and see if that adds
noise, and also switch the outputs of left and right from the MC1402B
chip to see if the white noise follows. If it does then at least i can
eliminate half the circuit i.e the TL084 Amp part. I still have the
bread board made up, so at least i have something to play with still.
Good plan! Stereo circuits really help with substitution. ;-)
Also going to run through the circuit to see if i can see any obvious
issues.
A good indicator that the noise is coming in on the power rails
is to bridge a large electrolytic from the supply rail to ground
(or both if bipolar) very near the audio stages. If you hear a
significant reduction in noise, the power is likely the problem
and regulating down is a good fix.
Another possibility is high frequency oscillation. High-speed
opamps can be unstable if they are not properly bypassed and
compensated. This can be a little trickier to find without an
oscilloscope. A low-pass filter will help here (as would slower
opamps ;-). Also note that an electrolytic bypass does not replace
a ceramic 0.1uF bypass at high frequencies--use both.
BTW, you need a low-pass to eliminate any frequencies above
half the sampling frequency, since they are noise. (My ears
have enough mileage on them that they low-pass everything, but
your's are probably in better shape. ;-)
In addition to an opamp buffer stage, a two-pole active low-pass
filter (plus a single RC pole) will help clean things up.
-michael
NadaNet and AppleCrate II: parallel computing for Apple II computers!
Home page: http://home.comcast.net/~mjmahon
"The wastebasket is our most important design
tool--and it's seriously underused."