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Re: Apple IIGS Stereo Sound Card - Interest?
Drew wrote:
On 24 Nov, 14:22, Drew <goggled...@gmail.com> wrote:
I was looking at another example Stereo card that was published in a
french magasine, and interestingly their design places an OP-Amp
(LM318) between J25-3 (waveOut) and the MC1402B and a SN7407 hex
buffer between J25-4/6 and the MC1402B IC...sounds like they did this
to clean up the signle before being split out.
Any noise on the +5 supplying the Ensoniq DOC chip will show up in
the analog output. Presumeably, the ROM 3 audio cleanup addressed
this issue. After the noise is injected, you won't be able to clean
up the part that falls within the audio band, so you can't do better
than the DOC output.
Using an opamp to buffer the DOC analog output prior to demuxing
in the 14052 shouldn't be necessary, and may introduce crosstalk
if the opamp bandwidth is limited.
Similarly, the TTL control signal does not need buffering. Seems
like the French design is a bit over-engineered. ;-)
The 14052 analog switch should not couple power supply noise into
its outputs, so you're good there.
The opamp circuits are also quite good at rejecting power supply
noise (though bypassing is always good), so I don't expect that
to be a problem.
If you were generating the analog inputs on the board, then power
supply noise would be a major factor, but the analog signals are
generated by the DOC, over which you have no control.
Hi, me again :)
Right i attached a floppy drive and using the drive whilst listening
to sound didn't make any difference to the white noise. I switched
round the output from the MC1402B (demux) and the white noise shifted
from right to left. This i would say eliminates the OpAmp TL084 imo.
I am at the moment just attaching a standard pair of headphones to
test with as don't actually have any powered speakers and the line in
to my pc doesnt seem to want to work lol (headphones work fine on
iphone...so don't suspect the headphones). I do have another pair of
better headphones that have a volume control and it would seem that
they don't have the white noise, well it doesn't seem so obvious.
Is it just because they are lower volume. or is there a real
difference in just the noise level.
This circuit is not designed to drive headphones of any kind--only
line-level inputs, so that is how you should be doing all your
testing. Old amplified stereo speakers are about $5 at most thrift
stores, and will work fine for testing.
Is it possible that though the card is outputting whitenoise that
certain speakers/headphones will take out the white noise?
It is possible that the headphones are causing the driving opamp to
oscillate, and that is showing up as increased noise.
(BTW, after looking at your circuit, I see that you already have
a 2-pole active low-pass filter--the first op amp in each channel,
so don't worry about my previous low-pass comments.)
I expect that everything is going to work fine as soon as you
connect the board to line-level inputs.
-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."