Re: Apple IIGS Stereo Sound Card - Interest?

[Drew <goggledrew@gmail.com>]

On 24 Nov, 19:01, "Michael J. Mahon" <mjma...@aol.com> wrote:
> 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."

Hi Michael,

Again thanks for your help :). I just bought a pair of powered
speakers from local supermarket and sounds much better, so much so
that i don't think there is a problem. In fact there is less white
noise than on my audio animator...which i was surprised to see..or
hear rather.

I think the headphones as you said were causing increased noise.

I think the problem is that i am a bit of a perfectionist and also
didn't want people buying the cards to be disappointed.

I would like to make a review board up with one of the remaining cards
(have some things to sort out first though for connectors and it wont
have a back plate at this point) and possible get it sent to someone
to play/test with and give me their opinion and possible compare it to
other cards to see if all is as it should be. If anyone is interested
in doing this drop me a message. I would like the card back, but if
you like it i am sure we can negotiate on the cost if you want to keep
the card (and I will send back plate once i complete them...though
assuming i don't get distracted with integrating a digitiser to
it...lol)

Thanks
Drew