Re: Apple IIGS Stereo Sound Card - Interest?

[Drew <goggledrew@gmail.com>]

On 24 Nov, 20:27, Drew <goggled...@gmail.com> wrote:
> 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

Request for testing of review card has been accepted.