Re: Apple IIGS Stereo Sound Card - Interest?

[Drew <goggledrew@gmail.com>]

On 24 Nov, 08:48, "Michael J. Mahon" <mjma...@aol.com> wrote:
> 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."

Wow...think most of that went way over my head lol. Thanks Michael
will digest all that tonight :).

I do really appreicate everyone's help on this and hopefully this is
just small stumbling block.

Cheers
Drew