[sdiy] LED current source needed for 'digital' outputs?

[cheater00 .]

When figuring out my low-noize mod for the 303, it turned out that
noise was generated exactly by the matrix switching of diodes. The CPU
itself wasn't an issue, the current consumed by the driver transistors
was. There was a local decoupling cap but it was ineffective and I
could hear the buzz at full scale in my audio if I put it through a
compressor-distortion pedal. I placed a 120 (?) Ohm resistor in series
with the diode that fed all of those driver resistors. This way, the
impedance towards the rest of the system was several orders of
magnitude higher than the impedance of the local capacitor. This
removed the noise in output. A somewhat smaller resistor leaked noise
through but I don't remember what other sizes I have tried.

On Tue, Mar 4, 2014 at 5:17 PM, Neil Johnson <neil.johnson71 at gmail.com> wrote:
> cheater00 wrote:
>> Similarly you could switch between a visible and invisible led, or anotger
>> diode.
>
> I used a zener diode.  You don't want a switch - too expensive, and
> you are still likely to get some sort of current spike during the
> switch transition which will generate HF noise.
>
> Neil
> --
> http://www.njohnson.co.uk