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

[cheater00 .]

To best control the brightness of an LED, typical advice is to use PWM
rather than vary the voltage. Not only does it make the response more
linear but also more predictable (even with good LEDs, I hear that
noticeable differences arise). How does one achieve good control of
LED brightness without injecting HF trash into their audio output?

D.

On Tue, Mar 4, 2014 at 5:24 PM, cheater00 . <cheater00 at gmail.com> wrote:
> 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