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

[rburnett at richieburnett.co.uk]

> ...How does one achieve good control of
> LED brightness without injecting HF trash into their audio output?

By limiting the slew-rate of the rising and falling edges?

As Neil said most regulators or op-amps have a fairly low DC output 
impedance so can deal with varying load currents and maintain regulation 
*eventually* in the long-term.  However, their AC output impedance often 
rises, so they are not as good at maintaining regulation in the short 
term after a disturbance.  If you limit the rate of change of current 
when you switch loads it makes life a lot easier for the regulator 
that's desperately trying to keep the voltage constant!

-Richie,