[sdiy] dare I say - decoupling?

[Steve Moorby]

I saw the trick of LEDs in series in a DDA mixing desk.  Essential trick to 
keep current down when there are lots of LEDs.  In this case I think the 
LEDs were across +/-15V

Steve

----- Original Message ----- 
From: "Oakley Sound" <tonyallgood at btinternet.com>
To: "Synth DIY" <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, September 04, 2005 9:11 AM
Subject: Re: [sdiy] dare I say - decoupling?


>> Would it be ok in the feedback loop of an op-amp like in some of the
>> old Oakley stuff?
>
> This would reduce ground currents, but at the expense of taking it out of
> the main supply rails. I tended to keep the maximum LED current to around
> 5mA which should have only a little affect on the stability of the rail
> potentials. But star power distribution [or as close to it as possible]
> was always assumed in an Oakley modular.
>
> Note also that when the input voltage to that circuit has small
> perturbations around zero volts, the output of the op-amp will rapidly
> switch between the two forward voltages of the LED. This will produce
> high frequency hash that will then be picked up by other parts of your
> circuitry. It is best therefore to slug the output response with a cap
> across the op-amp, from inverting input to output.
>
> What I did in one module with a lot of LEDs, the OMS-820, was to use a
> constant current sink set to 5mA. The LEDs were then wired in series from
> the sink to the 15V rail, each LED would be then turned on or off with a
> separate bypass transistor. This had two advantages; one, the current
> taken is constant; two, several LEDs can be on and take only 5mA from the
> supply.
>
> Tony
>
> www.oakleysound.com
>
>