[sdiy] Synth & LEDs

[Magnus Danielson]

From: "Paul Schreiber" <synth1 at airmail.net>
Subject: [sdiy] Synth & LEDs
Date: Sat, 24 Feb 2001 22:28:57 -0600

> >
> >     I guess I will have to go get that issue.  I stopped my subscription
> when
> > the DIY articles started to disapear (they even printed a letter to the
> editor I
> > wrote quite a while back lamenting the demise of DIY, good thing the
> internet
> > has brought it back :-) ).
> 
> The comment was made that the designer (that would be me) does not believe
> it tossing
> wheelbarrows of LEDs in a synth, bcause it injects noise into the *power
> supply rails*.
> 
> Driving LEDs in a -90SNR system is non-trivial. You have to use
> constant-current
> drivers with certain design rules to keep the current spikes out of the
> lines. You had better
> REALLY watch your grounding.
> 
> It's not a matter of 'distortion'.  Here is the quote:
> 
> "..each LED adds noise to the system in the form of added current drain and
> switching transients."
> 
> Now, just how *much* noise is a point for the system itself: most likely,
> you need a $30,000
> Audio Precision 2 test set to even *detect* it. But, it some cases, you
> could have a situation
> such that you hear 'strangeness' like VCOs FM modulated without LFOs patch
> in. Conversely,
> lots of pro audio gear (ie compressors) have a boatload of LEDs.
> 
> This is just one of my little personal quirks. MOTM does have LEDs. Just not
> everywhere.

As soon as you have any form of quick changes of voltage and current,
spikes may appear. In the case of LEDs, you could reduce the problems
by running them on a "dirty" or "digital" power. Decoupling, slew-rate
limiting etc. is also tools to use. This is not magic in itself, it is
just that it may not be considered by many designers. Sawtooth, square
and PWM waveforms are among the waveforms in a system where care
should be taken.

On the other side, consider the power-supply sensitivity, how
sensitive is a curcuit to power rail noise? Even if you can fix one
source of noise, have you fixed them all?

In the main rule of EMC:

   "There is a source,
    there is a transmission path
    and there is a receiver.

    The properties of these combine to transfer a disturbance from the
    source to the receiver.

    Let's make the source less disturbing,
    Let's make the transmission path provide less coupling,
    Let's make the reciever less sensitive."

I think the advice holds for this environment too. As you try to
acheive a dynamic range of 120 dB or better, then you learn plenty of lessons.

Oh, you don't need a Audio Precision, there are many diffrent tools,
but I'd allways wanted one never the less.

> >     The reviewer must have been somewhat technically oriented, but had
> never
> > experienced an analog synth before...
> 
> Errr...you couldn't be farther from the truth. The reviewer has 20 analog
> CDs out on 4 labels,
> has a degree from Stanford in music and did gratuate work with John Chowning
> at CCRMA. :)

That would count as merrits, allright ;)

Cheers,
Magnus