A cascaded down-regulation query

[Mark Hammer]

So I'm trying to cobble together a semi-modular MFOS Synthlab.  Those familiar with it, know the Synthlab is a single-board mono analog synth with moderately reasonable pitch stability/tracking for two oscillators feeding a single VCF and VCA, being modulated by a single LFO and attack/release generator.  I've added a bunch more stuff that I'll leave aside for now, and brag about (with pics) once it's done and actually working properly.  The Synthlab is essentially a self-contained "normalized" pre-patched thing, but I've installed (or will be installing) some jacks to permit more complex configurations of MFOS (and other) modules to work together.

The MFOS stuff runs off +/-12VDC.  I built a supply with four 1/4" stereo phone jacks for routing the power out to the various boxes I have the Synthlab and additional modules (ADSR, S&H, CV-mixer, LFOs, VCOs, filters, etc.) situated in.  They don't draw tons of current, so I figured a 300ma supply, using one of the little 12.6V transformers I picked up when Radio Shack here was ditching components, would be good enough to power maybe not everything, but a lot of stuff.

Since I had room, I figured what the heck, and decided to add a couple of 2.1mm jacks to the supply faceplate so that the same supply could provide 9vdc if I needed/wanted it.

So here's the thing.  The +/-12v portion is pretty much boilerplate - 1N4001 diodes, 1000uf on the inputs of the 1A 7812/7912 regulators, 1N4001 feedback/protection diode between input and output on the regulators, and 10uf+.1uf to ground on the outputs.  You've seen it a thousand times.

The +12V side, however, feeds a subsequent 7809 1A regulator (hence "down-regulating" from 12V to 9V).  Do I need more than the 10uf+.1uf to ground that I have there, or is that sufficient between cascaded regulators, given the 1000uf caps at the start, and the fact that it takes its 12V feed from a "pre-regulated" source?

Just trying to avoid misery.

[armdnrdy]

Taking all into consideration...I think/guess that the 10µf would be sufficient.

It is very probable that you will be feeding something that you built that already has on board power supply filter caps of their own.

Or.....you can follow "Keen's Law" and insert the largest BFC that space and funds will allow for good measure!

[R.G.]

ulp... here I am going against Keen's law... 

A BFC is not wrong, necessarily,    but it's not needed in this case. The second regulator will be fine with a small local cap and input from a regulated source.

The input capacitance for a three terminal regulator serves two different purposes. First is the suppression of ripple from rectification, and second is lowering the apparent impedance of the input voltage line. If the regulator is within an inch or so (as the electron flies) of the main filter caps, their low impedance keeps the apparent impedance seen by the regulator low as well, serving double duty. When you get further away from the caps, the inductance of the wires leading to the regulator can add enough impedance to make the regulator marginally stable. The local cap(s) shunt this impedance local to the regulator and ensure stability.

The app notes suggest a local 22uF cap or 2.2uF tantalum as I remember. 10uF and a 0.1 ceramic should be OK.

[karbomusic]

The +12V side, however, feeds a subsequent 7809 1A regulator (hence "down-regulating" from 12V to 9V).  Do I need more than the 10uf+.1uf to ground that I have there, or is that sufficient between cascaded regulators, given the 1000uf caps at the start, and the fact that it takes its 12V feed from a "pre-regulated" source?

Newb in the house... I just did very similar here using 317s:

http://www.diystompboxes.com/smfforum/index.php?topic=106755.0

Basically, I tested for a week or two on the breadboard with the scope and throwing everything I could at it. I could never find a need for subsequent filtering etc on the output side of the 12V regulator that was feeding the 9V. I haven't found a need post build either (it's in real world use now) that makes me want to rethink that.

[Mark Hammer]

Thanks, guys.

Tonight, the power will be mine!!