[sdiy] Starting a synth project

[Bill Thompson]

tlax at hellfish.org wrote:

>I've been thinking of starting a synth project. 
>
Do it!!!

>I thought it might be convenient to start from the power supply and surfed for them on the net. But one thing was almost never answered, what kind of transformers do people use in these? <snip>
>  
>
Power supplies are probably the best way to start any project, synth, 
preamp, toaster oven... anything!

Power supplies are pretty simple to get working, which is a big plus. 
But don't mistake "getting it working" for "getting as good as you can" 
<G>! There are probably hundreds (if not more) tweaks you can add to 
make the supply quieter, more stable, more protected, etc. And you don't 
necesarilly need to go all that far for the first pass, but the 
opportunities are there!

As far as a transformer for your power supply... well, that's half the 
fun really. The first task is to figure out the input and output 
ratings, and while you do this remember that you have to consider 
voltage, current, and power! Also, remember that if you are stepping 
voltage down, you are stepping current up, though it isn't exactly 
linear! If you want to get really deep into it, the turns ratio is only 
part of the equation, you also want to consider both DC resistance and 
AC impedance.

It can get really complicated if you let it. There are folks who spend 
their entire careers studying power supplies!

A couple of thoughts - first, I am a big believer in protecting power 
supplies from me. The extra parts to insure that you don't destroy the 
power supply while installing a module cost very little! And while it 
might appear obvious that in a production build environment those things 
aren't necesarilly necessary, in a DIY setting they are awefully nice to 
have!!!

Second, a lot of musical circuits take advantage of the sag of a power 
supply when large amounts of current are drawn. It's a cool sound, 
really, a kind of compression almost. BUT, if I were designing a power 
supply to power a modular synth I would NOT design in any sag. It is 
easy enough to emulate with a series resistor or choke if one or two 
modules can benefit from the effect. I'd design to make the rails as 
stable and clean as possible!

One more thought... you aren't building an arc welder, you might be 
surprised at just how much power (and therefore current) you really need.

As far as making it work on either North American or some other primary 
power source, that can be addressed almost entirely by swapping out the 
input transformer.

>Or maybe a better power supply schematics are lying on the net, but I just haven't runned into them. 
>
Well, I've never really surfed for a power supply schematic on the 
web... my starter circuit is based (loosely) on an old National 
Semiconductor application note. If you can find a copy of their older 
app-note book you might want to take a look. It may even be on-line 
somewhere.

>Might even buy a commercial one if I find a good one inside the europe and could move to the nice part of project like VCO. :)
>
I've also never looked into commercial power supplies, and I guess that 
could be a good way to skip a step and get right into the synthesizer 
itself, but I think there is a lot to be learned from building your own 
power supply!

Keep us informed...

Bill

-- 
Bill Thompson
Audio Enterprise
KB3KJF
--
"All truths are easy to understand once they are discovered; the point is to discover them."
Galileo Galilei.
--