Tuesday, February 26, 2019

Building a Low Cost 10V reference for Audio

UPDATE: The Balanced Modular referred to in this post is now done and working!  For Part II please go here.


I've been back at this audioDIY for a few years now and I am still looking for a really good balanced modulator.

A strange sentence right?

This is partially sentimental, my brother and I built a 1492 based PAIA 4710 Balanced modulator when we were very young indeed (12 and 14?).

Long Live PAIA!

I clearly remember how much I liked that module. The damn thing sounded frigging great! Metal klangs! Space gun laser sounds! Weird robot voices! Impress your friends! Baffle the girls!

Looking back, many of the modules in the PAIA 2700-4700 series sounded--well, bad, but not this one. Maybe my mind is playing tricks on me but the 4710 sounded better than anything I have in my rig now.  I have to admit it: I'd like a good, clean B-M.

So just rip off the 4710 schematic? Maybe...but 1492 IC's are hard to come by and hard to design around I think. The power (+18V, +/- 9V) isn't close to what I use in my DIY modular. Whatever.

I've tried recreating the good old days with AD633's as well as a Thomas Henry designed 3080 RM from his 3080 book but neither sound all that good. Also I fabbed up this Korg MS20 style RingMod which is cool but is pretty choosy about the incoming wave forms (ramp works best? Pulse?).  You can see my ring mod webpage here.

Hunting around more I found a AD533 based Balanced Modulator in Electronotes Preferred Circuits and thought this one might sound good!!

A lot of energy for the EN circuit is spent getting rid of "bleed through" which is the stumbling block of the latest B-M's I've tried to create. I'm not sure I use the right audio term for that. Crosstalk?  Here's what happens: The X or Y audio input can be heard, unadulterated, at output.  The digital (VST) BM's I use with Ableton never have that problem.

At output, I want pure, robotic, squeaky clean B-M sound. Looks like this circuit makes it happen.

For better or worse, the EN circuit requires an Analog devices 10V reference IC which is no longer made.  But! wait a minute, I already put together a working +/- 10V reference for a log converter I built several months ago. You can see that here. I will use that.

10V Precision Reference, finished, tests working

The ref circuit is simple.  It uses an LM397 zener at its core. The idea: you can heat up the 397 zener and nevertheless it stays pretty close to spec. So we have e-z super simple temperature compensation here, always a tough part, in my experience, of a good voltage reference design. Plug the Zener into the ADJ of a 317 and 337 v-reg, some glue trimmers and resistors, and the requisite coupling caps, and ahoy, you get a decently, pretty temperature independent steady voltage source.....

Here's a link to the datasheet for the LM329s.  I got some surplus for next to nothing, they are still out there. Can you make a more stable reference, put your circuit under a heat gun and have V++ stay steadier?  I'm sure you can.  But for a 1980's era Balance Modulator, I figure this design is good enough.

More next time: when I actually see if I can hook this to the EN Balanced modulator and make some sound. (update: works! here.) Until then, don't breathe the fumes.

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