Tuesday, February 26, 2019

Electronotes Balanced Modulator Part I: the 10V reference

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, since 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 remember with a great deal of fondness how much I liked that module. The damn thing, I remember, sounded frigging great! Metal klangs! Space gun laser sounds! 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.

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 even close to what I use in my rig. 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 waveforms (ramp works best?).  You can see my ring mod page 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 the bleed through which seems to be the stumbling block of the latest B-M's I've tried to create.

But, 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 was working on several months ago. You can see that here.  Let's use that!

10V Precision Reference, finished, tests working

The circuit is simple.  It uses an LM397 zener at its core. The idea is that 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 of a 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 steady voltage source.....

I got the LM329s surplus but you can still find 'em out there.

Can you make a more stable reference that you can put under a heat gun and have it stay steadier?  I'm sure you can.  But for a 1980's era Balance Modulator, I figure this is good enough.

More next time: when I actually see if I can hook this to the EN Balanced modulator and make some sound. We'll see. Until then, don't breathe the fumes.

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