Monday, March 18, 2019

Balanced Modulator Part II: Nothing like a good B-M!

Potty humor aside, I finished the Electronotes AD533 based ring modulator last week, and I am happy with how it turned out.

Front Panel is from Front Panel Express, post on that here

I was after a balanced modulator where the X and Y signals didn't leak into the output, like the good old PAIA 1492 based design I grew up with. The Electronotes "Preferred Circuits" design seemed like it would get the job done.

Good news: it does! This is a quiet, clear sounding module. Not much (if any) cross talk, cross-tweeze, bleed, high end roll off, low end humpty-humps and the other things that make some B-M's sound, pun intended: crappy.

I am yet to go wrong with any EN Preferred Circuits--the ones I've built are winners. If you are into reading about how the old stuff works, the publication is recommended--USD $39 for tons of great circuits is definitely worth the dough; get it here.

You can read part I of this post here where I build the reasonably stable and possibly unnecessary +/- 10V reference for this balanced modulator.

And--sound clip warning!!--hear the finished module here. For your listening pleasure (?) I tried to capture all the usual BM stuff: birds chirping, laser scifi sounds, gongs, bells, dumb Dr Who aliens, even a balanced modulated redux of a previous audiodiwhy sound clip.

You can get the Eagle files, PDFs, BOMs etc. from my website, if you want to make your own or modify this one.


Here is the PCB, it's small....

The hard to get part is the AD533, which hasn't been made for a few years, but I found them for sale in China via Ebay. The cans I bought ($2 each USD--cheap!) took a long time to get shipped to the US, but once here they worked fine. The AD533 isn't unobtanium yet--if you find them for a lot more dough keep looking.

Building both the reference board and the B-M itself was pretty easy, no trace cuts or whatever needed. Got 'em right the first time.

Nothing like a good sounding B-M using a can.....

I had to create a modified Eagle Linear library to accommodate this part; I modified the Eagle10 pin can for this application, and I figure I got it right--it worked.

For the .0001% of the readers out there who might want an Eagle lbr file for an AD533 I can help.....If you want a copy of the lbr comment below and I'll get you one.

Frac me up Scotty! As usual I had to mess with the boards to get them to sit behind a 1U Frak. The solution: the BM board is located behind the 10V reference on 1/4" 4-40 standoffs.

If you want to save more space the 3x op amp IC's could be combined into a surface mount TL084.

Can the trim: The preferred notes went through a couple of paragraphs re: how to trim out the AD533 balanced modulator, but not feeling like digging out the book and reading it again I put a 2K sine wave through X and trimmed the Y trimmer until I couldn't hear the tone at output, then did the same for Y and X.  I am not sure what DCTRIM does, so I put it at midnight and ignored it. Ha! Worked fine--no bleed. I am happy.

Another way to simplify this is to use a currently stocked 10V reference like the LT1012. You'd have to figure out a way to derive -10V from the 10V but that shouldn't be too hard.

OK another one in the bag!  Good to have a good sounding traditional balanced modulator in my rig.  I'm ready for scifi sound design!

One day I'd like to recreate the original PAIA build and see if it sounds as good as I remember, but, on to other things.

Next up: working on an old Tom Gamble diode filter.  Got it to work once; try 2 (new PCB) was a complete failure.  Working on try #3 with PCB #3 now.  Update: got it to work!  Dumb mistake on the PCB! Post is here.

Until then don't breathe the fumes!

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