EFM Diode Filter: Where have you gone Tom Gambalio?

UPDATE: sound clip for this VCF is posted  here.

OK let's turn the dial back to 2004.

That year, a big batch of inexpensive, low parts count synth kits showed up at my door after a long wait.

They were from an outfit called Electronics for Musicians or "EFM".

From the wayback website: EFM was an outfit run by a dude named Tom Gamble. Out of Houston Texas in the good old USA, I am guessing Tom ran the whole thing: circuit design, PCB layout, parts picking, bundling up kits, documentation, mailing out them kits rawhide, maintaining the website, taking orders, and responding to users--sometimes--who complained when their kits didn't arrive or even worse, didn't work.

If Tom had someone helping him I've never read about it.

My own take on the EFM diode filter, finished, tests working.

Yep, I built some his kits; Tom's website clearly indicated his kits were for experienced builders which of course I wasn't. I bought 'em anyway.

Some of  the EFM kits I built are long gone, some of them are still in my to-be-built box, and a few I still use: his LFO5A, and the dual ADSR Deg2A for instance.

My only EFM unbuilt kit that still has the all the parts....NOS in bag, circa 2004!

Still have sommore EFM, over 15 years old, unbuilt.  I imagine others out there do as well.

The best EFM module, that I built anyway, was Tom's VCF6C diode filter.

About the filter: Based loosely (I think) on the EMS Putney VCS3 filter the VCF6c is a simple and effective design. The VCS3 filter is discreet while Tom's uses a hard to find 13600 OTA. But Tom still cops the basic idea.

UPDATE: 5-28-20 13600 OTAs are becoming unobtainium, I tried out a 13700 OTA, much easier to find and it worked--I can't hear any difference.  13700 and clones can be had from places like Druid.

I think the VCF6C sounds pretty good. Maybe sounds really good! It's been my go-to filter for traditional subtractive synthesis in my modular for years now.

The EFM diode filter kit I built in 2004 is kind of wearing out: traces have corroded and have had to be replaced with bus wires, things like that. I wanted to make sure I kept this sound alive in my modular, so using the PDFs from this site and forum pdf capture I fabbed up another board.

No decals yet, but the filter works

My 2 cent soapbox take on EFM: Tom's work was always a bit--rushed? His kits were missing parts at times; there were frustrating IC-frying errors in the PCB traces, schemos and BOMS (but then again, who am I to throw stones?). Maybe EFM was a bit like a microscopically tiny version of Howard Dumble, i.e., he'd take your money then get you the goods--when he gets around to it--at some point in the future.

Customers tore Tom a new one in the audio forums, for his mistakes and long shipping delays and general indolence. It was horrible!

Now looking back, I feel sorry for the guy and feel really stupid for being a dick...I was out what, $15 USD for some kit I was trying to build without a decent DVM or scope?

Tom is (or was? I really, really hope he's still with us!) undeniably talented, produced a heck of a lot of designs and kits and I'm sure worked hard to keep the ideas and kits flowing. I can see him trying to stay afloat in his sweltering garage in Texas in July, gathering up parts, purchasing more caps from surplus places, having to dump out the bag to count the damn things again, while his wife yelled at him for not watering plants and letting their dumb dog pee on their shag carpet.

Right???

Tom must have loved audio DIY and electronics, why else would he be so prolific?  But perhaps his passion drove him crazy. One day I remember his kits just vanished, his site was gone, it's like he just gave up.

Now 15 whatever years later I have never been able to find anyone who knows what happened to EFM or Tom Gamble. I've talked to fellow synth geeks, college electronic music professors, audio electronics mensches, and asked the moderators on DIY forums. Many remember EFM and a few even built Tom's kits, but no one knows where he went.

I am curious: where did Tom study electronics--he knew his way around--did he go to a university or was he self taught?  Did anyone help him with any aspect of his work? Did he really have to wrap up all those kits by himself?

Well, Tom, regardless: thanks! I know the work may have been too much, and we, the nascent audio forum trolls were hard to deal with, but I can say with certainty you did a damn great job on your take of the EMS diode filter. I am happy to report that after 2 evenings sorting out my own trace errors and getting around to putting in the right parts, I got it my redux of your VCF6C working, it's in my rack now and it's sounding good.

About the board: Speaking of crazy, if anyone other than me does build this, you will need a big butt-load o' diodes (this is covered in the aforementioned VCF6c forum capture). Then get out your diode meter and measure each voltage drop across each diode without touching it. You want to match to a thousandth of a volt drop across the diode, so, .6132V matches  .6138 for instance. Put those up in each "horizontal pair" of diodes, which perhaps makes sense if you look at the schematic:

I have posted Eagle/schem/ blah blah for this VCF on my website: here.

If you don't do this the filter won't sound good; I was sloppy at first and had to pull all the diodes and replace with matched.

My buttload-o-4148-diodes. I have no idea where these came from but I have had this lifetime supply forever

There are also 4 .1uF caps between the diodes, get out a capacitance meter and match those too. I bought a bunch of 5% mylar .1uF and matched them to about .5%. Seems to have worked, the filter sounds better after the match.

There is one mistake in my PCB: the left side of R8 goes to ground but instead needs to go to -15V.  It took me about an evening to figure that dumb mistake out but once found: easily fixed.....a corrected version is available on my website.

UPDATE 1-22-20: I don't think I included how I wired up the VC rez pot in any of the docs here or on my site.  Might have been omitted from the EFM site as well? It's a 50K linear pot, wired like the PNG you see below.  There are lots of ways to do this; you can just use a pot if you want, so tie 5V to the "top leg" of the pot and omit the jack....but VC rez is a cool feature found in Tom's design so here it is.

Tom, if you're out there still, reach out to one of us sometime. I don't know about the others, but I feel for you, am grateful for your hard work, and want to know you're doing well.

Saving OTAs--One Chip at a Time--a Simple Hack

Quick background: what the heck is an OTA anyway?

It's a type of IC. For audio and especially synthDIY you see Operational Transconductance Amplifiers (OTA) all over. These chips form the heart of some good audio VCOs, VCF's and a lot of VCA designs for instance.

The idea: It's like an op amp, but current goes out, not voltage; and, there is a "BIAS" pin to determine how much current goes out. A resistor can be used to turn this output current back into a voltage. So: simply put, if you can work within their operating range, OTA chips are good for voltage control of whatever.

The problem with OTAs: These chips are fragile!  If you put too much current into the bias control pin you fry the chip. No warning, no buzz, no hum--the chip is dead!

The upper limit is 2ma. You probably want to stay well below that.

Blowing up a CA3080's or LM13700s, which are still pretty easy to find, is frustrating, but may not be game over--especially now that Coolaudio is making OTAs again. However, for something harder to find, like a LM13600, you really don't want this! The chip is becoming too rare.....

OK how to make sure your OTA is OT-A-OK in your new DIY audio creation?  You can get a current meter and test pins before dropping the chip into your circuit. But I'm too lazy.  Here's a simpler method:

First, build one or two of these:

So you are soldering a medium size LED to a diode in series. For this example I am using a red LED but any medium sized normal LED will do.

OK that took about 5 seconds.....

My finished doodad looks like this:

Next build your entire circuit, up to the point of dropping in the OTA chip.

Solder a socket in where your OTA goes (so: to the PCB, stripboard, perfboard, whatever) and shove in your 2 part masterpiece in like this:

For 13600-13700 you want to test both bias pins. The test rig goes between bias in and V--

Different OTAs have different pinouts, but 3080-13700-13600 is most common so that's what you get here....

OK now sweep your CV (from a voltage generator?  From another module?)  Whatever.

For me, 0V cv should look like this: LED is off!

'
As you sweep up the control voltage the LED turns on until, until!! at 5V, it glows faintly:

If the LED doesn't lite at all, you probably have something wrong with your control circuitry.  You won't blow your OTA IC up, but it's a fair bet your circuit won't work.

If the LED glows bright blazing red, you are probably going to blow up your OTA. Don't put your OTA chip into the socket yet! Instead put a current meter where the LED was and check--but my bet is that it will read more than 2mA. Bummer, but at least your expensive Ebay 13600 IC that took 4 weeks to arrive from Latvia lives to fight another day.

Fix your circuit until the meter reads 1.5mA or less.

I use this trick to test any OTA circuit I've been building, modding, or repairing; I have saved quite a few chips!

OK That's it. OTAs live! yeh baby!

UPDATE 5-28-20: E-M user a.dighera has posted a vid (here!) showing an OTA test LED at work, specifically to make sure a precious 13600 OTA doesn't get blown up while testing a VCF6C filter (blog post for 6C is here).  From my experience, the brightness of this LED looks about right. A lot brighter and you should check your buiild with a DVM first. Many thanks a. for making this vid. 

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.  Update 12-27-21 Get the Eagle LBR file, a library that contains the AD533 in the H10 can footprint, from my Github site, here.


ABOUT THE BOARD:

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!