Thursday, March 21, 2019

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. 

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