What to do with unused half of LM13700?

[Werebear]

Hi

dumb question time yet again!!

How do I tie off the unused section of an LM13700 OTA? Extravagant to waist one perhaps but these are cheap reclaims from China, I am building a three VCO box and pots are expensive in comparison to the crap I'm buying off China so a fourth osc is out and I don't need any VCA's for the kind of drone stuff I want to play with at the moment.

I'm sure I've seen how to do it on one of Ray Wilson's schematics but if anyone knows/can remember off the top of their head I'd be grateful.

Thanks

Da Bear

[Werebear]

Ha yeah right. Second schematic I looked at on Rays site gave me the info I needed - you tie inverting and non-inverting inputs to ground and let output and buffers float. Thanks Ray, Goddess Bless.

Sorry to waste time! Back to the soldering iron...

[wmonk]

Why not make a simple VCA out of it? There is always a use for an extra VCA
Even in drone stuff some extra modulation doesnt hurt.

[Werebear]

Well, I could... 'cept I've gone and built the board now!

The VCO is based on the EFM 4616:

http://www.modular.fonik.de/files/EFMforum/orgEFMfiles/efm-4600-doc_01.pdf

and scroll down a bit. It utilises half an LM13700 for the VCO and actually uses the other half for a VCA.

As I said I bought a lot of LM13700 from China quite cheap and when I've come to use them I realise they are reclaimed stock - one of 'em had solder bridges across the pins! - so I won't lose sleep over one amp.

Thanks for the input though!

[JovianPyx]

You can use an OTA as an op-amp by making 2 mA flow into the Iabc pin and then programming the gain the same way it's done on an op-amp. If you have a dual supply of +15 and -15 volts, you can get the 2 mA by connecting a 15K (1%) resistor from +15v to the Iabc pin. This is, of course, if you power the OTA with +15 and -15. Do not use a resistor lower than 15K because that would cause more than 2mA to flow into Iabc and that will fry the OTA. So there is another way to use an unused OTA assuming your design needs an op-amp.

[Werebear]

Thanks Jovian, but I'm running this on +/- 5V. The VCO is quite happy at that but I couldn't get the expo pair to work even with what I thought would be the right values for the resistors. But again at my level of DIY at the moment I'm quite happy with controlling the VCO with just a few resistors and the range is about five octaves though of course no V/octave scaling.

This thing is a glorified drone box. It will have three VCOs (built), each having a PWM output and a sub oscillator out from a handful of CMOS, a multi mode filter and perhaps as many as eight LFOs to modulate the hell out of everything. Working name is the Orrytron after a mate of mine.

Thanks for the input!

[Dave Kendall]

@ JovianPyx - great tip for op amp usage - Nice one