CV control with and without OTAs

[alkopop79]

I'm working on a project to create affordable synth modules for education. I wonder if there's a way to replace OTAs with transistors or op amps? I would prefer using TL07... opa amps every module to keep the cost and the BOM low. I saw some designs where the oscillator and the filter's cutoff are controlled by transistors, such as the Gakken Synth:

http://1.bp.blogspot.com/_FMBJEkaC8Lw/SLrL4GIAcJI/AAAAAAAAYNE/W8Mc8VW6GfY/s1600-h/SX150_SchemV2.PNG

Which do you think would be the easiest or cheapest option?

[fonik]

i am not sure if this is the correct place for your post. should be moved into the general DIY or developers sub-forum.

anyways, you might want to take a look into tom gambles 3500 series modules. he replaced all ota's by transistors for these:
http://www.modular.fonik.de/Page37.html

[alkopop79]

Will move the topic. Thank you for your kind help!

[Tim Servo]

Luckily, OTAs like the 13700 are still in production and still cheap. Still, if you want to go even cheaper or just look at alternatives for educational purposes, transistors and even diodes can be coaxed into varying their resistance in certain circuits. The infamous Moog VCF uses transistors as a voltage-control element, and other manufacturers did the same thing with diodes. One topic worth looking into will be the transistor "current mirror" which is a building block in a lot of voltage control circuits. Also, look up schematics to the old PAiA 2700 and 4700 modules (especially the low pass VCF) to see some examples of VERY low-cost synth circuits. I believe both the VCF and VCA use diodes for voltage control. Of course, you're not always going to get the range or stability you might with a more 'conventional' circuit, but still, they're worth looking at.

One other place to look will be here in E-M. In particluar, look up "A collection of circuits by Nicolas" for some wonderful economical synth circuits (several of which can be battery powered).
http://www.electro-music.com/forum/topic-34550.html

Tim (low cost, and worth every penny) Servo

[alkopop79]

Thanks Tim, fantastic!

[alkopop79]

I built this VCO circuit from the datasheet of the lm13700. It works fine, as long as there's nothing connected to the VC input (the 510K resistor). Once I connect a voltage source, the signal dies. I've tried potentiometers of different values and a function generator (as an LFO). I've checked the circuit millions of times and it uses the exact same resistor values. I suspect that the equations on the bottom has something to do with changing the frequency but I just cannot decipher them. Any ideas how to change the frequency? Where did I go wrong?

[elmegil]

What voltages have you tried?

What is the voltage at the junction where it shows Ic & If when you have nothing hooked up? It might be worth starting with a voltage near that, so that you aren't generating any Ic, and seeing what happens when you vary Vc slightly from that value.

For what it's worth I don't understand OTAs very well yet, so it may be that is a ground potential and you've already done this, I don't know.

[alkopop79]

Good tips, will try them! Thanks! I'm using 9V and an almost identical version of the circuit works perfectly with that.

[gdavis]

I'm still learning OTA's too, but I have designed a functional VCA with the 13700.

I think the voltage at the junction of Ic and If is about a diode drop above -15V.

If I'm interpreting the equations correctly, Vc = 0V should give about 100kHz. To get into the audio range, you need to get down to almost -15V.

You said the circuit that works is "almost identical", how is it different?