help with LM13700

[Dimitree]

hello everyone
my aim is to re-build the Korg MS20 "External Signal Processor" using LM13700s.

This is the schematic (quite edited):
http://www.ed-sounds.com/Immagine.png

as you can see there are 3 LPF and 1 HPF (no resonance).
They use 4 gang potentiometers. I have those pots, but I'd like to slim the whole thing using standard 9mm knobless pots.

So looking at the LM13700 datasheet and the article "Using Otas" by Ray Marston, I found this:


so it looks like I could replace the original filters simply using this LM13700 application example (2 of them in series for a 2 pole filter). .
But I have some doubts:

1) is it true that I can do that?
2) how should I calculate the right voltage that I need to supply to the LM13700 in order to get the narrowed frequency cutoff range as the original? (50Hz -> 2500Hz as opposed to 45Hz -> 45kHz) ?
3) since the original uses a reverse log taper pot, can I simply use a standard log taper and use it in reverse? (I don't want to build a "lin to anti-log" circuit.
4) I don't need C1 on the application example circuit, right? or maybe the capacitor C12 on the original schematic is a DC blocking cap so I need C1 too? (if yes, what value)?
5) how do I replace the trimmer on the last LPF of the original circuit?

thank you
Dimitri

[anotherjim]

Hello,
This kind of thing (the original circuit) fries my brain 

As to substituting with OTA, in principle you are right if you do it function for function.

Modifying the range is a bit outside my experience, but it seems to me that increasing the control resistor to OTA pin 16 (27k in Mr Marstons scheme) will reduce the change for a given change in CV - and reducing it increases change. Moving the actual "centre" frequency is probably as simple as change the C on the OTA pin 12. What changes to  make - well, I'd have to experiment if it was me.
Maybe someone who like calculating this stuff will come along.

There is the chance of changing the tonal characteristic if you change to OTA - they are fussy about peak-peak signal level so distortion can creep in.

But, if you just want to make the original smaller - How about making a multi-gang control pot by stacking vertical mount cermet trimmer pots? The kind where the screwdriver slot shows both sides. They are linear -  but if you read up on it ( "The secret life of pots"), you can probably modify them to have a log response.

[Dimitree]

thanks

There is the chance of changing the tonal characteristic if you change to OTA - they are fussy about peak-peak signal level so distortion can creep in.

yes I thought about that too, but I really don't know how to start to adapt the thing