[sdiy] MS 20 FILTER

[Tim Escobedo]

--- synth DIY <bbdsynth at gmail.com> wrote:

> hi! i have this MS 20 filter project to do and i've been have some
> problems:
> 
> in this schematic i don't really understand the meaning of the symbol
> in the
> output of U1a and U1b.

The LM13600 has two OTAs, hence U1a and U1b.
 
> "Vr" voltage is the ground voltage?

Yes. Sorta. Running off a single 9V battery, it's necessary to make a
reference voltage to act kind of like where ground would normally be
with a bipolar supply. The resistor/LEDs reference is pure hack but
does work. A regular op amp follower with the non-inverting input
referenced to 4.5V would probably be a more classy solution. 

> between pin 7 and 8 ( and 10 and 9 ) it's a buffer circuit. should i
> use any
> specific kind of buffer?

The buffer is built into the chip, at pins 7 and 8 (and 10 and 9). 
Just as shown.

> 
> Lm 136000 datasheet is not available, 

The LM13700 will work fine. ISTR JRC still makes their version, the
NJM13600, along with the NJM13700.

Tim E.

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