4 pole LM13700 filter explained (from Shruthi synth)

I have been on the look for a +-9v LP filter made from LM13700, tried several but they all didn't sound very good for some reason or another.
Knowing that the Shurthi synth sounds awesome, I looked for one of the filters schematics and discovered that it works with +-5v, so it's a perfect match for Arduino.
I am a beginner in electronics so I am "dissecting" the schematic in order to understand it better and to remove or change the functions that I don't need.

I have a few questions if anyone could help me:
1) Can feedback be replaced with a simpler op-amp (or method) so if I want to do a 2 pole only filter I don't need to use another LM13700? I really don't need CV controlled resonance.
2) The last op-amp (OUTPUT AMP) works like an amplifier of the signal or as a filter? What is the gain of it?
3) If I just remove the "4 POLE PHASE" and get the "RESONANCE FDBK" pin from the "2P OUT", will it work directly as a 2 pole filter?

Thanks a lot!

That feedback loop is essential to the operation of the filter, its a state-variable filter I believe,
which usually can be tapped for LP, BP and HP output all from one circuit. Some versions can
have the Q tuned separately from the resonant frequency (normally done with variable resistances), so
a great device for an analog synth.

Because this circuit uses transconductance amps its more complicated as the gain is varied by
bias currents, rather than variable resistors.

You could get a similar effect using normal opamps and digital pots for the variable elements,
but these days you'd do it in software, a state-variable filter looks something like:

float lowpass = 0.0 ;
float bandpass = 0.0 ;
float highpass = 0.0 ;

void loop ()
{
   lowpass += bandpass * alpha ;  // integration step, cutoff freq = alpha * sample freq
   bandpass += highpass * alpha ;  // second integration
   highpass = next_intput () ;       // next input sample
   highpass -= bandpass ;     // differencing stages
   bandpass -= lowpass ;
   output (lowpass, bandpass, highpass) ;
}

In other words its a fairly cunning IIR digital filter that splits the input into three. The
sum of lowpass+bandpass+highpass = original signal.

Thanks MarkT :slight_smile:

I really only need low pass at this point and the only CV I need it's for cutoff but I guess I could use all three LM13700 and have a 4 pole LP filter with CV for cutoff and resonance.

What about the other questions?

  1. The last op-amp (OUTPUT AMP) works like an amplifier of the signal or as a filter? What is the gain of it?
  2. If I just remove the "4 POLE PHASE" and get the "RESONANCE FDBK" pin from the "2P OUT", will it work directly as a 2 pole filter?

Dear Pancra,
for easy understanding for all stages look at this picture:
http://tinyurl.com/okfjsoo

The Shruti 13x00 Filter uses an additional stage of the 13x00 for getting a voltage controlled resonance.
Breadboard the two stages and try some things out with the resonance path (potentiometer, additional op-amp)
For myself, I've successfully adapted the V2164 multimode filter from the shruti.
To control the filter for testing, just use a potentiometer with a 1-10K resistor on the middle, driving the LM13x00. If you are not interested in super-fine-tuning, you can forget the whole "CV controlled cutoff" part and use following: ([RESISTOR]= 1K to 100K, try it out)

arduino PWM pin ---[RESISTOR]--+-----100nf----GND
|
LM13x00
Don't forget to setup a fast PWM frequency(64khz)!
I don't know, what signal you use (generated with arduino?), so if the signal is 0-5V, don't forget using an 1uf-3.7uf cap + resistor before the filter input!
Example:
arduino tone out ----CAP---100K----filter in

For summery: the shruti smr4 filter board uses three LM13x00:
two for the 4-LP filter stages
one half for the voltage controlled resonance
one half for the VCA
so if you wanna build a filter port with less parts you can do following:
2-LP stages + voltage ctr reso + VCA (two LM13x00) or
3-LP stages + manual reso + VCA (two LM13x00) or
1-LP stage + manual reso + VCA (one LM13x00) and so on....