I have this schematic of an MS20 Filter for a filter module I’m trying to construct from scratch, but I’m having trouble understanding how it works exactly. Mainly, I was wondering what the function of the two pnp mirroring transistors do in this circuit.
LMNC to the rescue?
3 Likes
1 Like
It would be so awesome if these texts were in German so that I could understand them better 
1 Like
German texts don’t turn up when I google.
Yeah, I know, I think I just wanted to moan a bit because my English is so bad 
1 Like
they are precisely matched BC558C transitors for fine perfeclty tuned oscillation. So you have to find BC558C"s and get the hFe’s to be close to exact as possible or buy them already done for more money . Matched trannys usually have emitters attached together.
It’s not as critical for a filter as for an oscillator, unless of course you intend to turn up the resonance and use the filter as a 1V/oct oscillator.
2 Likes
Thanks for the exponential circuits link! That helped a bunch. I kinda undestand the relationship between the inputs and the exponential converters, but I have another question on how the TL074 Op-Amp that’s connected to the transistors effect the circuit. How is it drawing power? Is it acting as the low pass filter? If that’s so, I thought that the resistor and capacitor had to be connected through the positive terminal of the Op-Amp.
He talks about that in the article. The op amp’s job is to maintain a constant current through the transistor (of 15V/470kΩ = 32 µA). The 4.7k resistor is to limit current and to reduce the voltage gain of the transistor, and the capacitor is for stability.
5 Likes
Ah! Ok, I see now. Thank You!
2 Likes
After running a few simulations on the schematic through Pspice, I realized that the circled potentiometer isn’t the cutoff knob for the VCF. Do you know how I can control the cutoff frequency in this schematic?
Yes, it is the cutoff knob. All the control voltages are on the upper left feeding into the transistors which form the expo converter: Two CVs with attenuator, one “pitch” CV with no attenuator which is V/Oct, and one knob that adds between -15V and +15V, the cutoff frequency knob.
4 Likes
Ah, I see. Thank You!
Is the pitch CV there to add filter tracking?
1 Like
So I ran some simulations through Pspice of the Korg MS20 Filter and for some reason the resonance control isn’t manipulating the frequency content of the signal before the cutoff. Instead, it attenuates the beginning of the signal the smaller the resonance knob resistance is.
Is there any way to fix this to where the resonance knob manipulates the opposite side of the signal near the cutoff?
(Thanks in advance you’ve been a big help and I greatly appreciate the feedback.)
Here is the output signal with the resonance knob set to 10k ohms:
Base on René Schmitz but with a LM13700 and its internal buffers
Cutting pieces of metal in old computers reminds me of doing this for my first machines / sequencers, now i really prefer to work with wood, more easy 
4 Likes
Love the simplicity in these vids.
That perfboard was perfect.
1 Like