This is a Moog-inspired Voltage Controlled High-Pass Ladder Filter clone, based on the High Pass Filter by EFM (Electronics For Music).
The final circuit is also directly inspired by the comments and suggestions from Kristian Blåsol in his video Moog Highpass Transistor Ladder Filter – DIY Modular in a Week 7.5.
This project is quite old now, and a couple problems have been pointed out since:
- The lower Cutoff frequency is quite high. Even when applying a negative CV and turning the “Cutoff” potentiometer, it stays high.
- The CV input has low impact on the Cutoff frequency.
- The resonance is almost always present, even on low setting.
These are the suggested modifications:
- R2 and R3 (100k) can be replaced by 10k resistors. The CV will have more effect, both on Jack input and on the potentiometer.
- Pin #1 of potentiometer R1 (“Cutoff”) is connected to GND. By connecting it to -12V, it drastically allows a wider and lower Cutoff frequency range.
- And for resonance, it’s possible to lower the value of R5, but I haven’t tested this modification yet.
The most difficult part is the modification of R1 pin #1. As pin #1 is connected to GND copper plane, it requires to cut 6 traces around the pad. 3 on top side and 3 on bottom side. Furthermore, the modification on top side requires to desolder R1.
Attention! Do not connect U1 pin#11 to R1 pin#1 without cutting the 6 traces around the pad. You would create a short circuit that could damage components!
Before powering up the module again, check for any short circuit between -12V and GND.