This module is based on the SSM2040 voltage controlled filter chip as used in some popular synths. The design here is, more specifically, based on the work by JÃ¼rgen Haible and RenÃ© Schmitz. Some time ago, JÃ¼rgen Haible designed the basic filter structure which matches the block diagram in the SSM2040 data sheet. RenÃ© Schmitz built another version with changes to the power supply configuration and a different exponential current sink. The changes included the use of a +5V reference that replaced ground in each filter stage. This allowed the current sink (bias current) to operate from ground. That version did not have the voltage controlled feedback that JÃ¼rgen Haible’s did. I have started with RenÃ© Schmitz’ design and added JÃ¼rgen Haible’s voltage controlled resonance circuit; also modified to work with the 5V reference. Other changes include: a summing opamp at the input; minor resistor changes for input, output, and feedback gain adjustments; and some new surface mounted matched transistor pairs in the discrete OTA sections.
The 2040 filter during testing
The picture above shows the mostly populated PCB of the filter I ran into some problems during the construction. Mostly caused by bad solder points on the tiny transistor pairs (SOT-363). The transistors used were DMMT3904W and DMMT3906W from diodes.com. These are tricky to work with. The difficulty is not so much in the pin spacing (0.65mm), but in the size of the complete part. I actually had to go out and buy a magnifying glass to read the orientation marks on the BJT pairs. An easier option would be to just use hand matched BC550/560s like the original versions. I used the diodes.com pairs mostly out of curiosity. I intended to take some measurements before building, but they were just to small to work with unless I had made a circuit board for this purpose.
The other SMD part (SOIC-14) is a HFA3046. This is a modern replacement for the CA3046. It is speced for high frequency and it does cost a few dolloars. I measured the matching on two of them a while back; they are comparable to the CA3046 and maybe slightly better. I have used the HFA3046 here because I had a few samples left over from some previous experiments.
I have not posted the complete schematic diagram for this module since I did not have much to do with the design. Below is a basic analysis for the filter core sections (linear approximations) and the exponential current sink.