You need to use well matching transistor pairs to make a good quality transistor ladder VCF.

Above is the classic methodology to find matching transistor pairs.  I tried this procedure with 2SC1815 and 2N3904 I have.  Next version of Analog2.0 boards will support the both types of transistors, so I wanted to know how different they are in VCF perspective.

Here is the result:

The result shows that 2N3904 has less variability.

This comparison might not be fair since my 2SC1815 transistors are very cheap ones purchased from Akizuki-Denshi, so these might have different quality control in manufacturing than usual retail transistors.  But at least I learned following:

• 2N3904 is good enough to be used for VCF (Naturally.  Actually, 2N3904 is more common than 2SC1815 to be used for VCF).
• It’s always better to check matching of transistors when you make a VCF.