LM13700 VCF (update)

[Tony Allgood]

Well I've built a two pole VCF using the LM13700 and I've found a
disadvantage with the internal buffers provided on the chip. With very
small values of controlling current, the darlington buffers do not
follow very well, the output voltage is a lot lower than expected. Far
more than the 1.2volts difference in input to output. In fact it is
possible for the buffer outputs to drop to the -ve rail. The OTA does
not produce sufficient output current to drive the base at low levels
of control current. The effect of this is that the control range of the
filter is reduced, in particular the bottom end. The VCF on the Fatman
will suffer from this, but it is not probably too bad if you don't want
your filter to cover the whole of the audio band. 

So call up the TL072, use this as a voltage follower instead of the
darlingtons and you are away. Loads of range.

Also, those linearising diodes are very good but they lower the
effective cut-off frequency for a given value of cap.

Tony Allgood.

Cumbria, UK

'I may live in the middle of nowhere, but the views are nice'