LM13700 VCF update no.2

[Tony Allgood]

Well I've built two of these low pass and high pass VCFs now. I've used
TL074 buffers instead of the darlingtons and all is well but...control
voltage breakthrough is terrible. One of the filters is worse than the
other. The LM13700s do have different batch numbers, so it may be case
of selection (yuk!!!). Oddly enough, I have a hunch that it may be the
linearising diodes. When these are in use Iabc seems to have more of an
effect on dc offset. I need to look at this more in detail. However,
what is strange is that the offset produced by the LM13700s depends on
the device. On one of my filters, the control voltage is in phase with
the output breakthrough, but on the other it is in anti-phase. Sounds
like a case of Iabc effecting the input offsets. Temperature seems to
have no effect.

Has any one any good ideas for reducing breakthrough in LM13700s, or am
I going to have to design some clever way of subtracting the CV from
the output.

Tony Allgood.

Cumbria, UK

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