LM13700 based filter

[Haible Juergen]

	>The thing is that the lowpass filter works fine but the high pass
gives some cracks when I turn the CV >almost to
	>-15V (the bias current is almost zero uA then). Has anybody got
ideas how this comes.

The LPF's mouth is fully closed when it dies from starvation, so it cannot
complain.
The HPF's mouth is fully open, so expect to hear some moaning.

	>And when I turn the CV even lower both filters stop working because
the outputs of the OTA's run into >one of the
	>supply rails. I have checked if this is because of a small voltage
difference betweent the inputs of the >OTA and this indeed is the case. 

I don't think so.
The reason is most probably the bias current of your buffer stage (internal
buffers ?).
Once your OTA runs on less current than the buffer needs for his own bias,
it will do the
"best" it can to provide that current, i.e. it runs to one of the rails.

> I wonder if this has something to do with the fact that I don't use 1%
> resistors.
> 
No.
What you need is a voltage controlled current source, preferably an
exponential one.

JH.