AW(2): 24 db/oct filter questions

[Haible Juergen]

	>Lastly, like many others, I too had to abandon the idea of using
the
	>buffers included on the LM13700.  I found they caused a nasty
clipping
	>of the output signal at very low cutoff frequencies (even at fairly
low
	>amplitudes) and caused a very large DC offset as the cutoff is
closed.
	>Replacing them with op amps did wonders!  I guess FETs work here as
	>well? 

You can get a very nice buffer by combining the internal darlington and and
an
external FET. This is especially useful if you have a finished PCB and
cannot
easily retrofit opamp buffers. (Happened to me when I built the JH-3 stuff.)
My favorite circuit is like that: OTA output / capacitor goes to FET base,
FET drain is on +15V, FET source has 33k resistor to -15V and goes to
darlington buffer input of 13700. Darlington buffer output has 10k or 15k
resistor to -15V. (A FET alone might have too high output resistance, but
then again I've seen filters with only one FET as buffer, too. (Roland ?)
The retrofit works especially nice on the "upper" half of a 13700, if you
use a FET with gate on the left side, like BF245A: the pins 13, 12, and 11
of the 13700 are just in line with the FET footprint.

	> I wonder why they didn't opt for a better quality buffer in the
	>chip.  Maybe cost or technology restrictions at the time the chip
was
	>designed.  Oh well.

It depends. Note that simple buffers have the advantage of being much faster
than garden variety opamps. If you're building VCOs, this can be important.
The 13700 is simply not intended for filters. The 13600 works much better.
For vocoders, it's just the other way round, btw.

JH.