Filter update

[Tony Allgood]

For those that are interested:

Following the lack of success in using the LM13700 for a post-VCA
filter, I decided to use the Moog ladder. Using the CA3046 as the
matched transistors in the ladder control current breakthrough was not
good. Following the advice given to me by several people, I decided to
find better matched pairs. Juergen and Martin told me to do this
myself, but I copped out on this because I realised that I had some
LM394 super-matched pairs loitering in one of my drawers. These are
matched to 50uV and as they share the same die they follow each other
very closely over temp. However, they are quite expensive. Some £2-50
each, now thats about 10 times the price of ordinary transistors. Hey,
it's only money!!

They do sound good, with only a very small amount of CV breakthrough.
When the filter is modulated from 100Hz to about 25KHz the amount of
breakthrough is about 5mV (peak). The VCF has a gain of one overall.
This is only audible if the sweep is carried out quickly, e.g.. fast
ADSR or square wave modulation, with NO audio input to the filter.

My filter is being modulated by a variety of sources, including AD/R
envelope, LFO and envelope follower. However, my next task is to get
the AD/R to trigger off the incoming audio signal. At the moment I'm
using a full-wave rectifier and 4-pole LPF at 33Hz to generate the
envelope follower. This works well but it generates too much of a lag
for a trigger circuit. So some more work ahead of me yet. I'll keep you
posted, but if anyone has had success with trigger circuits I would be
pleased for some help.

Tony Allgood.

Penrith, Cumbria, UK     

e-mail: oakley at enterprise.net

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