4600/5600 synth: any know more?

[Ken Stone]

The ETI filters are switched resistor, not switched capacitor. There is
resonance in the circuits, though it is not voltage controlled. Duplication
of the analog switch driving circuitry and addition of an analog switch in
the feedback path would allow voltage controlling of this too. Control
voltage response is linear.

There are two different filter dsigns in the book, the earlier 4600 filter
and the later 3600 filter. The latter is LP only, but with a better
roll-off. this is the filter I have, modified to be 12/24 decibel with a
switch, with resonance around the whole 24db path all the time, from memory.
The pcb was updated again after the book was published, though there is no
circuit diference that I can find.

Ken

>Y-ellow Y'all.
>        I'm kinda curious about the below statement.
>        
>At 08:24 AM 9/16/97 +1000, Ken Stone wrote:
>
>>It is also HL/LP/BP.  The later version used in the 3600 (LP only, 24db)
>>uses two 12 db filters in series. This module has to be my favorite VCF it
>>has great roll off, and as far as I am concerned, a good sound.  I have used
>>it to drive a sub-woofer before... nothing over about 30 Hz getting though!
>>Of note, the control portion of this VCF can be used to drive as many
>>filters as required, if a little creative circuit modification is done.
>
>Does this mean that there were several different filter designs for these
>synths. I'm curious as I've just gotten hold of a copy of the 4600/3600 data
>and it shows the filters to be of a semi-discrete switched capacitor
>variety. No resonance feedback and done with cmos bilateral switches. Now
>I've got now complaints about the method, I'd often thought of using an SCF
>approach, however without resonance they don't scream. If there was a
>revised circuit I'd be most interested.
>
>I think it is NSC that make a suitable SCF in a chip that, at a glance I
>once pegged as being a prime candidate. Being able to produce a multi-mode
>12/18/24 dB/oct filter. Given that some kind of feedback mechanism could be
>incorporated, and from memory it seemed likely, one could vary the Q of the
>filter at will. Making it likely to make an ideal DCF. Or with the addition
>of a VCO as in the case of the 4600 filter, a VCF. However if they have
>added a resonance scheme to the 4600 VCF I'd me most interested to learn how
>they did it.
>
>Thanks in advance.
>
>Be absolutely ICbeox.
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                Ken Stone  sasami at blaze.net.au
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