Fast analog sw. as VC resistor

[Tim Cockram]

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From: Mark Smart <smart at nn.com>
Message-Id: <199701021712.AA17804 at medusa.nn.com>
Subject: Re: Fast analog sw. as VC resistor
To: tim at redragon.demon.co.uk (Tim Cockram)
Date: Thu, 2 Jan 1997 11:12:12 -0600 (CST)
Cc: synth-diy at horus.sara.nl
In-Reply-To: <Chameleon.4.01.2.970102082745.tim at redragon.demon.co.uk> from "Tim Cockram" at Jan 2, 97 08:14:11 am
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> Hi Mark
> 
> You have got the math right in your post.
> 

I realized last night that it would be better to use an analog switch in
PARALLEL with a fixed resistor, rather than without a fixed resistor like
in my previous post. If you use a bilateral switch like in the 4066 and two
resitors, you can make a three-terminal pot very easily:

------+----//////-------+------//////--------+---
      |      R          |        R           |
      |                 |                    |
      +---------------o/  o------------------+
                    1/4 4066

This way, you can vary the resistance to any fraction of R, and just use
one PWM clock to control a pot.


The pot law could get interesting?

> Using Analog switches as Pots/ Attenuators is quite common. A US company called Innovonics use
> this in  broadcast processors and Studer (very expensive and good quality) use this technique for an out 
> put limiter in at least one of their desks.  In the synth world Roland effectively used this technique to 
> control the ADSR settings on the JP4.

Cool. Is that really JP4, or is that an abbreviation for Jupiter 4? I'd
like to get a service manual and see how they did it. The envelope
generators and filter resonance are all I need VCR's for.

Thats right the good old Jupiter 4. Roland use an OTA for resonance and separate oscillators to drive the ADSR.  I think I might 
have the circuit some where, I'll check at the weekend.
> 
> Think also switched capacitor filters eg the MF10.
> 
> You do have to take care to ensure that the garbage generated by the oscillator doesn't get in to the 
> rest of the circuitry (this includes filtering the output of the device!!!)

It seems like the filter could be pretty sloppy and still work, like maybe
just an RC lowpass?

If You can get copies of the Maplin synth kit schematics the VCFs used PWM technology although I think the sweep range was 
limited.


 and that the signal that you are 
> working on falls within the power supply range of the switch (4016/66 run quite nicely on +/- 7v5 or 
> you could use the more expensive DG or SSM series which have much greater).
> 

I'm avoiding specialized IC's due to the lack of availability.

The DG chips have been arround for years and are very much industry standards, so they shouldn't go away.

Thanks for the cool info!

> Regards  
> 
> Tim
> 



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