AW: SCR VCO?

[Tim Cockram]

The system 700 lfo uses a SCR too.


>
>	>After reading about the thyratron VCO, I thought: "Why not an
>SCR
>	>vco?"
>	>I have quite a pile of them of various ratings, and I wondered
>if
>	>anyone has tried this?  Assuming a saw waveform, the question
>(for me)
>	>is will an SCR discharge the cap fast enough to make a VCO with
>decent
>	>range?
>
>Most Korg VCOs actually use SCR circuits !
>
>(MS-20, Polysix, Trident, to name just a few.)
>
>Only that they don't use an SCR *component* - they
>build their own SCR circuit from two bipolar transistors.
>But it works exactly the same way: It starts to conduct
>when a certain gk voltage is reached, and it stops when
>the current thru the device reaches zero (i.e. when the
>capacitor is discharged).
>The first Moog VCOs used unijunction transistors - not
>the same, but similar in operation.
>An actual SCR component I have only seen in Roland 
>System 700 docs. Not in the VCO's, but in the envelopes,
>if memory serves.
>
>JH.
>
>PS: The Korg circuit looks very elegant, as you don't need
>any voltage comparator (it's buit in), nor hysterestic switch
>or monoflop (the end condition is "current == 0").
>Does this circuit also have some drawbacks ?
>Tolerances of trigger voltage ?
>Temperature dependance ?
>Any idea why it wasn't used by everybody?
>
>

-------------------------------------
Tim Cockram
E-mail: tim at redragon.demon.co.uk 

Phone:-UK-1222-384041
Fax:-    UK-1222-390326


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