sine wave mod for ASM-1?

[John Speth]

At 01:03 AM 3/16/97 -0600, you wrote:
>Anyone have schemos for a sinewave mod for the ASM-1?
>
>Roger

Coincidentally, lately I've been working on just this but I've really
gotten bad results.  Since the ASM1 VCO is an Electronotes copy, I figured
it would be simple to use the EN waveshaper circuits with little problems.
The triangle to sine convertor exploits the non-linear nature of a properly
driven FET to coerce a triangle wave to look like a sine wave with no more
than 1% THD.  I found that the one that I built really works poorly.
Here's what I did...

I built the saw-tri convertor no problem.  Then I built the tri-sin
convertor using the recommended circuit (1 Meg resistors and back to back
diodes at the FET gate, and 150 ohm resistors at the drain and source of
the FET, used 2N4391 FET).  What I saw was a "shaped" triangle but it
wasn't smoothly rounded at the top and bottom but rather it looked hard
limited (flat) on the top and mostly hard limited on the bottom.  I was
able to control the DC offset and the amplitude of the input wave and I
tried all combinations of the two.  
I could only see a hard limit window where the thing showed linear gain up
to a point then a delta gain of 0 after that point.  It's almost as if the
FET was showing way too much gain then saturating like a over driven op-amp
instead of gently easing into saturation.

Is anybody familiar with this circuit?  Could my problem be a bad choice of
FET (EN shows MPF102 and I used 2N4391)?  Or biased for too much gain?  Or
what?

John Speth (johns at oei.com)
Object Engineering, Inc.
Vancouver, WA