SEM VCO

[Haible_Juergen#Tel2743]

Hi!

Recently I built a SEM-like dual VCO for my Modular.
Made minor changes (FET input OPamps instead of
discrete FETs etc.) and ran into problems. I have found
some workaround, but nevertheless I decided to
rebuild the original - so this is another pcb that was
carefully designed, but is now scattered with additional
components on the copper side - oh well.
But now it works as intended, and it sounds *great*.
I have still some minor changes compared to the original
SEM VCO: I used LF353 for all opamps, and I used
BF245C FETs instead of the original ones. I used
a 560R tempco resistor instead of the original 1k,
and altered the other resistor of the divider, too.
I bent the tempcos over the transistor array, and filled
the space in between with much of this heat sink paste.
As the wheather is quite moody at the moment in germany,
I had the chance to test it with different room temperature.
I found that under these conditions, temperature stability
is excellent, nothing short of a 3340.
It sounds very, very nice. And it sounds quite different from
a 3340. A lot more high end in the discrete design!
(I did a 1:1 cross check with my 3340 based dual VCA, adjusting
the amplitude of the SAW wave to the same level, and
switched between the two VCOs - so the remaining signal chain
 - VCF, VCA - was identical for botth cases.)

Well, this won't surprise you - some of you have told me before,
but now as I have heard it, I believe it (;->).

JH.