[sdiy] 1.0 kohm "tempco" for VCO / sub octave question

[James Patchell]

At 12:34 PM 3/1/2004 -0800, Oren Leavitt wrote:
>Hi Thomas,
>
>
>5) There have also been some 'tempco-less' VCO designs discussed on the 
>List in previous threads.
>

         You can find one circuit here:
http://www.oldcrows.net/~patchell/synthmodules/vco.html

         I have found that it seems to work quite well....

>Hope that helps,
>Oren
>
>-----Original Message-----
>From: Thomas Dunker <dunker at invalid.ed.ntnu.no>
>Sent: Mar 1, 2004 11:15 AM
>To: synth-diy at dropmix.xs4all.nl
>Subject: [sdiy] 1.0 kohm "tempco" for VCO / sub octave question
>
>
>  Hello,
>
>  Just sent a big order for parts for my modular project...
>
>  One module that's been on hold for a while is the VCO:
>
>http://www.musicfromouterspace.com/analogsynth/August2003VCO.html
>
>  I'm looking for possible substitutes for a couple of "tricky" components
>(ones unavailable from ELFA in Sweden, my "regular" one-stop supplier):
>
>*The 1.0 kohm "tempco" to be thermally bonded to the CA3046 - the design
>  specifies the Precision Resistor Co. PT146, which has proven difficult to
>  find here (if anyone's got just TWO pieces to spare from their personal
>  stash, please let me know...)
>   ELFA, however, stocks a Philips part which I wonder if might
>   do the trick:
>
>   KTY 81-120 (SOD-70 package)
>   1000 ohms +/- 2%
>   Temperature coefficient: +0.75%/K (isn't that like saying 7500ppm/K?)
>
>   The PT146 has a postive temp. coefficient of 3350ppm, less than half
>   that of the Philips KTY81-120 - but are they otherwise (functionally)
>   the same kind of part? I have never worked with PTCs like these so I
>   have to ask, the part is to be in the feedback loop of a TL084 amp
>   at the CV input of the VCO, so I assume it must be a pure resistance
>   seen from the op-amp... Might the Philips KTY81-120 work?
>
>*Secondly, the MPF 102 N-channel JFET - looks like the only substitutes
>  available from ELFA would be either 2SK117 or 2SK170GR7 - would any of
>  these work? (I'm normally a tubes guy, so some times I just have to ask
>  when it comes to semiconductors...)
>
>  Another question kind of regarding the VCO is whether I should try to
>cram in a simple sub generator using the square output. Would this be as 
>simple
>as feeding the square signal to a counter and get the half (one oct. down)
>and quarter (two octaves down) frequencies from the counter? I realize
>that the counter would output a symmetric square regardless of the
>duty cycle of the square/pulse signal input to the counter, but that's
>okay. Since there's only one VCO planned for the synth so far, I want that
>one VCO to be as "fat" as possible, so a sub-generator would be welcome.
>
>  There's going to be two different VCFs on the synth, so one would be able
>to filter fundamental and sub frequencies separately but have the filters
>track each other and/or mix VCO waveforms ahead of the filters etc. for a
>maximum variety of timbral options.
>
>  Just looking to build as much sonic potential as possible into this
>nearly bare-minimum one VCO synth...
>
>  I'm not primarily thinking about playing the synth from a keyboard, it
>will typically be used as a "CPU" with a lot of peripherial signal
>sources, CV-generating sensors (pedals, photosensors etc.) to be able to
>modulate and add life to the various old Knob Controlled Oscillators
>(KCOs) that are piling up in the "lab"...
>
>  Also would like to know if there are any designs out there for a
>Theremin-style controller that will output pitch and volume CVs instead of
>(or in addition to) audio signals.
>
>Enough for now...
>
>Thomas Dunker

         -Jim
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