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

[Oren Leavitt]

Hi Thomas,

Some options for the tempco:

1) You can get away with using an ordinary 1K 1% resistor in place of the tempco if your use for the VCO doesn't require precise tracking - especially if you're using just one VCO in your synth.

2) You can get the PT-146 at:
http://www.precisionresistor.com
Just request a quote and they'll email you a price.
The PT-146 was USD 7.50 each the last time I ordered a small batch in October 2003.

3) The CA3046 can also be tempco'd by thermostatically heating it as outlined National Semiconductor ap note AN-299 using the other three transistors + an op amp (if they aren't already being used).

4) The KTY 81-120 will work if you add a resistor in series with it to buck the 7500 ppm down to about the 3350 ppm the VCO needs. You'll need to increase the values of the input resistors at the CV summing node, too.

5) There have also been some 'tempco-less' VCO designs discussed on the List in previous threads.


For the JFET - The 2SK... devices will probably work fine with Ray's VCO.
For sub-octaves, a 4013 dual flip-flop will give you the two sub octaves with one chip. Feed it directly from the output of the pulse comparator thru a diode to cut off the negative half of the signal so that it is within the 0 to V+ range of the CMOS.

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