Components
For general information regarding where to get parts and suggested part numbers please see our
useful Parts Guide at the project webpage or http://www.oakleysound.com/parts.pdf.
Some special considerations for this project
C14 is a close tolerance (2% or better) polystyrene capacitor or better. The PCB is laid out for
EXFS/HR series made by LCR. Farnell and others sell these. They are good components at a good
price.
The matched pair, THAT300P, can be difficult to trace, although Farnell and ourselves do carry
stock.
U2 and U3 should be low offset bi-polar dual op-amps. The LT1013C is to be recommended for
both its performance and cost. I used to recommend the OPA2277 but we now know of one case
where a particular batch of OPA2277 devices caused oscillation to the +5V reference voltage
The LM1458 is a real oldie, and some places don’t stock it any more. In the UK, Farnell and RS sell
the little chap for peanuts.
The LM13700N may be substituted with either the older 13600 (still made by JRC) and the well
and truly defunct Phillips’ NE5517.
The positive temperature coefficient (PTC) resistor is worthy of a mention. Meggitt make a
+3000ppm/K type that is a very good part. However, the actual theoretical value of compensation
should be closer to 3500ppm/K when used in the circuit that I have chosen. The Meggitt
part thus falls slightly short of this, and may cause the VCO to lose ‘V/oct’ tracking over the full
keyboard range unless the ambient temperature is pretty stable.
It is therefore preferable to get something closer to the ideal value. The ones I recommend are 1W
wirewound resistors with a nominal TC of +3500ppm/K. You can buy these from us, KRL-Bantry
in the US, or from Senso's Vintage Planet (www.senso.dds.nl/index.html). These are sold as
equivalents to the old Tel Labs Q81 series. Once fitted to the VCOs, they give better tuning stability
and allow the modular to ‘warm up’ quicker. They are also physically larger than the Meggitt type
which means they are more stable in air drafts too.
The PTC resistor is fitted to the board so that it lies across the top of the THAT300 NPN array. A
bit of thermal compound (eg. Thermpath) could be used to keep the two devices in contact. You’ll
need to carefully bend the legs of the TC to make sure it lies flat on the surface of the THAT300
and still fits through its solder pads.
The JFET is the J112, this is very common and is the same FET die as the original MiniMoog. But
take note, the Mini used the E112, and that has a different pin out.
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Summary of Contents for VCO 5U
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