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Analog Metropolis Design Tips
VCO Voltages,
Overview
This page has a few Synth DIY tips, based on my experience of recreating classic analog synthesizer designs. Enjoy!
VCO Voltages
The core of a VCO is very important and it needs to be designed for frequency stability and accurate frequency tracking. One key area that is sometimes overlooked when recreating old VCO schematics is the reference voltages used in the core and by the front panel frequency controls and on board frequency trimmers. This is a separate design issue to temperature stability, but an accurate volatge regulator brings the added benefit of a temperature stable voltage.
A typical VCO will need +5V for the comparator and +/-10V or more for the frequency controls. The voltage created by a voltage regulator or voltage reference chip will drift due to its (in)ability to manage the line voltage to a
target (say +5V) and due to temperature changes. A typical 1mV drift in the power supply translates straight into VCO frequency output drift, perhaps as much as 1 cent. This doesn't sound much as its within the "inaccuracies" of an even
tempered scale. However remember this drift happens every few seconds (usually up then down) and it DOES make a big difference when you are trying to tune VCO's together. A 1 cent drift translates into VCO's that will beat like mad, and
force you to kick in the Sync switch. Modern components mean we can push this drift down by a factor of 10 or more, for under £10 per VCO. One simple way of finding out whether your VCO supply is accurate is to meaure it with a 4.5 digit
volt meter, if its drifts by 1mV or more then you can improve it..
A precision VCO cannot be run from standard 3-pin LM7XXX voltage regulators, these devices are okay for producing the basic voltage rails in an analog synthesizer
but they are too unstable to directly run an accurate VCO from. In the 1970's the LM723 was an excellent performer giving 1mV drift on hand selected chips. It was therefore used in a variety of analog synthsizers including the E-mu Systems
Audity (along with a LM399 to give some added temperature stability) and it pops up in older DAC designs too. However IMHO this cheap chip is not sufficient for a modern VCO. :-)
 The next step up is a dedicated voltage reference chip such as the Analog Devices or Maxim REF01 with a typical price in
the UK of £2.50. They are often used as a reference voltage for DAC chips. However these don't outperfom the humble LM723 by very much and they should be ignored for modern prceision VCO designs. What we need is a precision voltage
reference chip such as the;
- Maxim MAX6175A (SMT only)
- Linear Technology LT1019A
- Linear Technology LT1027
The 1027 has better temperature stability and is the one I use. You'll usually need to buffer the output with a low drift Op Amp. A
dual Op Amp will also create the negative voltage reference. These chips are not cheap - typically £7.50 each, but the results are
very audible. The E-mu Systems VCO that I have recreated uses these precision voltage references and it performs as well as many modern VCO designs, except of course for the legendary MOTM VCO...
VCO frequency and temperature accuracy needs to be approached holistically, so its important to use precision components in other areas of the VCO circuit such as:
- 0.1% 15ppm resistors in critcial areas such as the control voltage inputs up to the expo generator
- Low drift Op Amps
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