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AM8027 Module - ARP 4027-1 VCO
Overview This is a clone
of the ARP 4027-1 VCO that was used in the ARP 2600 as well as the basis for the VCO's in every other ARP synthesizer from 1972. This replication has been carried out with care and detailed research, including borrowing a recently
serviced ARP 2600 to test out the capabilities of the VCO design, to ensure I could achieve or even improve upon the original ARP design.
Original Circuit The ARP 4027-1 VCO is a standard sawtooth core using a
capacitor and integrator. However it uses a NPN and PNP exponential generator with unmatched transistors. This design flies in the face of the standard dual PNP well matched transistior pairs (Moog, Oakley, MOTM), so let's see how it
works out...!
The 4027-1 is an improved design over the original 4017/4027 VCO's, as it has a fixed High Frequency trim circuit to ensure the VCO tracks accurately over a wide range of frequencies - with no adjustment!
The
ARP 2600 used potted sub modules for the VCO core, and the rest of the VCO circuit is on the main PCB. Odysseys and Quadras had no sub modules and used the circuit it is entirity on the main PCB. The ARP2600 had no VCO sync facility,
but the Odyssey does. This VCO design has been everywhere and lived on for 10 years...
AM8027 Circuit I have replicated the original ARP electronic design precisely, with only minor variations.
On board precision +10V and -10V Reference Voltages The Hard Sync facility added from the ARP Odyssey Removed the low frequency option 2N4392 FET's 25 turn cermet trimmers Metal film 1% resistors 1% Polystyrene integrator capacitor instead of Mylar Waveform mixing and switching, with LED indication
The AM 8027 VCO has the following controls:
FREQ, FINE pots control frequency FM1 and FM2 pots modify frequency PW and PWM pots control Pulse waveshape Waveform switch selects Triangle, Pulse and Sawtooth waves 3x LED indicators
The design is built over 3 PCB's; The VCO core as a sub module, there is an Analog PCB for the rest of the VCO and some wave shaping, and a Digital PCB
with waveform switching and audio signal mixing. The VCO has a single output, all 3 waves (Sawtooth, Trianle, Pulse) can be mixed together or separately selected, this increases the timbral palette of the VCO.
Prototype Clone
My initial AM8027 design used a transistor pair in close proximity to the Tempco resistor, just like the Odyssey. This proved the oscillator worked, but the frequency stability was poor when compared with my reference VCO, a
modified E-mu Systems VCO. This instability caused significant drift over time and made the VCO unusable from a modern perspective. I conducted a number of experiments to see how to solve the problem (bear in mind there are no matched
NPN/PNP transistor pairs on the market):
- A non-matched SMT transistor pair - the BC857BS, worked as the expo generator, but the HF tracking was out, although the frequency stability did improve. I didn't want
to re-engineer the HFT design, so this approach was dropped.
- The 2N3904/06 transistor pair was tapped to the Tempco and some heat transfer paste was used. This improved the frequency stability, but tests showed that the VCO core
has a number of temperature sensitive components as well as the transistor pair; the integerator capacitor and a few resistors.
- The next experiment was to go back to the original ARP potted design, with a sub-module VCO core PCB that mirrors the layout of the ARP 2600, fitted to a revised
analog PCB. This potted approach proved sucessful, with a big improvement in frequency drift, and an accuracy approaching my reference VCO.
Sub Module Potting The original 2" sqaure ARP sub modules were potted with silicon rubber and a final layer of epoxy. In 1971 you could buy
the 4027 sub module for 95 US Dollars. From the mid 70's they stopped potting except for the 4012 VCF and instead relied on clipping the transistors together. The potting was done to improve frequency stability, and maybe protect the
design from competitors.
The AM8027 has a VCO core in a 50mmx50mm ABS case filled with thermally conductive epoxy, these puppies will not be serviceable!. I have followed the original PCB layout to a large degree and the
sub-module clips into the main PCB via 2x5way headers and not the original pins or layout.
Whilst the potting of VCO's has fallen out of favour, it is interesting to see the improvements it does make. Please don't build an ARP
VCO using anything else but this design, the basic 4027's on ebay are going to be instable and unusable.
LFO Option
Okay, so I dropped the LF option, but the PCB's can be used to build a very nice dedicated low frequency oscillator with the same waveshapes and mixing. A 15nF integrator capacitor is used and the frequency range reduces to 0.02 to 50Hz. In fact you can do without the potting...just this time!
Digital PCB This PCB does some pulse waveform shaping, but its primary purpose is to select and mix the 3 waveforms together. An 8-way rotary switch drives a 40147 CMOS decoder, which drives a 4053 analog switch and
3x LED's. The PCB is connected to the Analog PCB via a 10 way cable.
Technical Bits There are a few technical details worth knowing if you want to build one of these VCO's:
- The design uses a MOTM style 4-pin Molex power connector (+/-15V).
- The 2N5910 transistor is hard to locate, but they can be bought on eBay or from me.
- The 2N4392 FET's may be hard to firnd, PN4392's are equivalent and easier to find. The PCB uses TO92 packages.
- The CA3046 is still available but obsolete in DIL.
- The triangle wave shaper needs a 741 Op Amp. Even then there is a slight waveform glitch between the 2 halves of the triangle, at the top of the wave.
- The design uses a high quality 10V Voltage Reference Chip.
- The CMOS chips are HCF4053 and CD40147 (15V types).
- The rotary switch is high quality = expensive one!
Current Project Status The VCO is now at production status - with everything working just fine.
Panel and Pots
The PCB's are designed for Alpha 16mm pots on 1" vertical spacing and for fitting into a FracRac panel. I am building 2x VCO's and one LFO for my FracRac ARP 2600 clone.
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