AM1045 Synthesizer Voice

ARP 1045 VC Voice

Overview This is a replica of the ARP 1045 Voltage Controlled Voice from the ARP 2500 Analog Modular Synthesizer. The module consists of three circuits in one module; 1004 VCO, 1006 Filter Amp and two 1003 ADSR’s. This is a very high density design which delivers a complete synthesizer voice in one 5U panel, the pinnacle of the 2500.

I am building two 1045’s and installing them with a 1027 sequencer and 1047 UAF into an Oak 6U Cabinet made in France. The bottom 1U is a power supply, along with a dual ARP quantizer, DIN Sync, and a Doepfer A160-5 Clock Ratchet. There is also an audio output stage and mixer with balanced XLR sockets.

1045 Voice Architecture

AM1045 Module My design uses the original ARP schematics, laid out onto three large 150 x 150 mm PCB’s. Metal film resistors are used, with no attempt to use legacy carbon composition resistors. The potentiometers are high quality CTS 270X models with 1/8″ shafts. These are PCB mounted which means the 3 “cards” are mounted straight to the panel with no metal sub assembly, which makes construction faster. Plastic M3 PCB stand offs hold the cards together as one unit.

I have used NKK MRA-112 1/8″ shaft rotary switches set to 5 ways for waveform selection, again rather expensive. They are available with coloured knobs which helps break down the mass of silver knobs on my 1045 modules, as I don’t have any coloured potentiometer knobs left.  A simple DPDT toggle switch turns the VCO on and off, with indication on a 5 mm LED and red lens.

Controls The module has the following front panel controls:

  • Fine Frequency
  • Frequency Range over 8 octaves using an 8-way rotary switch
  • 5-way rotary waveform switch
  • Toggle switch for VCO on/off with LED indicator
  • Pulse Width, FM1 and FM2 levels
  • Env Amp, Delay, Attack, Sustain, Release for VCA
  • Env Amp, Delay, Attack, Sustain, Release for VCF
  • Filter Cutoff and Resonance
  • VCA Gain and Mode switch
  • Ext CV amount to VCF and to VCA

Connections There 6x 3.5mm jack sockets mounted on the panel:

  • FM1 and FM2 inputs
  • OSC output
  • Gate and Trigger inputs (with LED indication of Gate)
  • Ext VCF and VCA inputs
  • Env1 and Env 2 outputs
  • VCA Output

Module Build The five circuits were laid out on three 150 x 150 mm PCB’s in Spring 2017, along with the smaller 30 x 30 mm sub module PCB’s (4001 and 4002), so that I could encapsulate these exponential converters at a later stage. I originally designed the 4001 and 4002 sub modules with THAT transistor arrays, but these did not work well, and I replaced them with Linear Systems dual transistors (LS120A/LS130A) which work perfectly.

The VCA, VCF and ADSR’s went into build and were tested in 2017 (and then again in 2019). Some minor corrections to the ADSR’s were needed, due to my mistakes in transcribing the schematics. To enable the pots to be PCB mounted I separated the VCA and VCF, which results in a long cable between the two with a low level signal. This was not a good design and it picked up noise, so the REV02 PCB’s added Op Amp buffers at the VCF output and the VCA input.

Simple so far, but the 1004 VCO is more challenging and a new design to AMSynths. It requires 100R Tempco resistors, which are tricky to locate, and put me off building one for many years. In 2017 I tracked down modern Panasonic 3300ppm SMD 100R and 220R (1047) tempcos – which are now sadly obsolete in 2018, rather than the original wire wound versions. I started to build the VCO card in 2017 and it was all starting to get exciting, but the project was paused through out 2018 whilst I moved into a new workshop, and I eventually restarted the project in February 2019, along with building the 1027 Sequencer and bought a 6U Oak case from France.

2019 Outcomes The 1004 VCO was the final 1045 PCB to be populated and tested, and I knew this was going to be a tricky module to get working, and there is a detailed explaination here. However after much hard work on 20 February 2019 I had at last got the VCO prototype circuit working perfectly, so I could order a production set of PCB’s (REV02). I reworked the PCB layout now that I understood the integrator and used modern Op Amps to gain as much stability as possible. The aim of the AM1045 is to achieve the highest quality rather than the retro perfection I have sought in most of the other 2500 modules.

On 21 March I completed the final setup of the VCO voltage scaling and achieved accuracy over the first 5 octaves and a stable frequency once warmed up. The high frequency trim does work but only compensates a few Hz, which is not enough above 1kHz. I have changed the frequency setting on the 1045 VCO to use octave range switching with 0.1% precision resistors and a reduction in the range of the fine control from two octaves down to one, this is due to the small size of the control knobs making it very hard to accurately set frequencies.  The octave switching also provides an on-board means of setting the scale accurately.

2019 REV02 PCB’s All three 1045 PCB’s needed to move to a REV02 version, with the VCO ordered in late March, and the VCF and VCA ordered in October. The REV02 VCO was built in October and adds a 8-way rotary switch for octave selection with an array of 10k 0.1% resistors and a precision voltage source (REF01). The VCO uses 4001 and 4002 expo generators which have the SMD 100R Tempco resistors on them (I moved some from the main board).

The VCF and VCA boards now have Op Amp buffers and the correct JFET pin outs. The VCF now uses tightly matched AS194 dual transistors and matched 100nF ceramic ladder capacitors. The VCA continues to use THAT quad matched transistor arrays, as they work well in the current mirrors.


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