ARP 400X Sub Modules

ARP 4001 Sub Module

Overview The ARP 2500 used a variety of potted sub modules to provide stable exponential generators and current mirrors. They typically contain matched transistor pairs and wire wound Tempco resistors. in a potted plastic container. Initially in the 1006 filter I put the 4001 circuit onto the main PCB and used a metal film resistor instead of a Tempco, and the circuit worked well as stability is not an issue in a low pass filter.

For the 1045 and 1004 VCO’s I wanted to use a more stable design suited to oscillators, and eventually by 2017 I had located SMD 100R Tempco resistors from Panasonic which avoided custom wire wound resistors and more expense. I created both the 4001 and 4002 sub modules, and used the 4001 in the 1045 filter and 1027. I also created the 4005 sub modules for the 1047 as I had located 200R Tempco resistors in SMD as well. Whilst less critical in the UAF, the potted sub modules and Tempco resistors provide extra stability in a precision filter.

P65 Op Amp Module

History The potted sub module goes back with Alan Pearlman to 1962 and the Boston USA company that he founded with Roger Noble, Nexus Research Laboratory, Inc. This was the early days of discrete Op Amps (like the P66) which Nexus built into modules for printed circuit board mounting, pioneering the use of rectangular, potted module packages. Many electronic companies were building circuits into modules in the 1960’s before silicon chip fabrication techniques at lower costs was available.

The sub modules used a standard square outline, with pins centered at 0.2″ in a 7-pin footprint in a 25mm square plastic box. The pin-out was extended by ARP to 9 pins and larger 1.25″boxes, with output/power on one side, and input pins on the opposite side. The sub modules in the 2500 are a design dating back to the early days of analog electronics and 1962. Later ARP sub modules for the 2600 expanded to 2″ square boxes and contained more circuitry, only the 4014 “large” sub module is used in the 2500.

The potted sub modules all have labels on the top describing what they are and the ARP patent they use. In the early Tonus days up until early 1973 they were black with white labeling and a plain ARP logo, they then were white with black lettering and the new ARP clef logo.

Hybrid 107C

Third Party Modules In the early days ARP used a third party 107C ring modulator module from Hybrid Systems Corp, based in Burlington, Boston. Hybrid were one of a number of small electronic companies in the Boston area, and they specialised in ADC and DAC modules through the 1970’s. The module is correctly labelled as an analog divider/multiplier.

This “off the shelf” third party module was used to speed up the development of the 1005 module and it was later replaced with the ARP 4014 module in 1971 as part of the 2600 project. The 4014 is functionally equivalent but unlikely to be the same circuit. The pin out positions changed and external X and Y trimmers were added which resulted in a PCB revision to the 1005.

Tempco The original ARP 4001 and 4002 use a 100 ohm wire wound Tempco, and the 4005 current mirrors use a 200 ohm version. I was fortunate to locate Panasonic SMD 3300 ppm Tempco resistors in both 100 ohm and 220 ohm.  Initially I could buy these in 0805 size, but once they became obsolete in 2018 I could only locate 0604 sizes. I have bought a small stock of these SMD tempcos for future use.

ARP 4001 Diagram

Matched Transistor Pairs The sub modules use a variety of PNP, NPN and NPN/PNP matched transistor pairs. Initially I used THAT300/340 in SMD format but the low gain of 75-100 Hfe did not always work once in the main circuit, even though the original TZ81/TZ581 have a gain of 60-100 Hfe.

I switched to Linear Systems LS120A/130A which have a higher gain of 200 Hfe and are matched to 1 mV. These have worked consistently well, and the sub modules include standard 2N390X transistors, where matching is not required. Resistors are metal film and mounted vertically to save space.

PCB & Pots The AMSynths 4001, 4002, 4005 pot boxes are 30mm square and 20mm high, which is a bit too high, but this does create more thermal mass and they are readily available. I designed a small 28 mm x 28 mm PCB for each sub module which slips neatly into the box, and used small mounting pins so they can be soldered straight into the main PCB. I test each sub module with flying leads before soldering into a module PCB. I will use pins and sockets in any production modules.

Potting and Stickers The sub module are carefully potted using Rabno PX700X black resin. I designed a set of stickers to go on the modules in Adobe In Design and had them printed as high quality plastic labels.

Build and Outcomes The initial prototype sub modules were built in 2017 and were not potted, so I could develop the circuit and ensure they worked. The production sub modules were created in March 2019 and have been fitted to all the AM2500 modules, replacing the prototypes.

AM4001 PCB

REV02 PCB’s In October 2019 I designed a REV02 version of the 4001 and 4002 PCB’s that make the potting process easier. The PCB drops into the bottom of the potting box, which can then be filled with compound. The PCB’s are available on OSHPark for anyone to purchase.

  • AM4001 VCO EXP-N is here
  • AM4002 VCO EXP-P is here

The PCB’s use 0603 SMD 100R Tempco resistors (still available in Autumn 2019), and nine 19mm length Harwin H23171-01 pins which are available from Mouser here. They both have dual Tempco resistors which avoids having one of them on the main VCO board. They work with the AMSynths 1004 VCO, and 1045 VCO PCB designs ONLY. The AM4001 can also be used in the AM1027 and AM1006 by leaving out one transistor and Tempco in the build.

2024 Update I developed a Eurorack version of the 1047 filter in the Spring of 2024, and created a full SMD version of the 4005 sub modules. There are two versions; 4005N and 4005P both fit within an 8-pin DIL footprint, and they are equivalent to a 1N and 1P. The modules are augmented with additional transistors on the main PCB to create 2N, 3N, 2P and 3P configurations. This helps reduce the size of the 1047 circuitry for a Eurorack module.

Copyright AMSynths 2022