AM1036 Dual S&H

ARP 1036 Module

Overview The AM1036 is a replica of the ARP 1036 Dual Sample & Hold from the ARP 2500 designed in June 1970. This is a pair of sample and hold circuits with clock oscillators and noise sources included.

The module is feature rich, having external voltage control of the clocks, trigger or gate external clocking, manual sampling, individual clock outputs and the ability for the second sample and hold to be driven by both clocks. This is a really interesting and complex module for 1970.

AM1036 Module I originally recreated the schematic in Eagle CAD in April 2005 but this version never made it to a prototype stage. The problem of sourcing 1/8″ shaft potentiometers at a reasonable price and finding the correct ARP style knobs put the project on hold until 2015.

The AMSynths replica uses the same ARP circuit but with some modernisation, as these are clock circuits that needs to be precise and tight. Metal film resistors have been used along with modern ceramic capacitors and 2N390X transistors. The original metal can tantalum timing capacitors have been retained, and polypropylene capacitors have replaced the polycarbonate versions in the clock and the hold. LM301 Op Amps have been retained and high quality LS4392 and LS3954A  JFET’s from Linear Systems, as they have fast 10ns rise times.

Noise Source This basic circuit is a transistor in avalanche mode, with a LM301 Op Amp to boost the level with very little filtering, It generates a raw white noise into the sample and hold of 10V peak to peak. There is a trimmer and front panel potentiometer to set the noise level, and the “noise” 2N5172 transistor is selected for maximum noise level. The noise source is internal for creating random sample and hold waveforms and there is no external output.

AM1036 Module

Voltage Controlled Clock This is a uni-junction relaxation design that produces a sawtooth waveform which is converted into a narrow 12us pulse waveform to create the sampling.  The clock frequency can be voltage controlled by an external control voltage. Each clock has a wide 100:1 frequency range; from 10Hz to 1kHz in x10 mode, and from 0.1Hz to 10Hz in x0.1 mode. I have upgraded the individual 2N4248 transistors to a Linear Systems matched PNP pair, you could go further and use a 1K87 tempo in place of R7.

The second S&H can be clocked from both clocks, which makes for some interesting patterns. ARP created not only a dual cross coupled sample and hold but also an amazing fast sample frequency of 1kHz which can trigger other modules. An amazing modulation source!

Sample & Hold This circuit starts with an buffer Op Amp using the LM301 with complementary output driver transistors pushing the signal into a LS4392 FET and a 15nF hold capacitor. This is then buffered by another LM301 Op Amp with the a dual matched JFET 2N3958 to ensure the hold voltage does not leak out.

Trimming As usual on the ARP 2500 modules there is a bit of component selection and trimming to be done;

  • Pulse width using a selected capacitor.
  • Output offset adjusted by a trimmer.
  • Overall audio signal gain set to unity by resistor selection.
  • Noise level adjusted by a trimmer.

Build & Outcomes In 2015 I restarted the AM2500 project and laid out the circuit on a 150 x 150 mm PCB , and ordered it in September 2015, and then populated the PCB with parts. Whilst the clock and noise worked ok, the sample and hold didn’t and if I recall correctly some transistors got hot. After tinkering around and checking the schematics many times, I gave up and went off at a tangent. I designed a PCB using LF398 chips as I was convinced I couldn’t get the original circuit working.

In 2019 I restarted the 2500 Project with sufficient time to complete it! I went back to the 2015 PCB determined to fix it! The clock worked fine, as did the noise source, although the source was only putting out 4V. The sample and hold circuit provided very tricky and after a lot of component replacement and swapping I could not get it to hold a voltage. I have reverted to the very similar ARP 4015 sub module which works well.

In March 2019 I ordered a revised set of PCB’s with 4015 sub modules laid out onto the main PCB and a Roland noise source that uses the famed 2SC828R transistor as used in the TR808, which provides a lovely strong 10V noise source. I don’t often give up on the original designs but just once I think I have created a better module in the AM1036. The PCB’s were built up in early April along with a nice front panel.

Copyright AMSynths 2017