AM8152 Ring Mod, S&H, Noise


Roland 100M – 150

Overview This module is a replica of the Ring Modulator, Sample & Hold and Noise generator in the Roland 100M (150 Module). It was built in 2009 with the three circuits on one PCB at right angles to the FracRac panel, held on with pot brackets over the rotary pots. The prototype needed a couple of fixes before production PCB’s were manufactured. The LFO from the Roland 100M was built into the AM8150 module alongside the Gate Delay from the 172 module.

In 2018 I started to redevelop the module as a commercial EuroRack product in 2018. However Behringer announced a set of 100M clone modules in 2020 and the project was cancelled. Let’s take a look at the Behringer 150.

Behringer 150 Module

Behringer 150  This module contains all four of the original circuits on the front of the PCB, interspersed with the hardware components. This makes for a very tight layout and makes servicing very difficult. The ring modulator uses the same 1496 chip in SMD and delivers the expected results. The clock oscillator for the S&H is a very simple Op Amp design and runs from 2.2Hz to 26.3Hz, as did the original.

The noise generator is a close copy of the original with nice bi-polar waveforms that are slightly lower in volume at ± 7.8V (white) and ±8.8V (pink) than the originals spec of ±10V. The Roland schematics have a well known error at R31 which is tied to GND rather than -VE, this cuts the waveform below 0V. Behringer have corrected this in their module, and the lower voltage levels is not an issue.

The LFO is a close replica of the original with only the trigger selection switch missing, and recreated on the rear of the PCB as a rather inconvenient jumper. With the jumper connected  a gate/trigger resets LFO phase and engages the “delay” feature, providing a fade in of the LFO output. With the jumper disconnected the LFO is free running. Behringer increased the value of the delay capacitor from 4.7uF to 10uF because they are using a 1MA potentiometer, rather than the 2MA in the original. The maximum delay time is slightly longer at 8 seconds, rather than 7 seconds as a result. Behringer also state a minimum time of 4ms, which is farer than the “0” in the Roland spec.

The LFO waveforms are clean and within tolerance of the original, both in terms of frequency range and voltage levels.

  • Sine is ±9.95V
  • Triangle is ±9.5V
  • Square is +10.1V
  • Sawtooth is +10.1V
  • Frequency range is 0.03 Hz to 26.3 Hz

The upper frequency is a bit lower than the original spec at 30Hz but this is not an issue.

The S&H circuit replicates the original  and produces high quality results, the “hold”voltage level shows no significant droop at very low frequencies below 1Hz. Using the LFO “down” sawtooth wave enables the S&H to clock down to low rates, the other waveforms are not as good at achieving this.

AMSynths Plans The Behringer 150 module performs very well, with no need for fixes or modification. This one is a keeper. There are no no plans to introduce an AM8150 module , however Maffez has documented a number of nice modifications to the Behringer 150 here.

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