AM8125 – 100M VCO

AM8125 Module

Overview The AM8125 module is a clone of the VCO which appeared without Sync in the Roland 110 module and as a  Dual VCO with Sync in the 120 module. It is this later circuit I have replicated (as a single VCO) but with some significant changes to the VCO core. This Roland VCO design originates in the analog modular synthesizer System 700 (1976), and has been used with minor changes in the SH-2, Jupiter 8, System 100. The VCO core is based on a heated UA726 dual transistor chip driving a capacitor based integrator that produces a sawtooth waveform.  My circuit is an improvement over the original design with a number of modifications to improve frequency stability and tracking.

Square Wave

Description  The dual transistor exponential generator has been retained but a modern matched BJT  is used (SSM2210, LS318) rather than the impossible to find UA726. The frequency control circuit has been modified to use a standard 1k Tempco resistor, and the octave switching has been retained using 0.1% resistors.

The high frequency trim circuit is based on the latest ideas from Rene Schmitz – which is another level of accuracy. The original Roland VCO was stable, thanks to a +/-10V reference power supply driving the core and +5V as a reference voltage for the comparator.  I have upgraded the design using high precision voltage reference chips and op amps. This further improves the stability of the power to the core and therefore the frequency.

The original wave shaping circuit has been carefully replicated using European transistors  rather than Japanese. The VCO produces the usual wave forms; sawtooth, square with pulse width modulation and triangle. There is a sync facility with Strong and Weak settings.

Module Outcomes & Availability  The AM8125 Single VCO was released in 2014 as a completed module, as well as featuring in my Roland 100M replica. A slightly revised version was available in 2018 (different control knobs). Behringer released its own copy of the 100M 110 and 112 modules in 2020, which means I have no plans to manufacture the AM8125 modules in the future.

In 2023 AMSynths launched a replica of the prototype Roland 100M 111 module which is a combined VCO and VCF module. The new AMSynths design is a V3340 based VCO and a V2044 based VCF.


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