AM8150 Module - Roland 100M LFO & Gate Delay
Overview This module is a clone of the Low Frequency Oscillator in the Roland 100M analog modular synthesizer, along with the Gate Delay module. This is a triangle core based VCO using a BA662 OTA chip, followed by waveform
shaping circuits and then a second BA662 OTA chip to provide the delay capability. There are 5 LFO output waveforms:
- Sawtooth Up
- Sawtooth Down
There is a main Frequency control, plus a Frequency Range switch and slide switches for full or 10% Output Level and Keyboard Triggering On/Off. There is a
LED indicating frequency and 2 LFO outputs.
AM8150 Circuit The circuit is the same as the original with the same Japanese transistors but the following upgrades:
- TL07X Op Amps
- BA6110 OTA chips
- 1M log pots instead of 2M log - simply due to availability
- Frequency Modulation potentiometer
I designed the module across 2 PCB's. The first contains the core LFO and the second contains the Delay circuit and the Gate Delay. The circuit is
relatively easy to put together and it works well. The LFO range is
The module has the following front panel controls:
There are four 3.5 mm jack sockets mounted on the left hand side of the panel, these are:
LFO FM Input
Gate Delay Input
Gate Delay Output
Outcome The LFO circuit went together well with the newer BA6110 chips, and the waveforms are accurate. However the frequency range was
rather limited, as I could get a maximum of 30 Hz but only around 1 Hz as a minimum. A simple kludge of adding an extra 220nF timing capacitor on the Medium and Low ranges to the circuit enabled a much lower frequency of 0.02 Hz to be
achieved. The adjustment within any range does not overlap very much and the both high and low frequencies can be produced. Production PCB's have this modification as standard.
The Gate Delay worked perfectly and provides delays up to 30 seconds.
Front Panel The front panel is a 3" wide FracRac with Alpha rotary potentiometers, with 15mm diameter plastic mixer knobs.