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AM8706 Module - Roland 706 LFO
Overview This module is a
clone of the Roland 703B 2-pole State Variable Filter that was part of the Roland 700 analog modular syntheszier. This is a traditional 2 stage OTA based design first seen in the Oberheim SEM in 1974. Roland's version is very similar
using CA3080 chips, LM301 Op Amps subsequently upgraded to the CA3140 CMOS Op Amps that became available in the late 70's. The design has adjustable Cutoff Frequency and Resonance, along with multiple signal and cutoff frequency
controls inputs. There are the usual Green and Red LED's for signal and overload monitoring.
Specifications
Frequency range: 0.01 Hz to 30 Hz Output: 10V peak to peak Control input sensitivity: 1 volt per octave Waveforms: Sawtooth, Triangle, Pulse variable rectangle and Sine Waveform reset on keyboardgate LED monitoring of output
AM8706 The AM version uses BA6110 OTA chips, simply because I have
them in stock and they are a better component than the CA3080. The circuit remains identical, with 3 signal inputs and 2 control voltage inputs. Filter response is selectable with a three way slide switch. Roland had made a nice
improvement on the SEM design by using a dual potentiometer for the Resonance control, this enables the volume of the filtered signal to be boosted when Resonance is turned up.
The module has the following front panel controls:
Signal Input Level (x3) Filter Response (LP, BP, HP) Frequency Resonance CV Level (x2) - this will be named KBD and MOD Red and Green LED's for signal and overload monitoring
There are six 3.5 mm jack sockets mounted on the left hand side of the panel, these are:
Signal Inputs (x3) VCF CV Inputs (x2) Signal Output
Outcomes The prototype PCB was designed in 2009 and I eventually got round to laying it out and buying the dual 50K potentiometer in March
2010. I am now waiting for it to be manufactured.
The front panel picture is of the prototype mounted to a PCB front panel and white card overlay. I use this approach to finalise the panel design before having an expensive
aluminum version manufactured.
Front Panel The front panel is a 3" wide FracRac with Alpha rotary potentiometers, and push fit mixer style knobs.
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