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AM8069 Module - Korg DW8000 VCF & VCA
Overview This
module is a clone of the Korg DW8000 Filter and VCA, which is a 2 and 4 pole OTA VCF design with a total of 3 onboard input and output VCA's. My module uses the original NJM2069 chip that many Korg synths and samplers used around
1985/86, such as the DW6000, DSS-1, DSM-1 and Poly 800. The rich analog filter counter balanced the digital waveforms in the DW/DS range.
I have kept the design simple and harnessed the VCF and final VCA, but not the signal input
VCA's. This means it can fit in a 3" wide FracRac.
The NJM2069 chip is hard to locate, but I managed to find one new and another eight from a scrap Korg DSS-1. I recovered the chips by carefully sawing up
the PCB down to cuts between every 2 or 3 IC pins and then desoldered each set of pins. This means the chips can be removed with no damage. I also retrieved the unusual M5201 Dual Op amps which have an inbuilt analog switch, these are
used to switch between the 2 and 4 pole sections. I also harvested the VCF capacitors and front panel LED's.
AM8069 Module The circuit is based around the Korg implementation in the DW8000, but with the input VCA's
held at 7.65V. The power supply is +12V and -5V and is onboard to the PCB. Two audio signals are buffered and level corrected before going into the NJM2069, the filter cutoff frequency and resonance, as well as the VCA level are
individually controlled by 3 buffered and level corrected CV signals. The additional VCF signal input and Log VCA pins are not used.
The filter response is switched between 2 and 4 pole by a front panel push button, that
triggers a simple CMOS 4013 latch to drive the M5201 on and off, and to switch the original Korg red LED on and off. I have put protection diodes on all the NJM2069 pins that are exposed to the potentially higher 15V signals of a
modular synth, we don't want to blow these rare chips up!
The module has the following front panel controls:
Signal Input Level (x2) Frequency Resonance CV Level (Frequency, Resonance, VCA Level) Push Button - 2 or 4 pole LPF LED for filter mode
There are six 3.5mm jack sockets mounted on the left hand side of the panel, these are:
Signal Inputs (x2) CV Inputs (x3) Signal Output
Front Panel
The front panel is a 3" wide FracRac with Alpha rotary potentiometers, the original Korg LED's are re-used, and there are push fit mixer style knobs.
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