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AM2400 Module - Eµ Noise Source
Overview This
module is based on the Noise Source module in the Eµ Modular. It produces random audio and control signals. Noise colour is variable and there is a Slow Random output for controlling other modules.
Circuit Design
Dave Rossum originally designed this module way back in March 1973. It was upgraded a number of times and I have based the clone on the Revision 3 schematic which uses digital techniques to create pseudo-random noise. I suspect
the original version used traditional analog noise generation techniques (transistor or zener diode in avalanche mode).
A digital approach to noise generation avoids the use of high gain op amps (which can cause
crosstalk). However digital noise generators can sound predictable, as the output pattern repeats itself every few minutes. For example the dedicated MM5837 chip used in the Audity has an audible and short cycle time. This chip was
also used in the MemoryMoog but replaced with the improved MM5437, which has a longer cycle time. Modern digital noise sources are much closer to being perfectly random
Dave's design is a simple circuit based on a CMOS clock and
two 18-bit shift registers. The final summing Op Amp is the unusual 556, see "AM Parts" in the sidebar for details.
The spectrum control varies the energy distribution of the audio signal from White through Pink to low filtered
noise. The Slow Random Noise signal varies from 1 to 10Hz. The audio filtered noise goes via a simple -6dB slope low pass filter. Rather than use a dedicated -3dB filter to create Pink Noise, Dave achieves the same result by mixing
White Noise with -6dB filtered Noise. Other designers such as Tony Allgood have taken a different approach and created dedicated Pink Noise filters. These are often more complex designs which achieve an accurate -3dB slope with minimal
ripple.
ARP 2500 Influence The Eµ design is a bit bland with just one control knob, so I have taken some ideas from the ARP 2500 Noise module (1016) and added them into the AM2400.
- Spectrum control controls MIX output
- Toggle switch selects Pink or White on Noise output
- Separate Slow Random Noise output
- Red LED flashes at the rate of the Slow Random Noise
- Volume control for the Slow Random Noise
These extra features means there is some more circuitry in the module, including a dedicated -3dB Pink Noise Filter. This filter uses shelving filters from
the E-mu Systems Audity.
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