for music synthesizers.
This module is a very standard pseudo-random digital noise source with a few enhancements. Instead of running a fixed high frequency clock, a VCO is used instead, allowing for unusual sweeps, and for reducing the speed right down to a series of random pulses. The internal linear VCO can also be bypassed so an external source such as a 1V/Oct. VCO or LFO can be substituted. It has pink and white noise outputs, and two separate (unique) digital outputs for use in triggering other circuits.
How to use this module:
Connect the CV inputs to a voltage source, and wind the speed input up to maximum. Take the required noise source from the appropriate output. The two LEDs indicate the activity of the two digital outputs. When an external CV source is used, the speed pot acts as an input attenuator. Adjust this pot for the desired effect. Alternately, feed an alternate clock pulse in to the external clock input.
A little on how it works:
The schematic of the Digital Noise.
The circuit consists of several distinct blocks. The first is the internal VCO and its associated parts. From the left, there is a voltage follower then the VCO itself, which is part of a 4046 phase locked loop.
The second circuit block is the timing generator. At the left, there is a comparator based around one half of a TL072. This is there to convert any external clock signal into something suitable for the remainder of the circuitry. With the values given, the sensitivity is set at around 2V, allowing triggering from the variety of signals common in modular synths.
The output of this comparator is fed into the clock input of the shift register, a 4006.
The exclusive OR gates form a complex feedback path and are responsible for the generation of the pseudo-random pulse train. The 1M and 0.47 mfd capacitor are there to start the sequence. A 1uF can be substituted if you are unable to obtain a 0.47uF electro. Reduce the 1M to 470k to compensate.
Several outputs are taken from different points of the feedback path, two being buffered use as digital outputs, the remaining being filtered to produce both pink and white noise outputs.
There are many similarities between this design and the ETI 4600 noise source, as this was developed from my heavily modified ETI unit.
Before you start assembly, check the board for etching faults. Look for any shorts between tracks, or open circuits due to over etching. Take this opportunity to sand the edges of the board if needed, removing any splinters or rough edges.
When you are happy with the printed circuit board, construction can proceed as normal, starting with the resistors first, followed by the IC sockets if used, then moving onto the taller components.
Take particular care with the orientation of the polarized components, the electrolytics, diodes, transistors and ICs.
When inserting the ICs in their sockets, take care not to accidentally bend any of the pins under the chip. Also, make sure the notch on the chip is aligned with the notch marked on the PCB overlay.
Make sure you insert a 100k resistor between the +15V supply and the normalized connection on the CV jack. This is because +15 will momentarily be fed out of this jack when you plug something into it, due to the mechanical nature of the contacts. There is no provision of the circuit board for this resistor. For those using banana jacks, simply connect the 100k resistor between the +VE rail and the CV input. Any signal present on the jack will swamp the current from the 100k, effectively removing it from circuit. The 100k pulldown resistor at pin 3 of the TL072 should be omitted.
There is provision on this PCB for an additional 7 MOTM style power connectors, to allow this board to be used as a distribution hub for nearby modules. Two links are provided to allow for insertion of resistors should greater isolation between the noise circuitry and the other modules. This is a noise source after all - probably not the best host for a power distribution point! Should you add these resistors (10 to 22 ohms), the eighth connector (nearest the circuitry) will also be part of the isolated section. Do NOT use that as your power input. You may find it a suitable point for connecting another noisy module. There is also provision for two extra decoupling capacitors (10uF). Neither is marked on the overlay. Take care with orientation if you install them.
To use the wire anchor holes:
This is a guide only. Parts needed will vary with individual constructor's needs.
If anyone is interested in buying these boards, please check the PCBs for Sale page to see if I have any in stock.
Article, art & design copyright 2001, 2010 by Ken Stone