The Serge noise module can be built in several ways. In it's simplest form, it contains three basic outputs, White noise, Pink noise, and a random voltage suitable for use with Sample and Hold modules. An optional Sample and Hold circuit is included on the PCB. Alternatively, the basic version may be coupled with the CGS92 Serge Smooth and Stepped Generator to make the Random Voltage Generator module.
It will work on either +/- 12 volts, or +/-15 volts by installing on-board regulators.
A little on how it works:
The schematic for the noise source.
Note that the orientation for the transistors on the overlay is for vintage 2N4250 and 2N3565. The pinout of modern PNXXXX devices is usually the reverse of what was used with the 2NXXXX devices. In this case, BC557B and BC547 variant can be used as direct replacements for the vintage transistors, or PNXXXX devices can be used, but installed in reverse with respect to the transistor outlines shown on the PCB. Whatever your choice, the tiny transistor schematic symbol within the "D" is correct and can be followed.
If you are sourcing your own parts, it is best to install a socket for the 2N4250 transistor marked N (for noise). You will need to try multiple devices to find one that is sufficiently noisy. Boards purchased from me will include a hand-selected PN4250 which can be installed with no further testing, in reverse to the D symbol on the PCB.
If you do not require the S&H, everything to the right of "D", with the exception of the 47uF capacitor, can be left off the board. (Viewed with the Power connector to the left.)
The LM or CA3130 is a very sensitive device. Use proper CMOS handling precautions when installing it.
The resistor marked 100k* on the PCB is not needed unless you are experimenting with isolating the S&H from the noise circuit. If a resistor is inserted between the S&HI pads, the resistor in the 100k* forms the lower leg of a voltage divider with it. The pair of resistors can then be used to adjust the range of the voltages being fed into the S&H.
The behavior of this circuit depends very much on the gain of the transistors used. It is possible if you have too much gain, the noise waveform will clip. A true RMS multimeter between 0V and output A (White Noise) will read between 1.3 volts and 1.69 volts if things are working correctly. Higher than 1.69 volts and clipping is likely. Lower than 1.3 volts, will merely mean you have a quieter/weaker output.
The tricky part comes in setting up the S&H. Connect output E or F to a VCO, scaled so you can listen to the output range. Connect an LFO to input G. If you find the VCO frequently playing the same low note, you are sending the CA/LM3130 to it's upper limit.
To deal with this, separate the circuits by cutting the short track between the two S&HI pads. The S&HI pads have an X between them on the overlay side to indicate where the cut should be made on the rear of the PCB. Connect the C output to the S&HI pad nearest pin 3 of the CD4007. You can then adjust the center voltage of the incoming waveform by installing the resistor "RPD".
This is a guide only. Parts needed will vary with individual constructor's needs.
Article, art & design copyright by Ken Stone