Noise Generators

Example:

Shin-Ei Volume/Surf/Hurricane/Wah

A bipolar junction transistor can make a great white noise generator.

Any diode reversed biased past its Zener voltage will pass current easily and this current will be noisy.

The amount of noise varies from device to device.

To use a bipolar junction transistor in this manner, use the base and emitter terminals and connect them so they will be reverse biased. For a NPN device, this means the positive voltage goes to the emitter, and the negative voltage goes to the base. The voltage must be high enough to exceed the Zener voltage of the device. This also varies from device to device. You don't want to connect the emitter directly to the positive voltage source. Instead, use a resistor of relatively high value (kΩs) to both limit current once the device "Zenes" and to create a useable voltage signal out of the noisy current.

Next, this noisy voltage source must be amplified. Using another discrete transistor is nice for minimalism, or an op amp is a good choice.

Troubleshooting

If you have an effect or synthesizer that uses a transistor noise generator, you should be able to scope the noise right at the emitter of the reverse biased transistor. The noise will be relatively small there, and then the amplifier output should be line level (for whatever internal level your device needs). The dc voltage at the noise device's emitter should be the zener voltage of that transistor. Something above 6V would be typical. If the dc voltage is at or near the power supply voltage, check for an open base-emitter junction (use a "diode check" on a DMM) or perhaps the base is not connected to ground or V-.

If you do have a broken or defective noise transistor, you may find the old part is obsolete. Fortunately, any transistor will make noise, but unfortunately some don't make a lot of noise. You may need to try a few if subbing in a common device, such as a 2N3904, for an obsolete device. Raising the gain of the amplifier is an option, but too much gain will clip the noise (the resistor in the emitter-base circuit also can be too large and clip) which won't be as loud and/or may sound strange.