V8 Noise Unit

Noise generation

There are many ways to generate a noise signal. They can be divided into digital and analog noise generation. For musical purpose you can forget about the digital way (many manufactures and technicians will object against this statement but they do not know what they are talking about besides cheaper production techniques for their equipment). Although the analog way counts not very many parts, the quality of the noise depends very much on the used transistor (must be selected) and also requires adjustment for optimum operation (and this is why the analog technique is not popular among manufactures).
When a current is send through the emitter-base junction of a transistor it starts to produce noise. This current is generated by the combination of R2 and the Voltage divider Pn and R1. With Pn, the applied voltage to R2 can be trimmed to set the current for optimum noise generation of the transistor (and this varies for each transistor).
There is a tricky relation between the applied voltage to R2 and the current through the which is necessary to generate noise. It is not only a matter of current but also a minimum applied voltage. For most transistors it is required to apply at least 10 Volts to Pn. I built the noise unit with a dual supply of 8 Volts and used a transistor that could generate noise under low voltage circumstances but, it is possible to uses higher power supply Voltage (up to a dual 15 Volts supply). Capacitor C1 is necessary to decouple the voltage that is applied at the trimmer center.
Any transistor can be used but they not all produce the same noise. Both the amplitude and the frequency spectrum can vary enormously. The only way to find the optimal transistor is to test many different ones (both different and equal types). The universal transistors Small Signal types (American, European and Japanese) should be the area to test. The target is to find a transistor that produces a flat frequency spectrum with high amplitude.
The bipolar capacitor C3 (build from two normal Elco's) separates the DC voltage at the emitter from the actual noise signal. A high value of C3 has the disadvantage that, at power start up, it takes time before the noise signal appears (because first the DC Voltage needs to charge C3). With the given value it takes some seconds. The advantage is that also very low frequencies find their way through C3. When these very low frequencies are not required you can decrease C3 (100 Nano for example)
R2', which is not used in my modification, serves the purpose to connect to a alternative gating circuit to switch on/off the noise generation. This could be useful when no VCA amplifier is realized on the board (minimized circuitry). How this gating must be realized will not be discussed now (simply because I never realized it).

Noise Amplifier
In preparation

VCA
In preparation

VCA Control
In preparation

Tone unit
In preparation

Output buffer
In preparation

Connection to Voice unit
In preparation

Vero board lay-out
In preparation