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Project Notes

#474 BJT White Noise Generator

Building a common white noise generator circuit and optimising it for 9V operation with BC337 transistors.


Here’s a quick demo..



Reverse-biased diodes and transistor base-emitter junctions exhibit a Zener effect when they breakdown and produce “shot noise”.

There’s a very common circuit using S9014 transistors as a white noise generator. One transistor produces the noise, and a few amplification and buffer stages are added to provide line and headphone output.

It shows up in kits like the EQKit “White Noise Generator”. This is widely available, for example from a seller on aliexpress. Martyn Davies has a very good construction tutorial on Youtube:


I started off simply testing the circuit on a breadboard - it works very well! However it does require 11-12V to get the noise started, so I set out to see if there are ways of producing the effect at lower voltages.

Performance is very particular to the transistor model, but I was able to ultimately discover that the BC337 NPN produces a noise effect as good as the S9014 but running at 9V.

S9014 Circuit Performance

The circuit has an intermediate buffer stage to support a line out, followed by a headphone output with volume control.



Running on a breadboard:


It produces a flat frequency response in the audio range; in other words good white noise:


Evaluating a Range of Transistors

Reverse biasing the S9014 to produce shot noise requires at least an 11V. To run the entire circuit on a lower voltage, I tested a range of other transistors. My Results are summarised in the following table:

Transistor Performance
S9014 very good > 11V
BC337 very good at 9V
2N3904 works but smaller noise signal; >11V
S8050 similar to S9014, but >=12V required
C1815 response is peaky; >9V
C945 response not flat; 9V
A42 squeaks and squarks
A42 squeaks and squarks
S9013 squeaks and squarks
S9018 no noise at 12V
2N5551 no noise at 12V
TIP122 squeaks and squarks
D882 squeaks and squarks

When I state response is not flat or peaky, I mean the response is not flat across the audio spectrum. For example, here’s the FFT for C945, showing a valley and peak < 1kHz.


Final BC337 Build

To my ears, the BC337 produces white noise just as well as the S9014. The BC337’s I have a category 25, with a ß typically 160-400.

I dropped the line output and intermediate buffer stages for this build, reducing the circuit to three stages:



Running on a breadboard:


I then decided to transfer to a little project box with power in one end and noise out the other, and ugly freeform wiring in between:


Hooked up for a final test:



While replacing the S9014 transistors with BC337-25 allows the circuit to run at a nominal 9V with only a 8mA, if the voltage drops much below 8.9V, it does not generate noise. This makes operation from 9V batteries a bit problemmatic unless they are brand new.

The noise generate has very flat response across the audio spectrum (measured at the audio output):


And this is what it looks like in the time domain:


Credits and References

About LEAP#474 AudioBJT
Project Source on GitHub Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.