Synth DIY: a White Noise generator (part 2 of 2)
In Part 1 I outlined how to make a very simple discrete white noise generator. In Part 2 I’m going to expand that into something a little more interesting.
To colour the noise, simple passive filtering is sufficient. Originally I was going to put a switch here with a couple of different RC networks in place for basic high- and low-pass, but instead opted for a potentiometer to fade between them. The filter circuit below is inspired by part of the classic EMS VCS3 synthesizer, though is not identical. The pot sweeps from high pass at one end, through to low pass at the other, with a balance in the middle.
R4/C4/R5 and R6/C5 form low- and high-pass passive filters respectively. The pot pans between them by proportionally grounding one, or the other, or somewhere in between. The two networks have the same cutoff frequency, and that remains the same as you sweep the pot. It’s really just a balance control, between high and low frequency content.
Now we just add a volume control and one more stage of amplification to buffer the output. The overall output from the whole circuit, with these components, is about 5v p-p maximum. This varies as you sweep the filter. Feel free to play with the gain – line level audio will hover around 1v p-p, modular signals nearer 10v p-p.
You can download the full schematic as a PDF here.
Please Note: the schematic below includes corrected R and C numbering in the first stage. The schematic posted in Part 1 accidentally starts the numbering at 2 instead of 1. The component placement and values, however, should be correct throughout.
CAN I USE A DIFFERENT VOLTAGE SUPPLY, eg. 12v or 15v?
The short answer is ‘yes’.
A single-sided DC supply is fine for the transistor circuits here, and 9v or more is sufficient to allow breakdown noise generation. If you use a dual supply, opamp stages would make for an efficient circuit with very easily controlled gain.
HOW DO I SELECT A TRANSISTOR?
Pick out any NPN you have at hand, connect its base to 0v and its emitter through a resistor to your positive supply. Leave the collector free. Power up and use a scope to monitor the noise at the emitter. Choose a resistor between 100k and 1M. Lower values give greater noise level, to a point. Too high resistance, and you won’t see a suitable noise source. The results listed in Part 1 for different devices were based solely on some quick practical tests, and are to be taken as a guide only.
You should be able to use pretty much any common general-purpose NPN in this manner, though don’t forget to pay attention to the pin arrangement as this varies across different devices.
Here’s a photo of the circuit in development on a breadboard. The scope shows the white noise output, the meter shows current consumption. Note the 9v PP3 battery providing power.
Finally, here’s a couple of photos of the finished device, housed in a stompbox-sized enclosure, and with an LED added to show power on. The audio output is a 1/4″ mono jack, and the power input is a 3.5mm minijack. Not the most common style for 9v, but they do crop up, on old DOD pedals for example, and some EHX. It’s just what I had lying around.
The second photo of the pair shows the interior. You can see how the circuit fits easily enough into a small enclosure. The eagle-eyed reader may notice a component difference or two – that’s because the black box is a version I made some time ago, and I decided to recreate and update the design for this blog.