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.

ADDING FILTERS

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.

Schematic diagram of the white noise colouration filter

Schematic diagram of the white noise colouration filter

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.

OUTPUT BUFFER

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 ā€“ 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.
Discrete white noise generator schematic

Discrete white noise generator schematic

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.

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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.

Photo of the white noise circuit during testing

Photo of the white noise circuit during testing

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 ā€“ 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.

Enjoy!

Photo of the finished white noise box

Photo of the finished white noise box

Photo of the finished white noise box - interior

Photo of the finished white noise box – interior

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16 responses to “Synth DIY: a White Noise generator (part 2 of 2)”

  1. Bobby Zincone says :

    Thanks! I’ve built this with a BC547. I have a couple of questions:
    – How much effect does the filter have on the signal? It’s not very audible for me but maybe I connected something wrong.
    – Is it possible to have more gain by using different values?

    • synthnerd says :

      Hi! I’m guessing you mean you used a BC547 as the noise source. In that case, I’d be looking at how much noise you’re getting out of that transistor, as the overall gain of the circuit should be plenty if that noise source is good. Make sure it’s connected the right way and play with its emitter resistor. As for the filter, it’s not extreme, but should still give a clearly audible rolloff of the highs at one end, and at the other it should be noticeably thinner and brighter sounding. Components will vary though. If you’ve verified that everything is connected right, try adjust the RC values according to taste.

      • Bobby Zincone says :

        Yes, BC547 is the noise source – but I also used BC547 for the other transistors. It works though, the volume level has no problems, I was just thinking if it was possible to make it even more louder with this specific buffer circuit. Thanks for the advice, I’ll check the connections again, and adjust the values if needed. This was so much fun to create!

      • synthnerd says :

        I don’t know if you’d be able to get much more useful gain with this design to be honest, it’s already putting out about 5V p-p on a 9V supply. You could try adding another amp stage, or running it on a higher voltage. If you want a noise source for something like a Eurorack synth to go alongside oscillators etc, you could do worse than try this on 12V and see what happens…

  2. laura says :

    hi! very useful design i specially love the filter. i was thinkin about doin this w a lm386 and was wondering if puttin que opamp stage before the filter would be enough or would i need to reamplify after it too? was just wonderin why you amp before and after the filter and not just before it

    • laura says :

      for reference im usin this schematic (w the transistor inverted ofc bc i have a bc 547 https://i1.wp.com/farm3.static.flickr.com/2774/4501573713_c3fbc6d5a5_o.gif

      • synthnerd says :

        Hi! I’ll be honest and say I don’t know much about the LM386 except it’s more of an output amplifier, but from that diagram it looks like they’ve set it up to take the noise directly from the transistor stage without more amplification between. The reason for my multi-stage design is that you can add or remove bits to suit your purpose. The noise from a transistor can be very low, though it’s always worth testing a few to see what you get as it varies a lot. My circuit buffers that and, then filters it, then buffers it again for output. If you want to add the filter to your 386, try it before the op-amp.

      • laura says :

        yes you are right is more of an amplifier than op amp. ok so will try the transistor into the filter into the lm386. thanks šŸ™‚

  3. nixnoise says :

    Thanks! I’ve build it with BC547 as the noise source and first thing I’ve noticed is that there is a huge difference between the individual units. Some produce much more noise than others… I also used an op-amp for the amplification since I need more V p-p for my eurorack! šŸ™‚

    • synthnerd says :

      Transistor noise is so variable! Always try a few before committing to one, for sure. Op-amp gain needs to be high for this kind of thing, and that can affect the frequency content. A story for another day. Enjoy!

      • nixnoise says :

        aha.. that would be interesting to know. how does the op-amp affect the frequency content? can we make this more controllable? šŸ™‚ cheers!

      • synthnerd says :

        Look up ‘gain bandwidth product’. It’s a non-negotiable property but we can afford to be not very fussy for a lot of basic synth circuits.

  4. J says :

    Hi. Iā€™m a newbie and wondered can you explain a bit more about how the hi/Lo pass filter works? Eg the capacitor C5 should have a resistor on the output side to act as a hi pass filter so is that resistor R5 plus RV1?

    There seem to be too many resistors. Also is there some power loss as the out put from one filter will not go to out but be recycled into the other? Thanks.

    • synthnerd says :

      Good questions. In the first instance, I’d say imagine you’ve got the pot panned hard left. This effectively grounds R6. What does the rest of this section look like to the input now? Then pan the pot hard right, which effectively puts ground between R4 and R5. What does the input see now?

      As for power loss, it’s only a simple circuit, there are lots of ways it’s not perfect. Adding the final gain stage brings the signal back up for running into a mixer etc.

  5. paula says :

    This is very nice, thank you! Kinda makes me want to make synths again

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