Let There be Noise!

Let There be Noise!

The blog has "noise" in it's name so it's about time to write a post about noise!

After finishing the 4xLFO module I started looking for another simple but useful module to build. After studying the ASM-1 schematics it was somewhat obvious that the noise module would be my next project. It's almost as simple as the LFO and significantly widens the sonic possibilities of my synth.

I only have 8hp wide blank front panels so a noise circuit alone felt like waste of panel surface. Of course the obvious companion for a noise module is sample and hold. By feeding white noise to sample and hold you get randomly changing control voltage. Feed that into the CV input of your oscillator and you get the classic "computer speaking" sound from the past.

White Noise

White Noise? Pink Noise?

Most common noise types in synthesizers are white noise and pink noise. White noise contains equal amount of all frequencies and sounds a bit like wind or ocean waves or something. Pink noise has more low frequencies and less high frequencies so it sounds a bit more bassy. Usually it's generated from white noise with a simple filter.

Where Does that Noise Come From?

After studying several noise module schematics I came to a conclusion that almost all of them follow the same principle. The source of white noise is a semiconductor with p-n junction. Either a transistor with one terminal unconnected or a diode. After that the noise signal is amplified heavily. Finally other types of noise can be generated from white noise by filtering.

I eventually returned to the first noise schematic I opened, the ASM-1 noise source. The biggest reason for the choice was that I had all the components available. The only problem with the schematic drawn by Magnus Danielson is that it misses one resistor value. Luckily there's another version of the ASM-1 noise schematic available from Elby Designs. There's some minor differences between the two but most of it is the same. I mostly followed the Magnus Danielsons schematic. For the op-amp I used TL074 instead of TL084.

Sample it! Hold it!

For sample and hold the most common way to do the trick is a FET transistor controlling the charging and discharging of a capacitor. When a square pulse is applied to the FET gate it lets the input signal flow to the capacitor so the voltage follows the input. When the gate voltage goes down the FET goes off and capacitor voltage stays at the "sampled" value. Of course there's other ways to implement it but this was the simple principle that I saw in several schematics.

My main options for S&H were:

• Super simple s&h from schmitzbits.de
• The Nor Eurorack version
• The YUSynth Noise & S/H module

None of these were exactly what I was looking for. Some had component types I didn't have and some had too many features. I have plenty of LFOs in my synth so no need to add an internal one to the sample and hold unit. I also have slew rate limiter already so no need for that either. 

I finally decided to go with the YUSynth version by Yves Usson but simplified and modified it quite a bit:

• Left out the slew rate part completely. (The U2c op-amp part). I take the S&H output directly from the output of the U2b. (Of course the 1k output resistor is still there)
• Left out the internal gate LFO (U1 and everything around it in the schematic)
• Left out the led and the transistor driving it (Q1). I was planning to have it but got lazy while building the module.
• I used J201 as the sample/hold FET (Q5) because I happened to have those around. Works fine!

Veroboard Software Madness

This time I thought it might be a good idea to do some layout planning before soldering anything. I decided to try a software for that. Fritzing looked most promising and I spent an afternoon drawing the noise schematic with it. Fritzing is somewhat simplified electronics design application and I liked the idea from the first moment on. I have studied electronics in the university but ended up as a software developer so I have never really done any real PCB design and don't know much about the fine art of layout design. Fritzing nicely hides lots of details and mostly defaults to values suitable for a simple hobby PCB. It's also possible to order the PCBs by clicking a button in the app. Quite convenient!

Fritzing also has a separate layout mode for designing veroboard or breadboard layouts. This was the most interesting part for me because I was hoping that I could use a stripboard for this project.

Fritzing veroboard layout design software

Unfortunately I tried the layout part only after spending several hours drawing the schematic. It pretty quickly turned out that the veroboard layout mode is pretty useless with a stripboard. It's pretty much impossible to modify how wide the resistors are. Very often it's necessary to mount resistors vertically and just connect them between two adjacent strips. Also the perspective of the components is problematic. Many big components are shown from side and once you add them to the board they hide everything underneath.

Anyway, I also tried the actual PCB layout part of the software and that seemed to work quite well. I might give it a try and design and order my own PCBs at some point. So Fritzing in general is nice but the veroboard part was a huge disappointment.

Veroboard Madness, the Traditional Way

So I ended up using the dotted veroboard once again and generating the layout while soldering. I started with a board about the size of the 8hp eurorack panel and marked the halfway. One side for noise and another side for s&h.

Noise part half done

Noise part ready

Bottom side of the Noise. There's a short circuit between the op-amp legs. I fixed that later.

At this point I got so frustrated with the crappy veroboard soldering that I almost gave up. If I hadn't already drilled the panel for both noise and sample and hold it would have become a noise only module. My next project is definitely going to have a better PCB!

Both circuits ready and connected to the panel. Once again I added some components directly to the panel. I'm not sure if it's good idea as there was plenty of room in the veroboard.

I had already drilled a hole for s&h gate led but got lazy and used it to attach the panel to the board.

The Result

I first tested the noise outputs. White noise and pink noise worked fine. Feeding the white noise to VCF input produced some really awesome sweeping sounds! The third noise output was not working and gave static +12V. After some debugging I found a short circuit between the + and - inputs of the op-amp, as can be seen in the bottom side photo above. After fixing that it started working. I had no idea what to expect as it just says "random noise" in the asm-1 documentation. It seems to be some kind of slow CV signal not audible at all but it gives nice bubbly randomness to the sound when routed to an oscillator or filter frequency CV.

The sample and hold also works nicely. That classic "computer speaking" sound works great and I'm sure there's tons of other cool ways to utilize this new module.

I wish I had added an internal routing from noise output to sample and hold input. Now I need to patch it externally with a cable. The jacks have built-in switches so it's just a matter of soldering one wire behind the panel. Maybe I'll add that later.

White noise to sample and hold

Triangle wave from LFO to sample and hold

The result.

And finally some instagram videos demonstrating the result:

https://www.instagram.com/p/BTUbH01FSk0/

https://www.instagram.com/p/BTUcYt7lZTF/

And here's a short sound sample:
http://tvainio.kapsi.fi/vihreateknokone/noise.mp3