REV 4 of noise board This is a pretty minor revision, and I have NOT tested this working (yet). I added a 6dB boost via op amp on the white noise side. I am pretty sure I got this right....I just took one more unused stage of the TL074 and used it to boost the White Noise gain.
PDF of the REV4 board schematic is here. You can get a PDF of a wiring diagram (not much for this circuit!) here. Eagle schematic is here and board is here. I will not post gerbers yet until I test this board, but since REV2 worked and this is just one more op amp it should work fine I figure. Somewhere down the line I'll create the gerbers and send this off to fab, we'll see how it sounds.REV 2 of the NOISE BOARD I have tested this version and it works. White noise outout is a bit low. Schematic is here and board is here. You can read either and make a gerber using Autodesk Eagle.
Board image looks like this:
While Schematic is as such:
Like so many old noise boards, this one depends on noise inherent in a NPN transistor between E and B and amplifies it. I set things up so you can put test leads on the "Transemt" and "Transbase" test points and clip it to your 2N2222 or whatever transistor. Hook the output up to your amp and monitor and try out transistors until you like the way the white and pink noise sound. Once you have a winner, solder the "winning" transistor into the T1 location if you want.
I can't remember where this design came from, I thought I saw something like it in an old tom henry design, but he might have swiped it from Electronotes? Anyway this is pretty much the "classic" way to get a good and truly random analog noise source.
For the build I ended up using, I tried a whole bunch (I mean, many, many) transistors.The one I ended up using had really good sounding pink and white noise, but the white was about 12DB too soft, stange because P2P was about the same, about 2v, when viewed on a scope. If I was building this again I'd take the unused op amp and put it in series with the white noise out so if you don't want to gain the WN up, just make it into a buffer; if you do, add some resistors to get x DB of gain.
Some of the things to mess with: R5 and R6 set the overall level for the pink noise. R10 and 11 for white. R1 and R4 set overall gain, you can set this for too much gain and get bad sounding noise. You can also cap couple the output--throw a 10uF cap on each output to make sure the noise centers around 0V P-P.