gihaume wrote:Sorry but what is PN ?
So, if you check the "NOISE" section on the Yocto diagram, it is responsible to generate the WN (white noise) used by SNARE, CLAP/MARACA and the PN (pink noise) used by all the TOMS/CONGAS. There is a famous quote from the 808 documentary about how critical this circuit section is to the overall sound of the unit, and it is the part responsible for driving and filtering the famous Q35 transistor. You can check here about it:
http://secretlifeofsynthesizers.com/the-strange-heart-of-the-roland-tr-808/http://secretlifeofsynthesizers.com/roland-tr-808-noise-transistor-update/As you can see in this article, there is multiple ways to try to get closer to the original noise sound. Sometimes changing the circuit, the transitor or tuning it. That is why the Yocto added a trim pot according to the "Roland TR-808 Service Manual" (
http://www.dinsync.info/2010/02/roland-tr-808-service-manual.html).
So, my logic goes like this:
The 808 circuit has basically 3 distinct NOISE generation circuits for the multiple voices:
- The "METAL" noise section is partially responsible for the COWBELL through the TP6 and TP7 output, it is fully responsible for the CYMBAL sound and partially responsible for the HATS sound thought the TO_HH output.
- The "NOISE" section is responsible for the SNARE, CLAP/MARACA through the WN output
- The "NOISE" section takes the WN output and filters to a PN (you can see that over the R133), and that is responsible for the TOMS/CONGAS
So, my first idea was quite simple. In the HATS switches I would first change the noise input from TO_HH (stock) to a WN from the NOISE section. Then, I thought that the other switch could flip WN to PN and feed the first switch, allowing me to choose between 3 possible noise sources for the HATS.
But then, I read this cool paragraph in this classic TR-808 mod page: "Cymbal / Hi Hat Alternative Noise Input" (
http://www.firstpr.com.au/rwi/tr-808/)
Basically he feeds an external input to the CYMBAL and HATS noise source and get multiple sound variations. It is a very cool idea.
But then, when I though about the infamous "NOISE" section that is so different between all 808s, it occurred me that instead of changing the noise source of CYMBAL and HATS (and I don't want too since that needs a lot of circuit changes specifically on the CYMBAL) and I was already doing a mod on the HATS to change it's noise source, what if I could completely change the WN and PN on the NOISE section with an external source? That would affect far more voices and you can use all sorts of noises, from synths to recorded sounds (like a snare recording on your phone).
Then, when I looked at the NOISE section, it is not too hard to isolate it from the circuit. Check this picture (I didn't soldered it yet, I was just test fitting):
Noise section isolation picture.Basically with those 3 JST-XH 2 pin connectors I can make a separate board for the noise source. I actually just need 5 pins: +B4 (get that from U24 pin 8), -B4 (get that from U24 pin 4), GND (get that from R131 S*), WN (get that from R130 S*) and PN (get that from U24 pin 7). I am wiring U24 pin 3 there just to hold the connector better, but that is not used for anything, so it will be NC.
Than, the idea is simple:
1) First, build an external board identical to the Yocto schematic, so, it will be "stock";
2) Second, build an external board with different features. For example, different noise sources or 3.5MM STEREO jacks to "override" the circuit WN and PN.
If everything works fine I can just offer the circuit files for anyone to print if they want. Maybe the Yocto community could get together to order a bunch of PCBs so the cost of each board goes down.
The possibilities are quite interesting... what do you think?
gihaume wrote:Not sure to understand fully as it seems irrelevant. You could just cut traces which feed noise to sections on the Yocto PCB and connect them to a switching jack..?
My personal goal is to not cut traces on the board. I want to keep the Yocto boards "stock". I know it is harder to do that way but I feel more comfortable about it
