So Mum’s just left the country for her yearly sojourn back to Canada and this leaves me with 8 weeks to scatter my stuff all over the place and actually get some stuff done. I know I’ve written about all these other new projects but this morning I decided to get an old one out that wasn’t finished and it wasn’t finished just before she got back last year.
Out it came this morning and it took me a little while to find my notes on it and it’s all fairly straight forward to get together and into a box. Of course, as soon as I looked at it I realised there were easier and simpler ways to do what I’d intended but thats what happens. The drum sounds themselves are from a schematic from a book both written and published in India which used to be available from Jaycar and has a bunch of archaic schematics that use hard to find and obsolete parts from the sixties and seventies. This one is the least archain except for BC148 transistors which are very hard to find but it doesn’t matter because modern equivalents will do the job regardless. The other tricky part is the 100mH inductor but I happened to have a bunch lying about that I swapped for something or other years ago because inductors in tone circuits are quite useful… including filters like the wahwah. Heres the schematic in all it’s olden day glory. If you can’t see it properly just click on it to go to the Photobucket site where it’s held or copy the image location and paste it into the browser address.
The way it works is by using something called a ringing oscillator which is an oscillator circuit that rings for a bit then dies. The infamous old Roland beatbox from the early eighties was made using the same technology and I’ve got the schematic for that somewheres and is a little fuller than this particular rendition but still was fairly blippy and beepy. This technology or angle I suppose was followed up to the end of it’s useful life which was heralded by sample technology which rendered this type of circuit somewhat redundant… UNTIL NOW!
Seriously though, I’m going to get this going to reaquaint myself with this ringing osc gyro stuff and look at making the oscillators ring a little longer.
So a drum sound generator device is the first part of the drum machine and the second part is a sequencer of some sort to fire the individual ringing oscillators and sequence them up to some sort of ordered output.
Above is that which is the drumatic! The board at the bottom is the finished indo beep and blip maker, six different sounds for triggering, and above that is the incredibly complex sequencer. The sequencer itself is pretty straight forward but my implimentation is as archaic as the Indian drum sound source. Essentially it’s a 555 to set the tempo, which can be adjusted from 10-140 bpm and then the trigger signal goes to a rotary switch which chooses between 4 different 4017 decade counters set up to count to 8 then reset and begin again. Each 4017 then has it’s own 1-8 count which then goes out to a transistor which eventually makes a connection to fire each drum sound. On it’s way to each drum sound the single out from each transistor goes to a switch block that can turn on each of the six sounds. Very confusing I know because I had to make it but suffice to say I can manually choose between 4 basic patterns then assign those patterns where I want. There is a logic going on and it’ll be alot easier when I can transfer that logic from a hands on switching and choosing system to something that works with actual electronic logic, which I’m not at all familiar with… as yet, and may even prefer the type of interface I’ve built here.
Once this is up and running I’m going to breadboard various other ringing oscillators to see if I can get them ringing for longer.
Meanwhile I’m having a bitch of a time getting this amp to work like it’s supposed and I have no idea why.
It’s a single IC amp, a TDA1519, which I bought originally as a kit from jaycar, which is designed for 12VDC car systems and does about 22W into a 4 ohm speaker at 1.5A but the bloody thing just doesn’t want to play. I had it going at 15V but it was only pushing about a half watt so I took it back down to 12V and now it does nothing. I also realised the chips I had, TDA1519’s, were actually quite different from the TDA 1519C which the kit came with. The TDA 1519 was a 6W + 6W, or 12W in bridged mode, while the “C” prefix is a later developement at 11W + 11W and 22W bridged but the pinouts the same and the only difference is the current draw to get to the max out. At this point I’m going to put the original chip back in a see what happens because I’ve got a power supply that works sitting on a 2A transformer so the thing should be getting all it needs and the max voltage for both chips is 17.5VAC so I might put it back up to 15 as well.