Keyboard and mixerI updated the keyboard by buffering the output with an emitter follower op amp. First I used a rail to rail op amp from microchip: MCP6271. I did forget that it had a maximum power spec of 6V, and I fed it 9V, so it started smoking after a while. It did work for quite a some time though... I replaced it with a ua741, but needed to fed that one with +-9V, to get a low enough output voltage. I really need to stock up on op amps...
The mixer was updated with some 1% resistors for the inverter. I should probably use 1% resistors for the unity gain input and feedback resistor, to get a gain closer to 1 for that input. I also AC-coupled the other input, since DC-coupled signals puts the synthesiser out of tune.
|Updated mixer schematic|
White Noise Generator
I also built a simple white noise generator. It works by putting a NPN transistors base-emitter junction in reverse bias and overcoming the breakdown voltage of about 6V. By doing this, the junction will create white noise. Since the junction will conduct at first, and when it conducts the emitter voltage will drop, causing the transistor to stop conducting, which builds up the voltage above breakdown again, and so on. This happens really fast and thus noise is produced. The noise is very low voltage. I couldn't pick up the unamplyfied noise with my scope, and amplifying it with a 10000 gain opamp amplifier barely made it visible above the noise floor of the scope. So I used two opamps and amplified to the whole swing af the opamps.
The noise can now be mixed with the VCO control voltage, but it really creates a very broken and harsh sound. So it's probably better to mix it with the VCO output. I'll try that next time. I should probably also build another mixer, so that I can mix a wave with the control voltage, and the noise with VCO output, now I need to choose...
On the next mixer I will get an op amp that can be turned off, so that I can use a gate signal from the keyboard to turn it on when playing a tune. As it is now, a high amplitude AC-coupled signal can have levels high enough to stimulate a VCO tune, which cause background noise when not pressing any buttons.
|White Noise Generator schematic|