Re: Korg Monotron

Have you checked the stability of the analog power supply? This is a single-supply design, which is always a bit iffy, so carefully check for noise on the V/2 Vbias line. Some opamp, or etc, may be oscillating at RF frequencies.

Two things stand out as possibilities: 1) The VCO signal generated by an MSP430G2231 controller, and 2) the VCF programmable output filter.

In each case you should be able to go in and listen to the raw signals involved and see if they are noisy. There are other possibilities, like the two multivibrators, but again, you can check them individually.

Are you using a fresh set of alkalines?

Also, maybe try taking it far away from any external switching power supplies (computers, TVs, CFLs, LED lamps, etc) to see if that makes any difference.

You might try connecting an external audio amp at the volume control (VR1A) to see if any of the noise is coming from the final audio amp in the Korg (IC6A).

You'll probably have to read the data sheet for IC1 to see if it can stand having power on its output pin with no input. If it can, you might try leaving the batteries out, and applying clean +5 V to VCC. If it can't, you might remove IC1 and then apply clean +5 V to VCC. If that helps, you have to figure out how to clean up IC1.

If clean +5 V helps, it looks like the analog supply voltage VA comes from the main supply VCC via R61, a zero ohm resistor; maybe replacing R61 with some kind of filter will help.

I *think* the lines coming off of pin 10 and 11 of IC2, the MSP430, are implementing TI's "Spy-Bi-Wire" debugger interface, but they might also be implementing JTAG. I think you can hold one of these lines high or low to keep the MSP430 in reset; see its datasheet. If doing that helps, then you have to figure out how to keep the digital noise from IC2 from getting into everything else.

If digital noise from IC2 is the problem, maybe replacing IC3A with a "real" voltage regulator might help, but probably costs you higher standby current. It seems a little weird to use IC3A as a voltage regulator, but MSP430s can be configured to use little current, so maybe it works out.

Also, VD (I beg your pardon) looks like it doesn't have much filtering on it at all - just 0.1 uF, probably near IC2. If IC2 doesn't draw much current, then it probably doesn't matter, but bumping this up to maybe 1 to 3.3 uF or so might help.

Matt Roberds

Thanks for the suggestions! I also noticed that there's an unused op amp section in the design that doesn't look like it's been properly set up in the schematic at least (unity feedback, non inverting input to ground). Naughty.

Excellent suggestions! Thanks!

Also, if it makes any difference, the type of noise is just a low level white hiss. Except for volume the control settings don't make any difference in the level or character of the noise - sounds with high harmonic content basically mask it if the output volume is up high enough, but when the VCF cutoff is turned way down the hiss is very objectionable.

Don't forget the switcher. That could be beating with .

Cheers, James Arthur