Can I Filter This Split Power Supply?

Question

I've got a battery powered synth that uses two 9v batteries to make a split rail power supply (±9 and ground). I'm trying to use a regular AC-DC power adaptor to replace the batteries. The adaptor outputs 12v and I'm regulating it down to 9v with the LM317 voltage regulator. To get the required -9v I'm using the ICL7660 CMOS voltage converter. The problem I have is with the noise created by the power supply, when i hook up the synth it works, but there's some oscillations and artifacts on the audio output. Does anyone know if there's a way I can filter out this noise?

here's the circuit so far -

Answer 1

Add capacitors between each power rail and ground. They'll act like short circuits to the high-frequency noise. I see that you have one between the -9 V rail and ground-- that's a good start.

The type of capacitor you use can make a difference. Electrolytic caps, for example, are relatively cheap, but they only deliver their rated capacitance at low frequencies, so they're not as good for filtering out high-frequency noise. I've had good luck with tantalum capacitors in this kind of application, but be careful that you don't plug them in backwards, as they will explode. Be double careful with the negative rail, because "backwards" might be the reverse of what you think. For both rails, you want the plus on the capacitor connected to the more positive terminal, which is ground for the -9 V rail.

Answer 2

I would use the ICL7660S instead. A couple of benefits --

• The S version switches at 35KHz which is above the audio range.

• The S version can switch 12V. If you did a +-12V conversion you could do a +-9V linear regulation on the output. This will reduce the noise.

You should be able to add a series inductor and another capacitor on the output. I am assuming you are using ceramic caps (X5R or X7R).

The load regulation of these switched capacitor converters is not that good under load. I am assuming that your synth only needs currents around 20mA or so. Check the specs for the voltage and current limit specs.

Answer 3

Not really an answer but an alternative.

For low current circuits you can use an AC adaptor without the rectifying bridge (the one that gives a low-voltage sine wave on it's output) or just a 220/110V->12V transformer and add two half-wave rectifiers (one diode for +12 and one for -12V). This way won't need two separate secondary windings on the transformer. Then you can regulate the voltage using one 7809 and one 7909 (but don't forget the capacitors on these).

PS. You could probably just remove some components from your adaptor but be extra careful when dealing with 220/110V circuits.