how to filter a switching PS?

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groggory

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If I wanted to use a computer switching power supply for audio, what would be a good, efficent circuit to filter out those 'pulses'?

I know how to filter out ripple, but these are pretty well regulated supplies, they just have that pulse nature to them that audio doesn't like.

So guys, any simple ideas? Could I maybe use some really small capacitors?

Next week I'm going to throw a few of my power supplies on a O-scope and see what frequency they pulse at. A couple cheapies, an antec true power, and an enermax.

Give your ideas, and I'll post specs when I measure them.
 
A 32 cent regulator will do the job very nicely. ;)
I have a bunch of alternatives to linear regulators on my system at home I will post later.
Are you having a specific problem or is this just a general information kinda thing?
 
Looking at computer psu schematics posted on the net, there's typically a choke and decent capacitor (eg 2200uF on the 12v line) filter already inside the box, plus all the extra mobo caps, but we're talking serious current with fast cpus.

What sort of audio amp are you thinking? The megawatt modules sold for ICE have an inverter built-in to provide the higher voltage needed, so they may not be so fussy about the 12v purity.
 
Originally posted by cpemma
Looking at computer psu schematics posted on the net, there's typically a choke and decent capacitor (eg 2200uF on the 12v line) filter already inside the box, plus all the extra mobo caps, but we're talking serious current with fast cpus.

What sort of audio amp are you thinking? The megawatt modules sold for ICE have an inverter built-in to provide the higher voltage needed, so they may not be so fussy about the 12v purity.
Click to expand...

I'm looking at integrating a headphone amp into the bay of my computer. But I don't wanna bother with batteries, or have to tap into AC lines and filter it myself. So I won't need much power. Maybe 750mA tops with the most inefficent design.
 
Check the PSSR rating of your opamps... it may be a moot point.
One thing that will come up is having enough voltage.
 
Originally posted by X86Dude
Check the PSSR rating of your opamps... it may be a moot point.
One thing that will come up is having enough voltage.
Click to expand...

It probaly is a moot point....but it might be something I can take out of the loop very easily. Might as well overdesign than underdesign.

As for having enough voltage. If I use the 12V rail as a source, that still gives me +- 6V for my amp. Most of the opamps I'm working with will run on as little as 4.5V. So that should be fine. I'd prefer more, but not a big deal.
 
I wouldn't worry about trying to filter the thing too much. I doubt that power supply noise is going to be your problem, esp. given that modern PSU's tend to oscillate beyond the audio spectrum.

My greater concern would be grounding issues, particularly if you're trying to run a dual-rail configuration. If you're getting your signal source from the PC, keep in mind that this signal is referenced to the PSU ground, and if youre virtual ground for your circuit is at 6v.... Well, I hope your design has AC coupled inputs.

Beyond that, if you're getting your signal from that PC, it's going to be damn noisy. I don't know where Creative gets off claiming 100dB signal to noise ratios, but I've never heard anything even near that come from a PC, particularly with headphones. Just a hint though, when you do get it put together, jack the volume up in software, (winamp volume slider) and leave whe windows volume as low as you can. This will help reduce all the noise that the signal picks up from inside the box.
 
I wouldn't worry about switching noise from a power supply rail getting into an op-amp. Just use decent op-amps with high gain / fast slew rate... OP275's from Analog Devices are probably one of the best IC's ever made for audio applications. Feedback with a decent (eg, non-JRC558) op-amp will cancel out any ripple and other crap you might get on a power supply rail.

If you're *really* worried, get a couple 7808/7908 regulators and drop down the +-12V rails on your computer with those.

Power supplies usually run around 40-50KHz, and Truepower supplies run their individual switchers at a few hundred KHz, pushing ripple well outside the audio bands. What I'd be more concerned about is load changes caused by the computer itself - eg, you'll probably find 1KHz crap caused by stuff running on a system timer interrupt, crap at your vertical refresh rate caused by a new 3D card doing heavy processing with Vsync on, and so forth. So, measure your power supply while it's in a computer.
 
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