Power Supply Noise Non-Analysis

When I started designing modules I decided it’d be handy to have a case to demo modules in. So naturally I decided to build one.

Not too long ago the power supply for this case arrived, and I thought, ‘wouldn’t it be great to compare the ripple of the new supply to the ripple of the ATX power supply I’ve been using to test modules.’

The ATX supply

Let’s start by looking at the ATX supply.

Where I out myself as a Rhode & Schwartz user

Wow look at all the garbage on there. Spikes all over the place. There’s so much high frequency noise that the oscillations blend together to form what looks like a solid 22 mV wide line. So what, if any of this, is coming from the power supply? Let’s start by taking a closer look at the high frequency noise. Enhance!

It’s funny because Oscopes over sample unlike cameras.

Well the high frequency noise looks suspiciously periodic. But with a frequency of 95.2 MHz, I’m pretty sure that’s a local radio station, not my power supply. How about the bigger spikey bits?

The frequency (~380 kHz) on these artefacts is a lot warmer, but they’re not the right shape to be power supply ripple. Power supply ripple should be triangular; this looks more like switch ringing. And sure enough just about all of the spikes go away when I turn off my bench lights, which are dimmable LEDs so they’re being switched on and off at some ridiculous rate.

Bench lights on
Bench lights off

But what’s this with those big spikes from the lights gone it’s easy to see an 85 kHz triangle wave. This could be power supply ripple; it’s impossible to know for sure without knowing more about the supply circuit. And the magnitude is *drum roll* 31 mV; barely larger than the background radio noise. nothing to be concerned about.

New Case supply

Now that we’ve established how little ripple there is with the ATX supply it’s time to fire up the new case supply and see what happens. I did hook it up to a few bus boards before taking measurements because who cares about consistency in experimentation.

Great, turns out a few bus boards more than tripled the background noise level to 70 mV. ~30 mV of ripple will easily be lost in this. At this point I stopped the experiment having determined that in order to meaningfully distinguish between the ripple of the two power supplies I would need to dig my power resistors up from wherever I hid them and try to limit background noise.

Parting thoughts

For me the most enlightening part of this to note that the power supply ripple is dwarfed in magnitude by ambient noise sources. Does this mean everyone with a mod synth should go buy a faraday cage. No, a few 10s mV ripple can be easily filtered by board level supply caps. And since the lowest frequency we looked at today was 80 kHz it’ll get filtered by our ears if nothing else. And we haven’t even started talking about the ambient noise suppression of all those op-amps in modules.

Is this a vote for or against using an ATX power supply to power synth modules? Neither, my understanding of why Eurorack systems shy away from using ATX power supplies has more to do with the current rating of the -12 V rail and the extraneous 3.3 V rail which isn’t in the Eurorack specification. Here’s the rating sticker from the supply I’m using.

300 mA of current at -12V is pretty reasonable for a small rack, but an ATX power supply would take up a substantial amount of space in a small case. So it’s usually a better idea to get a dedicated Eurorack case power supply. For a bigger system an ATX can’t supply enough current at -12 V to be useful.

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