I'm designing a power supply for multiple guitar pedals. To minimise noise I'd like the output voltage to be smooth when it's up and running. (I don't mind about small transient fluctuations when plugging in). I also want to eliminate ground loops by isolating each pedal's supply.

My design involves a toroidal transformer, bridge rectifier, smoothing capacitor, and an optional voltage regulator. This supplies multiple isolator assemblies.

The isolator assembly will include a DC/DC isolator plus the recommended inductor (in series) and capacitor (in parallel) to accompany it.

To ensure smooth output from the isolator assembly, should I add a regulator before the load?

A possible final regulator would be: Texas Instruments LM2940CT-9.0/NOPB, LDO Regulator, 1A, 9 V, 10.5 → 26 Vin 3-Pin, TO-220.

Otherwise, the isolator would be: Murata Power Solutions NMG 2W Isolated DC-DC Converter Through Hole, Vin 10.8 → 13.2 V dc, Vout 9V dc. (I know that wouldn't work in front of the TI regulator because it requires 10.5V in.)


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Hmmm... I looked at the Murata NMG1209SC datasheet. If I interpreted the line regulation spec properly, this is an unregulated converter. According to the datasheet the typical line regulation is 1.05 % / %. In other words, if your input voltage changes 5%, your output voltage will change by 1.05 * 5% = 5.25%. This means that if you don't have a regulator upstream, all the 120 Hz ripple from your rectifier + capacitor will make through the output of this regulator. Let's say you have 0.5V 120 Hz ripple on top of your 12V input, it gets translated into ~0.4V 120Hz ripple on the output. This sounds like a lot of buzz in your audio circuitry.

To mitigate that I would consider adding a real regulator after the rectifier and capacitor as you mentioned; cascade the isolated DC/DC with another regulator after it, as you also suggested; or I would look for a different isolated DC/DC that provides real line regulation.


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