Inside the Mugig 48V Phantom Power Supply

Image of cased unit

Earlier this year, before our lives were thrown into disarray, I had acquired a couple of new microphones to record a live concert - remember those? The microphones required phantom power and I was casting around for a suitable (and, preferably, cheap) inline power supply that would not necessarily require an available mains supply - so something with USB power or an internal battery. Amazon Warehouse were offering a USB-powered device branded Mugig for less than £10, so I thought I'd check it out. 

Amazon appear to have sold out of that particular brand, but there are others that look similar, so I thought I'd post this teardown anyway.

The first thing that strikes you on opening the case is that the circuit board is prominently marked with the web address - they are (or possibly were as I can't presently reach the website, though it was there when I received the device) a British designer of electronic equipment for musicians.

Power Supply PCB

Google does suggest that they at one time sold a phantom PSU with the model number printed on the board so the design has obviously lived on. The design appears to be a straightforward ladder voltage multiplier as you can see in this detail of the PCB which shows a chain of  diodes and capacitors:

Ladder Multiplier
Ladder Multiplier

The 74HC240 is an octal buffer, presumably acting as an oscillator to drive the ladder. The output voltage was measured at a respectable 45.7V:

Using the supplied mains adaptor, I noticed significant hum on the microphone signal. Using a USB powerbank, the hum was eliminated, but the device draws so little current that most powerbanks will shut off after a few seconds. The solution turned out to be a cheap £2 powerbank from Poundland - it happily powers the phantom supply without turning off!
Using a scope, the AC component of the supply was observed:
You can clearly see the residual ripple from the oscillator feeding the voltage multiplier, but it is of low relative magnitude and its frequency is way beyond the audio range.