Voodoo Labs PSU works but an ordinary 9V PSU doesn’t

jaymenon

I wired up an acoustic guitar with an LR Baggs anthem such that it gets 9 V DC phantom power from a TRS cable.

I plugged it in today using a Zoom 9v 300mA centre negative PSU - which is the correct polarity - but got an almighty bass ‘noise’ from it (no signal). The power LEDs on the anthem preamp unit lit up just fine though…

Tried a different TRS cable, no joy.

Before I opened things up I decided to try one of the power cables from my electric pedalboard PSU into my acoustic guitar phantom supply unit  (9v DC from a Voodoo Labs PSU). 

Worked just fine.

The current draw of the on-board pick up system is only something like 10mA. The Zoom PSU is a 230v job (not universal 100-240v voltage)

I wonder what’s happening here?

pic:
https://i.imgur.com/Qq0MrcF.jpg
https://i.imgur.com/5XAbaTC.jpg

jaymenon

The Zoom is reading 12.5 V DC on a multimeter (though the printed voltage is 9 V)

Another Salter Weigh-Tronics PSU made in Germany rated at 9 V DC is actually reading 8.8 V DC. And it’s working - but with considerable hum.

https://i.imgur.com/WTmo1Xo.jpg

when I take a power supply from the voodoo labs it’s dead quiet.

MARVlN

Not sure about the hum I'm afraid, but stupid question - are you not meant to connect the input power to the 9V connection rather than 9V through as shown? I'd assume the 9V was power in and the through is so you can basically daisy-chain to another pedal. Of course, it mght not matter anyway depending on how it's wired.

steamabacus

The Zoom PSU is probably a non-regulated supply - as the current draw increases, the voltage drops proportionally until it's 9V at the stated maximum current draw. As you're only drawing a tiny current, the power supply will be providing close to its maximum voltage. 12.5V sounds about right. This is just like the old ACA power supply used by older Boss pedals which is why they need 12V from a modern isolated multi-output unit.