Cheap "Wall Wart" power supplies

FenderBender · 2010-03-16T15:45:28+00:00

I had a customer bring in some DIs for repair that are creating a hum on the desk. The first thing I noticed was that each one had a switchable wall wart power supply connected to the battery terminals. I decided to do a few measurements to check the supply itself, since I was certain that this was the cause of the noise. The supply has a switch for 3, 4.5, 6, 9 and 12 volts. I tested only the 9V, 7.5V and 6V output. I loaded the supply lightly with the DI and measured the DC output with a Fluke 115 multimeter and the AC ripple with a Philips PM3230 oscilloscope. The 9V output produced 14V DC and the AC ripple was around 600mV! The 7.5V output produced almost 12V DC and the AC ripple was a fraction under 600mV! The 6V output produced 9.7V DC and the AC ripple was around 450mV! What does this mean? The gear on the other end of the power supply is running at too high a voltage and this could stress the components. The massive AC ripple will induce nasty hum on your signal path. Batteries are the best supply for your pedals and gear, but... If you want to use AC power, your wall wart must be destroyed and you must use a good supply!! Boss power supplies and the like are not cheap, so I have been looking around for a good alternative. I have found a really good (on paper), low cost, reliable PSU that I will be testing soon. It delivers a regulated 9VDC at up to 2A with very little ripple. It will handle two to three digital pedals and lots of other analogue pedals at the same time. I will post here again after the tests!

FenderBender

Ok here it is!

I finally got the power supply I was going on about earlier this week.

It's really small and connects to the mains with a kettle plug so IT HAS AN EARTH PIN!

Another nice feature is the noise suppressing ferrite ring molded onto the output lead, just before the plug.

I decided to check the beast out with the obligatory fluke and philips combo and this is what I found:

DC output measured up at 9.15V when powering the DI's (as per my first tests on the JUS PSUs) and the ripple was not even measurable on the scope!

I decided to slap a fat 8 Ohm resistor on the ouput to get some current flowing and then measured the output voltage and ripple again.
DC output dropped to 8.5V:

The ripple measured was again negligible!

Ripple is what causes the trouble and hum with equipment. The 0.5V drop on the output is no cause for concern as most 9V batteries get there pretty quickly when being used anyway.

After replacing the output plug with another 2.1mm unit (in order to swap the polarity as it comes standard with a centre positive plug), I tested the supply on a few pedals and she's a beaut - no noisy business here.

The unit I tested is model number ES18A09-P1J.
They are available from the agents, Current Automation (http://www.rectifier.co.za/), or any branch of Mantech. I paid R100+VAT, which is a bargain in my book, although they are cheaper direct from the agents.

Squonk

Thanks for all the info, so these wont blow up my pocket pod then? ?

IceCreamMan

excellent info, group buy anyone?

FenderBender

Squonk wrote Thanks for all the info, so these wont blow up my pocket pod then? ?

Nah it won't turn it into a rocket pod hehe!
When comparing it to a standard wall wart, think of two kids - one with a box of matches and one without ?

EDIT: I forgot to mention, that at R54 or whatever Current Automation are selling them for, you can afford one for each pedal in your tote ?
Seriously, the best way to go is multiple isolated supplies. I noticed that they also have a smaller 12 Watt unit that puts out 1.3 Amps - A possible cheaper option if you want to go the "seperate supply per pedal" route. Model GI1209A-P21. Not sure what they go for.

nick

Ive got one powering my pedals and its brilliant, 2amps is more than enough! even convinced Reinhard to buy one. I paid R140 from Mantech in Epping 8)

Reinhard

nick wrote Ive got one powering my pedals and its brilliant, 2amps is more than enough! even convinced Reinhard to buy one. I paid R140 from Mantech in Epping 8)

Thanks to Nick, I was able to build a VERY economical pedal board. I should actually buy another as a backup.

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