Ground loop hum using step-down converter

[beatnik]

I have just bought a 220V to 110V converter for a couple of fx units i have

This thing makes a lot of hum

There is some ground loop going on but i don't know how to eliminate it other than cutting the ground termination on the power cord. not really the safest option.....

What would you suggest ?

 

[ruffrecords]

It should not be causing a ground loop. Make sure there is continuity between the mains input earth and the output earth. I bought one of these years ago from Maplin. It hums acoustically but it has never cause hum in the system.

Cheers

Ian

 

[beatnik]

the fx units outputs are connected to the audio interface inputs

it hums a little. if i unplug the converter from the mains outlet the hum stops. if i turn on the converter it hums a lot.

the fx units get grounded through the output cable on the record inputs so there is continuity between the mains earth and output earth,  it's also grounded through the power converter mains wire.

i did upgrade the fx units with a 3 pin power cord grounded the chassis like i do on all my diy builds, i suppose without this mod there won't be any noise, but not really safe isn't it ?

Cheers

 

[JohnRoberts]

the fx units outputs are connected to the audio interface inputs

it hums a little. if i unplug the converter from the mains outlet the hum stops. if i turn on the converter it hums a lot.

the fx units get grounded through the output cable on the record inputs so there is continuity between the mains earth and output earth,  it's also grounded through the power converter mains wire.

i did upgrade the fx units with a 3 pin power cord grounded the chassis like i do on all my diy builds, i suppose without this mod there won't be any noise, but not really safe isn't it ?

Cheers

I am not a fan of defeating safety grounds, but there are lots of legacy products that have pin 1 problems.

ASSuming your mains are protected by RCD (ground fault interrupters) using a capacitor in place of the hard wire safety ground could effectively open up the ground loop (i hate calling them loops because many are not actual loops), while drawing enough current to trip the RCD during a fault.

In the US at 60Hz to draw >5ma to trip a GFCI requires a 0,15uF cap. You will need to research what your RCD trip current is, and scale the cap size larger for 50Hz .

Stay safe.

JR

 

[beatnik]

ASSuming your mains are protected by RCD (ground fault interrupters)

not sure about what that is

 

[JohnRoberts]

ASSuming your mains are protected by RCD (ground fault interrupters)

not sure about what that is

RCDs are a safety device installed in many mains distribution branches to protect humans from current leaking from the mains. They compare the current leaving to the current returning at the outlet and as long as that current nets out to zero, all is good, If current is going somewhere else other than returning back into the outlet, the RCD opens the circuit, just like a fuse or circuit breaker.
====
Alternately adding audio transformers in series with audio paths that are picking up hum, is a more expensive solution but avoids thinking about the mains power entirely.

JR

 

[beatnik]

right. well, i am in a rented space so i don't really know about the electrical wiring, but i should hope it is meeting all the necessary safety requirements....

the problem happens on three different 110v fx units i have

i did modify all of them replacing the 2pin power cord with a 3pin grounded one

i am wondering this caused the problem in the first place - they didn't have the chassis grounded through the power connection in the first place...

i have tried moving the power converter to different sockets than the recording system rack, and i can actually find a configuration that has less hum component in the noise, but there is still a constant buzzing that makes the fx units unusable for recording

 

[gridcurrent]

if the 220:110 converter is an autoformer,
your inexpensive options are limited.

if the converter is in fact a transformer,
make sure the converter is indeed grounded but re-install those nasty 2 wire power cords.
your fx will be at least as safe as taking a trip in a Boeing 777.
for those that want to argue, crunch the numbers.

 

[JohnRoberts]

UL gear sold with 2 wire line cords are double insulated so safe from mains faults.

JR

 

[beatnik]

well, the chassis is metallic... i think it isn't any different from all the modern units i have

since day 1 they always told me do not break the ground connection- never...

i thought these units had the 2 pin power cord just because were old. and changing to a 3 pin would be a standard safety upgrade

i would feel safe if i use this gear myself, but maybe in the future i will sell it, so i would rather fixe the noise without touching the ground connection on those pieces of gear

the problem comes from the 220/110 converter - if i power the fx units with the variac there isn't any noise

would be a viable option to modify the converter unit to add noise filtering ?
need to check if it's an autoformer or a transformer

 

[JohnRoberts]

OK what step down convertor are you using? They are not all the same...

Is it a heavy transformer fully isolated, less heavy auto-former, or cheap light weight electronic (half wave diode)?

======
regarding 2wire line cords,,, if the products were originally 2 wire and UL approved, the primary circuits are double insulted so completely safe.

JR

 

[beatnik]

here is the one i'm using

https://www.amazon.co.uk/1000w-Mains-Converter-Transformer-constant/dp/B001AVHDG2

 

[JohnRoberts]

CAVEAT I am not expert on UK safety regulations or mains wiring practices...

=====

OK they say its a transformer, it might still be an auto former but for your application either should work fine.

It appears to have 3 pin input plug and output sockets.

You might start by checking for continuity between input and output safety ground, and if those grounds are isolated from neutral.

If neutral current is flowing into ground that could corrupt the ground quality.

JR