Hi,

I've never been particularly good with earth problems so I hope someone can
help.

I'm in the UK and I'm tring to sort out someones problem. They have a Boss
digital multitrack connected directly to an Alesis RA500 power amp - they
don't have a mixer. The Boss has a separate switch mode PSU (big wall wart)
conected via a 2 pin DC plug and socket. When the 2 earths are connected
via the signal cable, large amounts of power supply and disk drive activity
noise come out of the amp. I have discovered that the Alesis signal input
earths are not connected to the mains input/chassis/power supply earth but
via a 4.7 ohm resistor in parallel with a cap, situated on each power
amp/input PCB. Presumably these 4.7 ohm earth loops are the problem. (The
Boss is quiet enough on my bench, but I can still hear the PSU and disk
drive noise, but only as loud as the inherent hiss and hum).

So, why do Alesis do this, and can I safely short out the resistor, or would
this render it liable to RF oscillation or something? If it was my system
I'd suck it and see, but I'd rather not "have a go" with someone elses.

Any info appreciated!


Thanks,

Gareth.

try strapping an external ground wire between the units.
see if that stops the ground noise.

Doug

"Gareth Magennis" wrote in message
...
Hi,

I've never been particularly good with earth problems so I hope someone
can help.

I'm in the UK and I'm tring to sort out someones problem. They have a
Boss digital multitrack connected directly to an Alesis RA500 power amp -
they don't have a mixer. The Boss has a separate switch mode PSU (big
wall wart) conected via a 2 pin DC plug and socket. When the 2 earths are
connected via the signal cable, large amounts of power supply and disk
drive activity noise come out of the amp. I have discovered that the
Alesis signal input earths are not connected to the mains
input/chassis/power supply earth but via a 4.7 ohm resistor in parallel
with a cap, situated on each power amp/input PCB. Presumably these 4.7
ohm earth loops are the problem. (The Boss is quiet enough on my bench,
but I can still hear the PSU and disk drive noise, but only as loud as the
inherent hiss and hum).

So, why do Alesis do this, and can I safely short out the resistor, or
would this render it liable to RF oscillation or something? If it was my
system I'd suck it and see, but I'd rather not "have a go" with someone
elses.

Any info appreciated!


Thanks,

Gareth.