Hum problem

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Can someone throw some light on this?

I've just bought a Benchmark DAC1. I connected it to my Audiolab 8000A
amp via the RCA sockets. There was a hum at the speakers.
When I disconnect the earth wire from the amp power cord the hum
disappears.
I'm assuming the DAC1 is earthed.
The man from Benchmark said "It sounds as though the problem amp is
not tying chassis ground to signal ground. This would account for the
hum. I would contact the amp manufacturer and find out what they
suggest"
What do other people think?
I should mention, I had the phono sockets replaced about 18 months
ago. Also, no other devices attached to the amp produce a hum, but I
know they are not earthed.

[jamesgangnc]

You have a ground loop. This is why commercial equipment uses three wires
instead of two. The problem amp is tying signal ground to chassis ground
and that's the trouble. Do some searchs on "ground loop". There are a
number of solutions.

> wrote in message
...
> Can someone throw some light on this?
>
> I've just bought a Benchmark DAC1. I connected it to my Audiolab 8000A
> amp via the RCA sockets. There was a hum at the speakers.
> When I disconnect the earth wire from the amp power cord the hum
> disappears.
> I'm assuming the DAC1 is earthed.
> The man from Benchmark said "It sounds as though the problem amp is
> not tying chassis ground to signal ground. This would account for the
> hum. I would contact the amp manufacturer and find out what they
> suggest"
> What do other people think?
> I should mention, I had the phono sockets replaced about 18 months
> ago. Also, no other devices attached to the amp produce a hum, but I
> know they are not earthed.

[Bob Eld]

> wrote in message
...
> Can someone throw some light on this?
>
> I've just bought a Benchmark DAC1. I connected it to my Audiolab 8000A
> amp via the RCA sockets. There was a hum at the speakers.
> When I disconnect the earth wire from the amp power cord the hum
> disappears.
> I'm assuming the DAC1 is earthed.
> The man from Benchmark said "It sounds as though the problem amp is
> not tying chassis ground to signal ground. This would account for the
> hum. I would contact the amp manufacturer and find out what they
> suggest"
> What do other people think?
> I should mention, I had the phono sockets replaced about 18 months
> ago. Also, no other devices attached to the amp produce a hum, but I
> know they are not earthed.


Yep, it's a ground loop as the other poster said. Actually the problem is
that the signal common (ground) is tied to the power ground in MORE than one
place. This creates a loop and the loop intersects stray magnetic fields
from the transformers and power wiring inducing currents into the signal
grounds, thus the hum.

The solution is to break the loop. The signal ground should connect to the
power ground in only ONE place so no loop can form. When you disconnect the
earth wire you are breaking the loop. However, that is a safety issue so
that is the wrong place to break the loop. Try disconnecting the ground on
one end of the signal leads.

The guy from Benchmark actually has it backwards, the problem IS connecting
the signal ground to the earth ground. It's important to only connect it in
ONE and only ONE place.

[gareth magennis]

"Bob Eld" > wrote in message
...
>
> > wrote in message
> ...
>> Can someone throw some light on this?
>>
>> I've just bought a Benchmark DAC1. I connected it to my Audiolab 8000A
>> amp via the RCA sockets. There was a hum at the speakers.
>> When I disconnect the earth wire from the amp power cord the hum
>> disappears.
>> I'm assuming the DAC1 is earthed.
>> The man from Benchmark said "It sounds as though the problem amp is
>> not tying chassis ground to signal ground. This would account for the
>> hum. I would contact the amp manufacturer and find out what they
>> suggest"
>> What do other people think?
>> I should mention, I had the phono sockets replaced about 18 months
>> ago. Also, no other devices attached to the amp produce a hum, but I
>> know they are not earthed.
>
>
> Yep, it's a ground loop as the other poster said. Actually the problem is
> that the signal common (ground) is tied to the power ground in MORE than
> one
> place. This creates a loop and the loop intersects stray magnetic fields
> from the transformers and power wiring inducing currents into the signal
> grounds, thus the hum.
>
> The solution is to break the loop. The signal ground should connect to the
> power ground in only ONE place so no loop can form. When you disconnect
> the
> earth wire you are breaking the loop. However, that is a safety issue so
> that is the wrong place to break the loop. Try disconnecting the ground on
> one end of the signal leads.
>


Noting that they would have to be reconnected if the OP wanted to use any of
his non-earthed other sources.



> The guy from Benchmark actually has it backwards, the problem IS
> connecting
> the signal ground to the earth ground. It's important to only connect it
> in
> ONE and only ONE place.
>
>

[Jim Gregory]

> wrote in message
...
> Can someone throw some light on this?
>
> I've just bought a Benchmark DAC1. I connected it to my Audiolab 8000A
> amp via the RCA sockets. There was a hum at the speakers.
> When I disconnect the earth wire from the amp power cord the hum
> disappears.
> I'm assuming the DAC1 is earthed.
> The man from Benchmark said "It sounds as though the problem amp is
> not tying chassis ground to signal ground. This would account for the
> hum. I would contact the amp manufacturer and find out what they
> suggest"
> What do other people think?
> I should mention, I had the phono sockets replaced about 18 months
> ago. Also, no other devices attached to the amp produce a hum, but I
> know they are not earthed.

In my book, the screen [or shield] is there to protect a signal Input wire
from picking up any intrusive hum, so anchor it to the earth [or ground] at
the Receiving end only. If, wrongly, it is connected at either end to
apparata interfaces, it will create a 'loop' of voltage difference between
ground levels, which gets amplified and, worse, gets noticed.
So cut the unwanted outer connection at the signal Sending end!
Jim