ground lift hum stopper?

[peter]

At a theater when I get an audio feed from the house mixer, it always come
with a bit of hum.

While reading about ground lift hum stopper, I come across a bunch of boxes
selling for around $30. Some (all?) contains isolation transformers. I
understand how this works.

I also find some device under $10. E.g.
http://www.fullcompass.com/product/290797.html
It looks too small (and too cheap) to contain a transformer. If I may hazard
a guess, all it does is disconnect pin 1 (ground). But if this works, why do
other hum stoppers bother with an isolation transformer?

[Paul Stamler]

"peter" > wrote in message
news:MLCvh.27267$uC6.14771@trnddc02...
> At a theater when I get an audio feed from the house mixer, it always come
> with a bit of hum.
>
> While reading about ground lift hum stopper, I come across a bunch of
boxes
> selling for around $30. Some (all?) contains isolation transformers. I
> understand how this works.
>
> I also find some device under $10. E.g.
> http://www.fullcompass.com/product/290797.html
> It looks too small (and too cheap) to contain a transformer. If I may
hazard
> a guess, all it does is disconnect pin 1 (ground). But if this works, why
do
> other hum stoppers bother with an isolation transformer?

Because some audio devices (e.g. unbalanced circuits) don't work when you
disconnect pin 1.

Peace,
Paul

[Mike Rivers]

peter wrote:

> http://www.fullcompass.com/product/290797.html
> It looks too small (and too cheap) to contain a transformer. If I may hazard
> a guess, all it does is disconnect pin 1 (ground). But if this works, why do
> other hum stoppers bother with an isolation transformer?

Because sometimes lifting pin 1 doesn't solve the problem. But you
should know that sometimes just putting a transformer in line doesn't
solve the problem either. It's handy to have gadgets like this in your
tool kit, because one or the other might be a quick fix. But often you
have to dig deeper.

[Scott Dorsey]

In article <MLCvh.27267$uC6.14771@trnddc02>, peter > wrote:
>At a theater when I get an audio feed from the house mixer, it always come
>with a bit of hum.
>
>While reading about ground lift hum stopper, I come across a bunch of boxes
>selling for around $30. Some (all?) contains isolation transformers. I
>understand how this works.

This is what you want. I urge you to spend the additional money and get
a Jensen, though.

>I also find some device under $10. E.g.
>http://www.fullcompass.com/product/290797.html
>It looks too small (and too cheap) to contain a transformer. If I may hazard
>a guess, all it does is disconnect pin 1 (ground). But if this works, why do
>other hum stoppers bother with an isolation transformer?

Hosa COULD be using a transformer in that thing... it would be a pretty
godawful one, worse than the one in the $20 Radio Shack unit.

It could also be a device that breaks pin 1, which strictly speaking is
called a ground lift adaptor.

Sometimes for hum problems, a ground lift adaptor is all you need, because
it allows you to break the grounds and allow the common mode rejection of
the input stage to eliminate the noise. The problem is that most equipment
today uses electronically-balanced inputs, which don't always have the best
noise rejection.

But if you have a console with transformer-isolated inputs, which used to
be the norm a few years ago, a ground lift is probably all you need... no
need to throw an additional transformer in front of the one that is already
there.

I always keep a dozen or so isolation transformer boxes in the back of the
truck, and I always do broadcast feeds through the things, just because it
eliminates any possibility for finger-pointing when things go wrong.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Julian]

On 30 Jan 2007 09:04:56 -0500, (Scott Dorsey) wrote:

>In article <MLCvh.27267$uC6.14771@trnddc02>, peter > wrote:
>>At a theater when I get an audio feed from the house mixer, it always come
>>with a bit of hum.
>>
>>While reading about ground lift hum stopper, I come across a bunch of boxes
>>selling for around $30. Some (all?) contains isolation transformers. I
>>understand how this works.
>
>This is what you want. I urge you to spend the additional money and get
>a Jensen, though.

In a perfect world...

>Hosa COULD be using a transformer in that thing...
>It could also be a device that breaks pin 1, which strictly speaking is
>called a ground lift adaptor.

Look at the link he sent. It says right there - "Ground Lift Hum
Stopper". It just lifts the ground.

>it would be a pretty
>godawful one, worse than the one in the $20 Radio Shack unit.

Come on, it ain't that bad. 'bout the best sounding $20 audio device
around.

>I always keep a dozen or so isolation transformer boxes in the back of the
>truck,

Yikes, some truck you have!

Julian