[Bob F]

Don Pearce wrote:
The schematic here seems completely wrong to me.
http://www.mcmelectronics.com/conten...Sheets/50-7725
.pd
f

http://canada.newark.com/mcm-custom-...alun-rca-plug-
rj4
5-shield/dp/96K0889

Looks OK to me. It's configured as a series common mode stopper.
What's the problem?

Will this design eliminate ground loop problems? My concept of
baluns was that
they isolated the two ends by placing them on opposite windings
of a transformer. I am no expert, which is why I asked.


That is a different thing. You need a transformer to kill a ground
loop entirely, but yes, provided there is enough inductance in the
coils this will still make a good job of getting rid of hum from a
ground loop.

If there's enough inductance to attenuate 60 (or 50) Hz from the
power line, why won't it do the same for similar frequencies in
the desired signal?


No, because of the way the windings are arranged their inductances
cancel each other out for the audio signal, but not for the induced
hum.

I'd sure like to see (and measure) those inductors. My "spidey sense"
(combined with the fact that they don't bother to spec the CMRR at
50 or 60 Hz.) tells me that they're probably way too physically
small for that to be the case.


Spidey sense is not always very useful.

The downside of the transformer method is that lower bass
response is usually somewhat compromised.

For any sort of well-designed and properly terminated transformer,
the lower 3dB point will be well below the frequency of any
"musical note" you'll ever want to pass through it. So, no, bass
response won't be compromised at all.

Effects strat becoming apparent well before you hit the 3dB point.

Know anybody who can hear the difference *on actual program material*
between flat to 5 Hz and -3dB at 5 Hz? Because the little thumb-sized
not-at-all-special transformers I use in a homebrew groundloop killer
have that measured characteristic. It's just not hard to find decent
transformers for audio.

Isaac

Construction of these transformers doesn't have to follow the normal
rules. They don't need good linearity, so very high permeability
ferrite can be used for the core in order to get a high inductance
value. This is because the audio doesn't have to pass through the
ferrite, it bypasses it.


Doing a little research, I found the following page. It seems these baluns are
"current mode" baluns.

http://vk5ajl.com/projects/baluns.php