[Scott Dorsey]

Phil Allison wrote:
Scott Dorsey wrote:

Remember the *only" connector is a HP jack.

IME, such units do not have enough metal shielding to exclude buzz and hum
from fluoro and dimmed domestic lighting.

I have a little box that has 600:600 transformers in it and a shunt load
on the transformer secondary. The idea being that with a lower impedance
load, the keyboard noise is less severe. I have really only had to use it
with $10 Casio keyboards, gameboys, and the like, but it helps a bit for
the really cheap stuff.

** Grounding the common rail inside such contraptions often makes injected electrical noise from the environment worse.

What do you mean by "grounding?" The grounds on the input are not bonded
to the grounds of the output. (They can be with a switch if necessary).
The idea is to float the two grounds.

The OTHER marvelous benefit of the transformer is that it provides a free
low-pass filter to reduce RF noise, which is one of the biggest problems
with these things. Switching supplies produce RF trash, it gets into audio
electronics and nonlinearities detect it down into audio.

When no such ground exists ( eg when the unit is on battery power ) signals tend to inject into all wiring equally, leaving little differential anywhere.

Same goes when coupled through a 1:1 transformer, with low capacitance to earth.

These things are true. You can't do anything about the noise on the unbalanced
input side other than to low-pass some of it and to provide a low enough
load impedance that capacitively-coupled noise is reduced.

But you CAN split grounds between the balanced and unbalanced side, and you
CAN balance the signal as close as possible to the instrument.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."