"david" wrote in message
om

Could anyone please offer me some constructive suggestions:

Almost a year ago, I bought an Echo Mia soundcard and installed it in
my computer. Up to now I have not got it to work properly, suggesting
that my set-up is incorrect.

Obviously, the soundcard is not receiving the "balanced" signal for
which it is designed,

Ironically, the Mia does not have *real, active* balanced inputs, just input
jacks that would be capable of handling the outputs from a balanced source.
It doesn't have *real, active* balanced outputs, either.

However, you could still have all of the problems you have, even with real,
active balanced inputs and outputs. For example, your most obvious problem
exists at the input to your amplifier, not at the input to the sound card.

but I cannot imagine how to generate a "balanced" input to it.

You don't need to.

And the manual does indicate that it should
work with an unbalanced signal of the kind that I am supplying to it.

The manual is telling you like it is.

First off, you've confused inputs with outputs. I read your post as saying
that the sound card makes your system hum. Problems with balanced inputs
would make your recordings hum, but you seem to be saying that anything you
playback hums, and in fact you've got a hum even where there is no playback
or recording active.

When connected in this way, there is a near-constant hum in my system,
at a level high enough to be distinctly irritating.

You've got a ground loop!

When the soundcard
outputs are connected to the amp, the hum can be heard through my
loudspeakers, whether or not the computer is turned on, or even
connected to the mains power supply.

A ground loop is due to two or more ground connections between two pieces of
equipment. The basic trick is to reduce that to one good ground.

My take is that there is something else connected to the computer that
effectively grounds it, even when disconnected from the mains. The mains
connection is probably the third or fourth or whatever ground. With the
mains disconnected, there are still two or more, one being the connection to
your ampliifer.

The computer is normally
connected to the same power supply as the amplifier, and so they should
be grounded at the same level.

But, the amplifier is also connected to the computer through the audio
wiring, so there are your 2 or more grounds.

However, unplugging the computer (so
that of course it is no longer grounded) seems to make no difference
to the hum.

That just means that there is a third and perhaps also a fourth or fifth
ground, as well.

Similarly, whenever the amplifier is connected to the soundcard's
inputs the signal that it receives includes a constant hum, so that any
recordings I make with it from my sound system are marred by the hum
that is recorded along with the music.

All things considered, that is to be expected.

Please let me know if you have any suggestions for reducing or
eliminating the hum.

Please read these documents:

Introduction:

http://www.smr-home-theatre.org/Ground-Loops/

Meat of the matter:

http://www.epanorama.net/documents/g...oop/index.html

I have recommended these two documents dozens of times, and know of no
situations where they did not fail to shed light. You may still have some
questions after you read them, but reading them will explain a lot of things
to you.