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.

My set-up: the soundcard is installed within a new-ish Dell Dimension
PC. It is connected to my stereo via two pairs of mono leads, each
manufactured with an RCA jack on one end and a phono jack on the
other. These are plugged into a (high quality) DIN-phono adapter,
since my amplifier has five-pin DIN inputs and outputs. The amp
operates at DIN level (a lower signal level than the line level used
by
modern equipment) and is a Sugdens A48 made in 1976. The soundcard is
connected to the amplifier via the Tape input/output.

Obviously, the soundcard is not receiving the "balanced" signal for
which it is designed, but I cannot imagine how to generate a
"balanced" input to it. And the manual does indicate that it should
work with an unbalanced signal of the kind that I am supplying to it.

When connected in this way, there is a near-constant hum in my system,
at a level high enough to be distinctly irritating. 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. The computer is normally
connected to the same power supply as the amplifier, and so they
should
be grounded at the same level. However, unplugging the computer (so
that of course it is no longer grounded) seems to make no difference
to
the hum.

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.

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