Re: Weird problem with M-Audio Transit & signal strength meters

On Thu, 05 Jan 2006 22:28:46 GMT, Jonathan Sachs
<xxxxxxx@xxxxxxxxxxxxx> wrote:

>On Thu, 05 Jan 2006 13:04:42 -0800, Loren Amelang <loren@xxxxxxxxxxx>
>wrote:
>
>>Did this high meter reading happen instantly, or did it take a few
>>seconds/minutes?
>
>Instantly.
>
>>Could you still hear clean music through the line in,
>>or were you getting loud noise as well?
>
>Clean.

Ok - you don't have the "hissing" problem I had. (Which as someone
later pointed out was probably mostly ultrasonic, with just a bit of
modulation that I could hear.)

I'm betting on the theory that the 2.2 V mic power is feeding back
into your meters. Testing with a series capacitor in the audio line
would confirm or disprove that.

>>If both your computer and the connected audio equipment are grounded,
>>try disconnecting the computer from everything but the cable to your
>>audio interface (if you can),...
>
>The tape deck can't be grounded unless it's grounded through the
>Transit itself, since it doesn't have a grounded power plug. There's
>nothing like a grounding post on the rear panel, so it's not clear how
>I would accomplish this task. Attach a wire to one of the cover
>screws? I suspect the mechanism is double-insulated, so that wouldn't
>do any good.

Not that it matters to your problem, but everything with any
connection to mains power is grounded to some degree. If there is no
third prong ground and no transformer isolation, the chassis may be
connected directly to one side of the AC line (hopefully with a
polarized plug to try to grab the low side, and hopefully only in
grossly antique and obsolete equipment), or may be connected to the AC
line via a small capacitor or large value resistor or both. If there
is transformer isolation, it may still be bypassed by a capacitor or
resistor, and if not, there is still a tiny bit of "grounding" via
capacitive coupling through the transformer. Even that tiny bit of
grounding was enough to cause my "hissing" problem!

Your tape deck has its idea of ground, your computer another. The
shield of your audio cable to the Transit follows the tape deck, the
USB power ground to the Transit follows your computer. The tiny wires
in the USB cable are probably the weakest link, so the power the
Transit sees is compromised. I had some success connecting a heavy (12
AWG) grounding wire between the computer chassis and my external audio
source, (as you say, via the cover screws), but it wasn't a complete
solution.

The fact is, "ground" is a most elusive commodity. We can only
approximate it, no matter how deep the ground rod and how large the
wire connecting it. As the wires branch out toward our audio
connections, often the best we can do is to try to pull the branches
that matter a bit closer together, or disconnect whatever is pulling
some of them away from ground.

Loren
.

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