[sdiy] Protective Earth on PC soundcard GND?

[Magnus Danielson]

From: "JH." <jhaible at debitel.net>
Subject: [sdiy] Protective Earth on PC soundcard GND?
Date: Thu, 27 Jan 2005 20:11:54 +0100
Message-ID: <000f01c504a4$15114c50$0200a8c0 at jhsilent>

> Hi,

Hi Jürgen,

> still considering various PC recording options, here's a stupid question:
> 
> Is the GND of typical PCI soundcards floating, or is it connected to
> protective earth inside the PC? (speaking about European mains voltage,
> German in particular)

Yes or at least more or less connected (few ohms).

> Background: I wonder if it's worth to invest in a balanced in/out card,
> because my mixer has unbalanced (RCA, -10dBm) inputs and outputs for the tape
> machine (wich will be used to interface a 8in/8out soundcard).
> 
> I'm aware that I could take advantage of balanced TRS inputs on the PC even
> then, connecting the hot RCA pin to the TRS tip, and the RCA-GND to the TRS
> Ring, leaving the TRS sleeve unused (or connected via resistor), thus
> avoiding a ground loop.

Indeed. This is what you should do if you don't have a propper module on the
side with it's own power and ground which naturally is the ideal. Remember my
reasoning about not wanting to climb behind the computer to often.

> But the other side, PC balanced TRS outputs to unbalanced RCA inputs on the
> mixer, would only work if (*if* !) these TRS outputs would really be
> "floating balanced" (as with a transformer, or with a special cross-coupled
> opamp output). But I suspect "balanced output" all too often means just
> complementary signals which are both referenced to GND, in which case a
> tip/ring to RCA connection will be fatal (or at lest not break a ground
> loop).

There is ways to actively drive wich allows the drive to be diffrential while
the common mode may be controlled or indeed acidental as it would become with
ground loops and other imperfections in offset. Within the (little) range of
the op-amps it works, but the cost is headroom.

> OTOH, if the soundcard has no connection between GND and PE, all would be
> fine.

I would not count on it.

> That's why I am asking.
> 
> (I still have this unexpensive M-Audio Delta 1010LT in mind, but I'm also
> asking if there is a general convention of connecting, or not connecting GND
> and PE in a PC ausio system.)

Ground should be connected at all times, but with real balanced systems you may
cut it when that aids in lowering the hum.

Cheers,
Magnus