Poor quality MBox 2 SPDIF out signal.

[danlea]

I need to use my MBox 2 with my Cambridge Audio Dacmagic and have connected it via the Coaxial SPDIF out, however the signal is really poor, i.e. the DAC has trouble locking onto the signal, particularly when operating at 48kHz, but also at 44.1kHz. The clock is set to internal and I'm using a decent digital coaxial cable that works fine with my CD player and the DAC. The DAC also operates solidly via its own USB connection using the same port on the PC (48KHz, 16 bit only).

Is this just a defective MBox, or is there anything else that's likely to fix the issue?

Dan.

[Kris75]

Just to make sure,
Are you using a 75ohm cable?

[danlea]

While I don't have the packaging for this cable any more, it's definitely a 'digital audio' coax cable, and so will be 75 Ohm.

[danlea]

OK, it seems that the steadiness of the signal is highly variable. Position of the hardware and cables does appear to make a difference, but no configuration consistently provides a good signal lock.

This suggests RF interference, and so perhaps a better cable is the solution, but I'd like to rule out an earth loop. Does the S/PDIF out have an isolating transformer like it should?

[danlea]

Right, it's an earth loop. Connection is fine when I unplug my active monitors. That's not particularly useful mind you, but at least I've pinned down the problem.

[danlea]

For posterity, here's the precise cause of my problem and the solution:

MBox 2 -- Coaxial S/PDIF -> Cambridge Audio DacMagic -- TC Electronic Level Pilo (passive line-level volume control, XLR-XLR) -> Yamaha HS50M active monitors.

The issue seems to be that the Level Pilot connects the earths of both speakers, causing an earth loop that compromises the digital signal from the MBox (disconnecting one of the speaker connections on the speaker side of the volume control fixes the signal lock).

The simple and cheap remedy (bearing in mind that the MBox 2 does not have an optical out) is to buy an earth lift XLR coupler (in-line or short cable) for one of the speakers. I should note that I do not experience any noise on either speaker, with or without this fix (in a small studio).

[memoryman1978]

Hi, just purchased my first MBox (3rd gen). Was wondering if there is a noticeable improvement in sound quality, stereo imaging, detail etc by running an outboard DAC between my MBox (SPDIF out) into an outboard DAC like the Matrix Mini-i or a DAC Magic (SPDIF in)???

Anyone have any advice for a newbie?