The perils of impedance matched direct outs

[bergante]

Hi,

Just sharing a problem I had last Saturday hoping it can be helpful for others.

I own a WZ3 14:4:2 purchased back in 2006. I have used it very little so it is almost brand new. Last weekend I have taken it to our jazz club because the old Mackie is in really bad shape and it needs a replacement.

Everything was working like a charm until, suddenly, we noticed a metronome like interference coming out through the master bus. I had the desk almost maxed out in connections using 10 channels with 7 phantoms on, a compressor on a channel insert, two graphic equalizers (one on subs 1/2 configured as aux masters and another one on the master insert) and the 10 direct outs with the default pre-eq configuration connected to a MOTU Traveler and a Behringer ADA8000 using a balanced multi-core.

Despite having all of this powered from the same power strip I begun disconnecting compressor, equalizers, etc with no success until I disconnected the direct out jacks, which stopped the interference. Actually the first problem cause I suspected was the power supply but turning out phantom power didn’t make a difference either.

After completing the sound check and inspecting the recording setup better I noticed that the Magsafe connector of the Macbook Pro used to record the concert was damaged and it wasn’t making a proper contact. So, it seems that there was a one-second cycle of connecting the power supply, checking power was valid and disconnecting it, which was causing the Traveler (first version, bus power) to restart. Once we reseated the Magsafe connector carefully everything worked perfectly and we could complete the concert *and* record it without issues.

However: I was rather puzzled at the vulnerability of the MixWizard3 direct outputs to interference coming from the direct outputs. Of course I made sure that phantom power wasn’t enabled in the Traveler (it has actual physical switches for phantom power so I don’t think it was sending brief phantom “pulses” while initializing).

Is this to be expected? I had been considering a second hand GL2400 for the club (although we don’t use many microphones I already maxed out the 14:4:2) and I see that they share the basic architecture. As the live output of the direct outs seems to be buffered according to the block diagram it would seem that the interference was coming in through the connector pin tied to ground through a resistor.

No harm done apart from the sound check trouble and the anxiety caused by the paranormal phenomenon until the cause was located, though!

And the old mixer performed like a charm.

[bergante]

I mean balanced, not matched.

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