Re: [LAU] Behringer and Linux

On Thu, April 2, 2015 10:39 pm, mark hadman wrote:
> Len:
>> I was looking at the X(R)18. Which acording to Sweetwater has 16 i/o
>> but on the Behringer site says 18/18. I would hope the manufacture
>> is the right one. Actually, the sweetwater site says 16 USB channels
>> in one place and 18 io half a page away... so I can see both numbers
>> at the same time. :P
> Fons:
>> just a quick remark on USB and Behringer. At my new workplace our
>> boss, in a fit of enlightment and for reasons only known to himself,
>> decided to buy a cheap Behringer mixer with a USB interface. Nobody
>> here needs it and it remained in its box for months.
>> Some week ago I had some spare time and decided to have a look at
>> it. Connected the USB to my Linux workstation and connected some
>> headphones to the mixer. Result: with all faders down and just
>> the HP volume turned up there was a high level 50/100 Hz in the
>> headphones. Removing the USB connection was enough to stop it.
> Patrick:
>>> That is not bad considering that only a few years ago the idea of an
>>> affordable plug and play digital hardware mixer on Linux was just that,
>>> an idea.
> Fons:
>> It's not a digital mixer, nor did I suggest it was.
>
> Just re-editing and recapping this, as I too was confused.
> Fons was not talking about the XR18 or any of Behringer's new digital
> range.
>

Now that has been resolved, here's some suggestions for resolving the issue.

(1) The mixer and the Linux box both use switching power supplies. They
are plugged into the same outlet strip. This will occur with most
switching power supply audio devices and most laptops. Much less of a
problem with workstations with large well filtered motherboards and power
supplies. Less of a problem if the devices are on separate power
circuits. Not a problem if the audio device is powered from a linear
supply or battery. Often goes away even if just the laptop gets powered
off its battery.

The two switching power supplies operate at frequencies that are close but
not identical and not synced. The noise generated is very buzzy.

(2) If Fons had audio cables from his workstation plugged into the mixer
(even if the faders are all down) with the USB plugged into same, then the
odds are he had a "ground loop" as the "digital ground" and "analog
ground" of the workstation do not reference the same points (i.e. are not
the same "ground"). This will happen with ALL gear that does not have
audio isolation transformers on the inputs and outputs. The solution is
to either use external ones (Jaycar and Altronics in Australia, and many
other outfits supply them.) The other solution is to not use metal
connected USB but rather fibre optic SPDIF or ADAT (the reason for their
existence!).

Ground loop noise has its primary frequency the power line frequency and
may have a large second harmonic component. Fon's description of the
noise is consistent with this problem. If his Linux audio workstation is
a full-out desktop workstation with a big high-end power supply and the
motherboard has a premium audio chipset, then the above "ground loop" is
almost certainly the problem.

Both of these, are not really Behringer problems. It is nearly impossible
to sell new audio gear with linear supplies due to regulations about power
efficiency in many countries. Only very exotic (and correspondingly
expensive!) high end studio gear has either (and sometimes both), high
quality audio transformers on all analog inputs and outputs, and/or common
mode rejecting (true-differential) semiconductor circuits on all analog
inputs and outputs.

Marschall Acoustics Instruments P/L (*) electronics always has
true-differential semiconductor inputs and outputs, with additional
isolation transformer coupling available on request at extra cost and
physical device size (not to mention weight). MAI equipment also only
uses linear supplies (not switching). It is usually sold as "research
instrumentation" for import purposes, but could never be sold to consumer
retailers.

(*) http://hydrophones.com

--
Patrick Shirkey
Boost Hardware Ltd
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