Soundcraft GB-Pultausgänge

Zitat von "Fränkie"

Dort sind zwar XLR-Buchsen drann, aber der Pin3 ist dort über einen Widerstand auf Masse gelegt.

Hast du dir mal den Wert angeschaut?

Hallo Fränkie,

keine Panik wegen der Ausgänge, diese machen im täglichen Betrieb genausowenig Probleme wie "richtige" symmetrische (und wenn du nicht ins Schaltbild geschaut hättest, wäre es dir sicher auch gar nicht aufgefallen stimmts ?). Beschalte sie einfach genauso, als wären sie symmetrisch, und alles wird gut. Solltest du einen unsymmetrischen Eingang damit ansteuern, so wird empfohlen, die Leitung trotzdem mit 2 Adern plus Schirm auszuführen und die Brücke zwischen Pin 3 und 1 beim XLR am eingangsseitigen Ende der Leitung zu löten (falls nicht im Gerät!) bzw. die kalte Leitung mit dem Schirm zu verbinden, im Falle eines Klinkensteckers. dadurch werden Störanteile, die die Leitung auffängt, mit kompensiert. Den Schirm darf man zur Vermeidung von Erdschleifen unterbrechen (Groundlift), da er nicht als Signalrückleitung dient.

HTH Jörg

Da hat der Hias recht. Das gab’s in den 80ern schon mal, sogar bei ‘richtigen‘, voll modularen Pulten der Größenordnung 30.000,- DM und mehr (z.B. bei TAC/ Amek). Funktioniert in der Praxis problemlos, auch wenn es ganz sicher nicht ganz die feine englische Art ist. Ein wenig an den Haaren herbeigezogen könnte man sogar argumentieren: ein mal weniger überflüssiger Weise symmetriert = ein mal weniger Übernahmeverzerrungen im Nulldurchgang...
Andererseits: bei einem Passat oder gar einer S – Klasse wär’s ein Aufreger. Bei einem Trabbi nimmt man’s achselzuckend zur Kenntnis - falls man es denn überhaupt bemerkt.

Mit freundlichem Gruß
BillBo

Zitat

Ich stell mir mal vor jemand schliesst so ein Pult an einen nicht Phasen gedrehten Klark an, da gibts bestimmt erst mal lange Gesichter, :lol:

Nur der Richtigkeit halberer: In der Polarität gedreht!!!
Ist ein großer Unterschied

Jawoll, wegen dem "impedance balanced" würde ich mir keine grauen Haare wachsen lassen. Natürlich ist die CMRR etwas schlechter als bei einer Schaltung mit Differenzverstärker oder Übertrager, was allerdings in 98,5 % der Fälle bei line Pegel keine praktische Rolle spielt. Allerdings fällt dafür ( wie billbo schon sagte ) die Verzerrung durch den Übertrager oder den Differenzverstärker weg, ( die ebenfalls kaum eine Rolle spielt :wink: ).

Auf jeden Fall war der Stereo Master Out meines ersten Soundcraft 1S vor ( im Januar sind es genau ) 32 Jahren ebenfalls 'impedance matched' und ich kann mich nicht entsinnen, das es bei Anschluß an einen symmetrischen Eingang einer Weiche mit 50m Core dazwischen je ein Brummproblem auf dem Output gegeben hätte. :wink:

Der Thread ist zwar schon etwas älter, aber dieses Statement von Graham Blyth (der das Pult entwickelt hat) kam mir gerade wieder in die Finger:

Zitat von "Graham Blyth"

The world is a very dangerous place today if you are an innocent
piece of clean audio trying to get from A to B.
The air is full of electromagnetic radiation that exists over a massive range of
frequencies all the way from A.C. mains up to mobile phones and even beyond
into radar, and we have to find ways of keeping this interference out of our
pristine audio. We are mostly concerned with the audio engineer’s old enemy,
buzz from lighting and mains cables, but we also cannot ignore the effect of
frequencies beyond the audio spectrum.
Old science to the rescue.

Most of our audio cables are screened, and this does a decent job in keeping
the Radio Frequencies at bay, but doesn’t do anything much to keep out the
signals that we can hear, because these attack our audio largely by long wave
length electromagnetic fields which go straight through the expensive foil
wrap and braided screen protection around our cables. Our telephone lines,
however, run over vast distances but are unscreened, yet work just fine, and
the BBC used to run hundreds of meters of unscreened cable all around its
Langham Place complex with no problems. How? Very early in the history of
telecommunications it was realised that if you drive audio across a pair of
wires and twist these wires together, either wire has the same statistical
chance of picking up some unwanted mush as its twisted partner. The
resulting signal appearing equally on both wires is known as a Common Mode
Signal, and how well it is rejected by the audio circuitry is known as the
Common Mode Rejection Ratio or CMRR.
What do we mean by balanced?

When buzzy interference is heard, most people blame the input stage, but it
doesn’t matter how good the CMRR of your input stage is if the unwanted
signal on your audio cable isn’t Common Mode, so how can we make sure that
it is? These airborn signals neither know nor care how the audio signal got on
the two wires, what level it is or what colour the wires are, because they
cannot see these things. All they know is how easily they can impress
themselves onto these wires and this is entirely controlled by the Impedance
that each wire presents to the unwanted signal. It is therefore essential that
these two impedances are ‘balanced’ and if they are, then you have a truly
Balanced Line. It is very easy to make the source impedance of the hot and
cold signals equal by connecting a 75ohm resistor between the opamp output
pin and the hot wire, and a matching resistor between the audio ground
nearest to this opamp output and the cold wire. This is known in common
parlance as Impedance Balancing, and is tremendously effective, takes no
additional circuitry and is therefore the most cost effective solution.
You might be surprised to know that the vast majority of condensor
microphones, including some of the most expensive on the planet, use this
method. Now if this is acceptable at mic level then, Q.E.D, it must be brilliant
at line level, so why use any other method?

Going a step beyond

Apart from using a transformer, which does a marvellous job in input and
output stages but is large, heavy and expensive, the “other” method is
Differential Balancing.
In this method, two opamps drive the audio signal to both hot and cold wires,
the signal to the cold wire being an inverted copy of the hot. Let’s be clear
from the start here: if the source impedance of each of these signals was not
identical i.e. balanced, the method would fail completely, the matching of the
differential audio signals being irrelevant, though desirable for headroom
considerations. However, there is an interesting technical reason that
supports the differential solution, particularly over long distances. At
frequencies above the audio range, the output impedance of the opamp
driving the hot wire in our simple classic Impedance Balanced circuit begins
to rise, whereas its balancing resistor in the cold wire stays substantially the
same, thus creating a very small inbalance. However, in the differential
solution the cold wire is driven from an opamp that is identical to that driving
the hot wire, thus preserving the balance over a wider frequency range. Over
long distances this can make a difference in difficult environments, hence you
will find Differential Balancing on the main outputs in more expensive
consoles that might be used in bigger installations and touring situations
where cable runs can be considerable.

Two other features make the differential method attractive. Firstly, you can
drive 6dB more level down the cable before dropping it 6dB at the other end,
thus gaining 6dB of common mode rejection, but this is almost never done for
standardisation reasons. Secondly, most implementations of differential
balancing can be unbalanced by shorting out either cold or hot to ground, with
no change in signal level. This can save the day if someone has picked up a
wrongly wired or faulty cable, though it can hardly be said to encourage good
practice on the road.

Ignore the hype

It is rare to find unanimity amongst audio gurus, but without exception they
will all say that the only true Balanced Line is an Impedance Balanced Line.
Unfortunately there is a lot of marketing hype and uninformed peer pressure
that will try to tell you that the only true balanced line is one that has opposite
phase audio signals on each wire. This is NOT the reason that makes
Differential Balancing worth paying more for over the simple elegance of
Impedance Balancing. If your cable runs are going to be really long (more
than 100m), then you might benefit from the balance in hot and cold
impedances across a wider bandwidth that differential balancing offers, but
for line level signals it’s very much a marginal call.

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