Men det er nok muligt at VdH har taget det for givet at et balanceret signal også er symmetrisk og at det måske er udgangspunktet for at han skriver dette om det med at forbinde skærmen i begge ender af kablet. Som du kan se antyder han også muligheden for RF problemer:
If the cable is to be used for microphone or non mains powered musical instrument connections, i.e. with low (non line level) signals, the cable shield should also be linked to the signal sending side connector’s ground pin. There are two reasons for this:
1. To extend the shielding function to the microphone or instrument’s housing.
2. And also with phantom powered condenser microphones: To provide a phantom power supply current return path.
In all other cases (i.e. line level connections) leaving the cable shield disconnected at one side of the cable (drawn above) is safe practice. This to avoid ground loops with non properly internally grounded equipment. If with these “one side disconnected shield” connections RF interference problems are experienced, reconnecting the shield at the side where it was disconnected by means of a series network of a 47 Ohm resistor and a 10 nanoFarad ceramic disc capacitor is advised. Latter may be standardly applied for all balanced line level interconnections.
Reference: K.R. Fause. Fundamentals of Grounding, Shielding and Interconnection. Journal of the Audio Engineering Society, Vol. 43, No. 6, June 1995, p. 498-516.
Jan Didden som har udviklet det nye udgangstrin til Behringeren skriver om den "balancerede" udgang:
On the balanced issue, there are two issues really. Firstly, why balanced lines? Because if we run two signal wires closely twisted, AND we make sure that they have identical impedances to ground, any noise, interference, etc will be impressed identically on each wire. At the receiver end we will have a differential stage that amplifies the signal only and not the noise etc because that is common mode.
Note that there is NO requiremment for the signals to be equal in amplitude! If we do drive the signals in antiphase, we get 6dB more output level from a given supply voltage which may get you 6dB more S/N ratio, depending on other factors. But it has NO effect on the advantages of balanced interconnects to reject noise, hum, buzz, etc. In fact, you can get impedance balance with one wire driven from an opamp through a defined output impedance, and the other wire to ground through the same impedance. In this scenario, only one wire carries the signal but you still have the full advantages of fully balanced interconnects.
Jeg havde overset den røde sætning og hvor vigtig den var.
Og nu tilbage til målemikrofoner
