Signal to noise ratio: -100dB with respect to 1V output, 4.7kW input
Frequency response: -3dB at 2Hz and 200kHz.
Total harmonic distortion: less than .001% from 20Hz to 20kHz with a 1V
handling: supply dependent; requires 30VDC or ±15V for 9V RMS signal
Professional audio gear invariably has balanced inputs and
outputs. However, what if you want to connect standard audio equipment that has
unbalanced outputs to equipment that has balanced inputs? Alternatively, what if
you want to connect a balanced output signal to an unbalanced input? Either way
this Balanced/Unbalanced Converter project can do the job.
The reason professional audio equip-ment utilises balanced
inputs and outputs is quite simple. It’s done so that audio connections can be
made over quite long distances without adding extra noise to the signal. These
balanced connections use 3-pin XLR plugs and sockets and screened twin-core
Fig.1 shows the basic arrangement. Basically, the audio output
signal is coupled to two separate amplifiers and these drive the two signal
leads in the cable in anti-phase (ie, the signals have opposite phases). In this
case, Amplifier 1 has an output signal that’s in phase with the input, while
Amplifier 2 has an output that’s opposite in phase with the input.
The output impedance of each amplifier is the same and the
twin-core cable carries the signal to the equipment at the other end. However,
in some cheaper balanced line drivers, one core does not carry any signal but is
grounded instead. So in this case, Amplifier 2 is left out and the lefthand side
of resistor R2 is grounded.
In operation, there will be some noise and hum pickup over the
length of the cable even though the cable is shielded. However, because the
cores in the cable are close together, any signal that is picked up will be
common to both.
At the receiving end, the signal in each of the two cores is
subtracted to produce the original audio signal. At the same time, this also
removes most of the noise and hum that was picked up in the leads, since the
same noise signal is present in both.
If one of the cores is grounded, as in the cheaper type of
balanced driver, then the signal level after subtraction will be the same as the
signal in the main core. Alternatively, if anti-phase signals are applied to
both cores, the subtraction process produces an audio signal level that’s twice
the level in the individual cores.