A small desktop console for converting balanced and unbalanced signals.
Balanced line is one of those elegantly simple engineering solutions that benefits greatly from the existence of the integrated op-amp. It relies on a differential amplifier's common-mode rejection (CMR) capability, in which the amplifier outputs the difference between its positive and negative inputs, and rejects anything that appears in common on both. The result is an efficient way of transmitting signals over long distances without significant degradation.
Typically a balanced audio signal is tranmitted in normal phase on one wire designated "positive," and 180 degrees inverted on a second wire as "negative." The amplifier acts on the incoming difference signal, and two benefits are realized. First, any noise picked up by the signal wires will be in common phase on both wires, and is rejected. Second, the cable shield does not directly contribute to the signal, and hum loops generated in the cable are not reproduced in the next processing stage.
The concept became a standard in professional audio but remained esoteric in consumer kit. At the dawn of consumer audio, the implementation costs outweighed the benefits on typical short cable runs, and modern consumer audio is rapidly transitioning to wireless standards. For professional use, wireless technologies are taking over as well, but multiple cable runs of many meters are still common and noise pickup is problematic, especially with sensitive microphone circuits. Balanced line remains the standard way of mitigating it.
Sooner or later, that equipment will end up on the bench for repair work, and now we need a way of testing it.
A balanced line input can be driven by an unbalanced signal if one input pin receives the signal and the other is grounded to the shield. The signal can also be passively converted through a 1:1 audio transformer (a "balun" in audio speak). Either approach drives the downstream amplifier at half the normal peak-to-peak voltage. However, it would be nice to have an active benchtop tool that can convert both ways on demand. It should also provide inline volume control, since many rack amplifiers rely on external attenuators and will fire the cone right out of a typical workshop test speaker if connected directly to a source (to say nothing of what will happen to the technician). And thus a new audio tool was born, using Elliott Sound Products' Project 87.
Figure 1. Interior of the console.
The balanced line converter occupies the left side, accepting unbalanced RCA inputs and delivering balanced line on XLR outputs. The unbalanced converter occupies the right side, and does the opposite. In both cases, the unbalanced inputs are guarded from accidental DC with coupling capacitors, and a 50kΩ log attenuator allows for volume control in series with the unbalanced jacks.
A regulated power supply lives in the center, based on ESP Project 05. Only four dual opamps are being driven and the regulators do not require heatsinks. The application of an external plugpack eliminates the need to have a transformer, IEC jack, and earth-grounding provisions inside an otherwise compact console.
The fully assembled unit, viewed from the business end:
Figure 2. Control face of the console.
Again, unbalanced-to-balanced I/O is on the left and balanced-to-unbalanced I/O is on the right. The knobs controls the attenuators. The XLR jacks are non-locking types, which are quite adequate for temporary test use. The power switch is at top-center and a nearby green LED indicates the power status.
Figure 3. The rear side of the console.
There's nothing much to see on the back. 18VAC enters through a standard coaxial adapter jack on its way to the voltage-doubler rectifer at the front of the power supply.
Finally, an action shot:
Figure 4. Testing a PA rack amplifier.
Here we see the new tool being used in transmit mode to drive a recently repaired PA rack amplifier. An unbalanced source feeds into the console, and the balanced output drives the amplifier. Prognosis? Both console and amplifier worked fine.
The same function could be obtained using a small commercial mixer, but this DIY console is cheap and has the advantage of being easily disassembled and repaired should a worst-case failure occur in the test device. Such cannot be said for many mixers, even at the lower end. It is worth building for that reason alone. §
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