Quick answer: An electronically balanced output uses two amplifiers per channel to drive the hot and cold legs, either from an inverted copy of the signal or from a cross-coupled network that derives the cold leg from the hot. It costs less than a transformer, has no low frequency saturation, and does not isolate the two chassis. The rejection is good at low frequency and falls as the frequency rises.
Electronically Balanced: The Output Stage Explained
An electronically balanced output drives the cold leg with an amplifier instead of a transformer winding, and the difference shows up in the cost, the rejection, and the behaviour when something goes wrong. The stage is built from two amplifiers per channel, one driving the hot leg and one the cold, with the second either fed from the inverted signal or derived from the first by a cross-coupled network. The arrangement gives a balanced output without the size and the price of a transformer, and it gives up the galvanic isolation that the transformer provides. This guide covers the topologies, the cross-coupled variant, the rejection figures, and the comparison against iron.
The Three Output Topologies
The output stage in a balanced device is one of three arrangements, and the name on the specification tells the buyer which. The single-ended stage driving a balanced connector is the cheapest, and it puts the signal on the hot leg and the ground on the cold, which is an unbalanced output in a balanced shell. The two-amplifier design drives both legs from separate amplifiers, one inverted, and it is the common electronic arrangement. The transformer design drives the two legs through a winding, and it is the one that isolates.
Single-Ended Driving a Balanced Line
The single-ended stage is the arrangement that the connector hides, and it is worth identifying because the marketing does not always say. One amplifier drives the hot leg and the cold leg is tied to ground through a resistor or directly, so the receiver sees a signal on one leg and nothing on the other. The connection works and it is not balanced in the sense that matters, because there is no cold signal to subtract. The specification that reveals it is the output level quoted as the same figure for the balanced and the unbalanced socket.
The Two-Amplifier Design
The two-amplifier design has one amplifier for the hot leg and one for the cold, and the second is fed from an inverter that copies the first with the polarity reversed. The two legs therefore carry equal and opposite signals, and the receiver subtracts them to get twice the amplitude. The design is the standard electronic balanced output, and its weakness is the inverter, because any difference between the two amplifiers appears as a common-mode error that the receiver cannot remove. The matching of the two amplifiers is the quality of the stage.
The Cross-Coupled Variant
The cross-coupled output is the refinement that derives the cold leg from the hot without a separate inverter, and it is the arrangement in a large part of the pro equipment. The cold leg is driven by an amplifier that senses the hot leg’s output and generates its inverse, so the two legs track each other even when the load is unbalanced. The arrangement is more complex than the two-amplifier design and it behaves better in the real world, where the load is rarely perfectly balanced.
How It Lifts the Cold Leg
The cross-coupled stage senses the voltage on the hot leg and drives the cold leg to the opposite value, so the differential output stays correct even when the cold leg is loaded or shorted to ground. The property that follows is the one that matters in practice: the output can be connected to an unbalanced input without the level dropping to half and without the distortion rising. The stage does the work that the transformer’s second winding does, with transistors rather than turns of wire.
The Fault Behaviour
The fault behaviour is the difference that a service technician notices. An electronically balanced output that is shorted to ground on one leg survives, because the amplifier limits its current, and the cross-coupled version keeps the other leg correct. A transformer output that is shorted survives as well, because the winding has resistance. The failure that matters is the one where a leg is shorted to a supply rail, and there the electronic stage can lose the device while the transformer rides through. The protection circuit is the answer in both cases.
Rejection and Bandwidth
The rejection is the property the balanced connection exists for, and the electronically balanced stage delivers it with a frequency dependence the transformer does not have. The common-mode rejection ratio is high at low frequency, where the two amplifiers track closely, and it falls as the frequency rises, because the small differences in gain and phase between the two legs grow with frequency. The transformer’s rejection is set by the winding symmetry and it holds further up the band, which is the reason the transformer survives in the measurement market.
CMRR at Low and High Frequency
The rejection ratio is quoted in decibels, and a good electronic stage reaches eighty to a hundred decibels at fifty or sixty hertz and falls to fifty or sixty decibels at ten kilohertz. The transformer can hold ninety decibels across the audio band and beyond, which is why it is used where the residual hum has to be inaudible. The numbers are not a quality ranking, they are a description of where each device is strong. The electronic stage is strong where the noise is the mains hum, which is the low frequency case.
The Output Impedance Match
The output impedance of the two legs has to match for the rejection to hold, because a difference between them converts the common-mode noise into a differential signal. The electronic stage achieves the match with trimmed resistors, and the match drifts with temperature and age. The transformer achieves it with the symmetry of the winding, which is fixed at manufacture. The practical consequence is that a thirty year old transformer still rejects, and a thirty year old electronic stage may not.
Against the Transformer
The comparison is not a winner and a loser, it is a fit to the installation. The electronic stage wins on cost, size, weight, and low frequency extension, and it has no saturation point at the bottom of the band. The transformer wins on isolation, on rejection across the band, and on the ability to break a ground loop without a circuit. The choice follows the problem: a hum that survives a good electronic stage is a loop that needs isolation, and a stage that has to fit in a rack unit needs the electronics.
Where the Electronics Win
The electronics win in the equipment that has to be cheap, small, and repeatable. A mixing console with sixty outputs cannot carry sixty transformers at the price of the iron, and the electronic stage is the only practical design at that scale. The low frequency extension is the second advantage, because the transformer’s core sets a limit that the circuit does not have. The third is the weight, which matters in the portable rig and in the rack that has to be carried.
Where the Iron Wins
The iron wins where the two chassis must not share a conductor, which is the ground loop case and the reason the transformer survives. It also wins where the rejection has to hold at the top of the band and where the equipment is expected to last for decades without adjustment. The cost and the weight are the price, and the market that pays it is the one where the residual hum has to be below the noise floor of the recording. The transformer is the specialist part and the electronic stage is the general one.
Electronic Against Transformer Output
| Factor | Electronically balanced | Transformer | Practical effect | Choose when |
|---|
| Isolation | None | Galvanic | Ground loop | Hum survives |
| CMRR at 50 Hz | 80 to 100 dB | 90 dB and above | Mains hum | Either |
| CMRR at 10 kHz | 50 to 60 dB | 90 dB and above | RF and buzz | Transformer |
| Low frequency limit | Set by the circuit | Set by the core | Sub bass | Electronics |
| Cost and size | Low | High | Channel count | Electronics |
| Drift over time | Resistor match | Winding symmetry | Long service | Transformer |
Real-World Application Scenarios
The mixing console is the case the electronic stage dominates. Sixty outputs cannot each carry a transformer at the price, and the cross-coupled stage gives the balanced output with a few components per channel. The mastering chain is the second, and there the transformer is the choice, because the residual hum has to sit below the noise floor and the two chassis are often on different mains circuits. The interface in the project studio is the third, and it uses the electronic stage with the balanced connector and the matching output impedance. The location recording is the fourth, where the transformer’s isolation is worth its weight in the bag that has to be carried.
Related Guides on This Site
The related guides on this site cover the cable and the connector. The balanced cable that the output drives is set out at balanced XLR cable, the run length that the rejection supports is at long XLR cable, and the pin assignment at the connector is at XLR wiring.
Frequently Asked Questions (FAQ)
Q: What does electronically balanced mean?
A: It means the cold leg of the output is driven by an amplifier rather than by a transformer winding. The stage uses two amplifiers per channel, one for each leg, with the cold leg derived from an inverted copy or from a cross-coupled network. The result is a balanced output without the cost, size, and isolation of a transformer.
Q: Is electronically balanced as good as a transformer?
A: It is as good at low frequency and it is not as good at isolation, because the two chassis still share a conductor. The rejection of a good electronic stage reaches the transformer’s figure at the mains frequency and falls short of it at the top of the band. It is the right choice where the hum is not the problem and the wrong one where the loop needs breaking.
Q: What is a cross-coupled output?
A: It is an electronically balanced stage that senses the hot leg and drives the cold leg to the opposite value, so the differential output stays correct even when the load is unbalanced. The arrangement lets the output drive an unbalanced input without the level dropping to half. It is the design in a large part of the pro equipment.
Q: Can an electronically balanced output drive an unbalanced input?
A: It can, and the cross-coupled version does it without the level loss that a simple two-amplifier stage shows. The cold leg is either tied to ground at the input or left floating, and the output stage tolerates both. The six decibels of loss apply to the simple designs, not to the cross-coupled one.
Q: Why does my balanced output hum when the transformer version does not?
A: The hum is a ground loop that the electronic stage cannot break, because it passes the signal with a shared reference. The transformer couples magnetically and leaves the two chassis isolated, so the loop has no path. The fix is an isolating transformer in the line, not a different cable.
