Quick answer: A balanced amplifier uses a bridged output stage, with two amplifiers driving the two ends of the speaker in opposite phase, so the load sees twice the voltage swing and up to four times the power. Neither speaker terminal is grounded. The speaker connects across the two outputs, never to ground, and the input side may be balanced or unbalanced independently of the output.
Balanced Amplifier: Bridged, Not the Same as Balanced Line
A balanced amplifier is an amplifier whose output stage is bridged, and the word balanced here means something different from the balanced line at the input. In the bridged stage two amplifiers drive the two ends of the load in opposite phase, so the speaker sees twice the voltage swing of a single-ended stage from the same supply. That is the same arithmetic as the balanced headphone drive, applied to a low impedance speaker rather than a voice coil. This guide covers the two meanings, the bridged stage, the speaker connection, the power it delivers, and the safety rules that the floating output makes necessary.
Two Meanings of Balanced
The word covers two different things in audio, and the confusion is worth clearing before the rest. A balanced line is an input or output arrangement where two conductors carry the signal in opposite polarity and the receiver subtracts them, which is about noise rejection. A balanced amplifier is an output stage where two amplifiers drive the load from both ends, which is about voltage swing and power. The two can appear in the same product, and neither implies the other.
The Line Stage
The line stage is the input side, and its job is to receive a signal without picking up noise. The differential input subtracts the two legs, the common-mode noise cancels, and the stage passes the difference to the voltage amplifier. The arrangement is the one in every pro input, and it says nothing about the output stage. A balanced input with a single-ended output is a common and perfectly sensible design.
The Bridged Output
The bridged output is the power side, and its job is to put more voltage across the load. Two amplifiers receive the same signal in opposite polarity, one drives the positive terminal and the other the negative, and the load sees the difference. The arrangement doubles the swing for the same supply and gives up the grounded output. The name balanced comes from the symmetry of the two halves rather than from the input arrangement.
The Bridged Stage
The bridged stage is two complete amplifiers per channel, and they are driven from the same signal with one inverted. Each amplifier sees half the load impedance, because the load sits between them rather than from one to ground, and the current through the load flows through both. The voltage across the load is the difference between the two outputs, which is twice what either produces alone. The price is the doubled component count and the loss of the grounded output.
Two Amplifiers, One Load
The two amplifiers are identical and they share the same supply, and the load is connected between their outputs rather than from one output to ground. Each amplifier therefore drives half the load impedance, which is the reason the bridged amplifier is rated for a higher minimum impedance than the same circuit in single-ended form. The current path is through both amplifiers in series with the load, and the protection circuits have to account for the doubled current. The design is the standard way to get more power from a given supply voltage.
The Voltage Doubling
The voltage doubling is the point, and it follows from the anti-phase drive. When one output rises by half the swing the other falls by the same amount, so the load sees the full swing rather than the half. The power into a resistive load rises with the square of the voltage, so the same supply can deliver up to four times the power into the same impedance, and in practice the supply and the thermal design limit the gain to something less. The arithmetic is the same as the balanced headphone drive at a much larger scale.
The Speaker Connection
The speaker connection is where the bridged amplifier differs from the ordinary one in practice, because neither terminal is at ground potential. The speaker connects across the two output terminals, and both of them move. Connecting either terminal to ground, which is what a common ground speaker switch or a headphone adapter does, shorts one half of the amplifier. The connection is a floating one, and the wiring has to respect it.
Neither Terminal Is Ground
Neither output is referenced to ground in the bridged stage, so the black terminal is not a ground and the red terminal is not a signal against ground. The pair is a differential output, and the speaker is the load between them. A meter from either terminal to ground reads half the signal, which is the measurement that confuses the installer. The pair is what matters, and the chassis is the only ground in the amplifier.
The Four-Wire Link
The four-wire link is the wiring arrangement that keeps the two channels separate in a stereo bridged amplifier. Each channel has its own pair, and the four conductors run from the amplifier to the speakers without a common return. The arrangement is the same as the four-wire headphone connection in principle, and the reason is the same: a shared return would couple the two channels and, in the bridged case, would short the outputs together. The two pairs are kept separate at both ends.
The Practical Questions
The practical questions are the power, the impedance, and the safety, and each has a short answer. The power rises with the square of the voltage and is limited by the supply and the heatsink. The impedance has to be at least twice the single-ended minimum, because each amplifier sees half the load. The safety rules follow from the floating output, and they are the part that the installer has to know.
Power Into the Load
The power into the load is set by the voltage swing and the impedance, and the bridged stage doubles the swing for the same supply. The heatsink and the supply current are the limits, so the four times figure is the theoretical ceiling rather than the rating. The practical gain over the single-ended stage is between two and three times into the same impedance, and the amplifier’s specification states the figure for the bridged mode. The load impedance that the bridged mode supports is the one to check.
The Safety Rules
The safety rules are short and they are not optional. Never connect either output terminal to ground, because the short takes out one half of the amplifier. Never use a speaker switch or a headphone adapter that ties the two channels to a common ground. Never bridge two channels into a load below the rated minimum, because the current doubles and the protection may not save the devices. The floating output is the design and the rules follow from it.
Single-Ended Against Bridged Output
| Factor | Single-ended stage | Bridged stage | Effect | Rule |
|---|
| Output reference | One terminal at ground | Both terminals floating | Wiring | Never ground a terminal |
| Voltage across the load | 1x | 2x | Power | Check the rating |
| Power into the same load | 1x | up to 4x | Headroom | Supply and heatsink limit |
| Load seen by each amplifier | Full load | Half the load | Minimum impedance | Double the minimum |
| Speaker wiring | Two wires, one grounded | Four wires, no common | Channel separation | Keep the pairs separate |
| Bridging two channels | Not applicable | One load across two channels | Mono power | Check the mode switch |
Real-World Application Scenarios
The subwoofer is the case the bridged amplifier is used for most often. A single channel has to deliver the low frequency power, the bridged mode doubles the voltage, and the four ohm driver sees the amplifier as a two ohm load. The PA rack is the second, where two channels bridge into one horn or one monitor and the extra voltage is the difference at the long throw. The car audio installation is the third, where the supply is twelve volts and the bridging is the only way to reach the power the subwoofer wants. The home stereo is the fourth, and there the bridged mode is rarely used, because the load impedance and the power are already matched in the single-ended mode.
Related Guides on This Site
The related guides on this site cover the wiring and the cables. The cable and the impedance behind the speaker link are set out at amp speaker cable guide, the gauge for the run is at 12 2 speaker cable, and the question of using XLR connectors on a speaker line is at XLR speaker cable.
Frequently Asked Questions (FAQ)
Q: What is a balanced amplifier?
A: It is an amplifier with a bridged output stage, where two amplifiers drive the two ends of the load in opposite phase. The load sees twice the voltage swing of a single-ended stage from the same supply. The word balanced here refers to the symmetry of the output, not to the balanced input, and the two features are independent.
Q: Is a bridged amplifier the same as a balanced input?
A: No. The balanced input is a differential receiving arrangement for noise rejection, and the bridged output is a power arrangement for voltage swing. A product can have either, both, or neither. The input and the output are separate design decisions, and the specification lists them separately.
Q: Can I connect a bridged amplifier to a common ground speaker switch?
A: No. The speaker switch ties the two channels to a common ground, and the bridged output has no ground terminal, so the switch shorts one half of each amplifier. The result is a failed output stage. The switch has to be the type rated for bridged amplifiers, with the channels kept separate.
Q: Why does a bridged amplifier double the minimum impedance?
A: Because each amplifier sees half the load. The load sits between the two outputs, so the current through it flows through both amplifiers, and the impedance each one drives is half the speaker’s. The minimum impedance for the bridged mode is therefore twice the single-ended figure, and the protection is set for the higher current.
Q: Does bridging improve sound quality?
A: It changes the operating point rather than the quality. The higher voltage means the amplifier works less hard for a given level, which can reduce the distortion, and the doubled current means the protection is closer. The bridged mode is a power tool, and the sound depends on the design rather than on the mode.
