Balanced And Differential Audio Interfaces
Two Related Properties
A differential receiver responds to the voltage difference between two conductors:
Vdiff = Vplus - VminusA balanced source presents substantially matched impedances from those conductors to the relevant reference so external interference tends to couple similarly. Equal and opposite signal voltages are useful, but they are not the definition of balance.
Common Implementations
| Implementation | What it means | Compatibility question |
|---|---|---|
| symmetrical active | both legs carry opposite signal voltages | may either leg be grounded or left open? |
| impedance balanced | one leg carries signal; both legs have matched source impedance | does the receiver preserve the impedance balance? |
| cross-coupled or servo | output adapts when one leg is grounded | what connection does the manufacturer require? |
| transformer balanced | magnetically coupled, often floating within ratings | what are level, frequency and loading limits? |
| differential but not balanced | two-leg voltage exists without matched impedance to reference | how much common-mode rejection remains in the installed path? |
Shield Is A Separate Conductor
In a properly implemented balanced interconnect, the two signal conductors carry the signal and return current. The cable shield controls electric-field and radio-frequency interference and is normally bonded to chassis at the equipment entry according to the device and applicable interconnection practice. It should not be silently repurposed as one signal leg.
Common-Mode Range Still Matters
A receiver can have excellent small-signal CMRR and still clip when both input conductors move outside its allowed common-mode range. Measure each conductor to the receiver reference as well as the differential voltage. This is especially important with factory speaker outputs, floating sources, single-supply circuits, adapters, and equipment powered from different references.
Balanced To Unbalanced Is A Design Problem
Do not assume a pin adapter performs the required electrical conversion. Determine whether the balanced output tolerates an open or grounded leg, whether the unbalanced receiver expects its shell to be signal return, and whether the shield and chassis paths remain valid. Use the manufacturer wiring method, a proper active receiver/driver, or galvanic isolation when needed.
Verification
- Record output type and source impedance from each leg to reference.
- Measure both conductors to reference and the differential signal.
- Verify receiver input impedance and common-mode range.
- Measure CMRR with representative source imbalance and cable capacitance.
- Test the specified open-leg, single-ended or adapter condition only if supported.
- Verify shield, chassis, noise, clipping and transitions in the installed system.