Checking Source Impedance Balance at an Audio Line Receiver

A balanced audio receiver can have excellent intrinsic common mode rejection yet perform poorly with an unbalanced source impedance. Interference is rejected only after the complete input network has presented it to the receiver. Examine both signal legs, including cables and protection parts, before attributing hum to the amplifier IC.

Draw both source impedances

Include output resistors, coupling capacitors, connector contacts, and the source’s active output impedance. The two legs need not carry equal and opposite wanted signals to have balanced impedances, but their impedances to the relevant reference matter for interference rejection.

The THAT 1200 series data sheet explains how source impedance mismatch combines with receiver common mode input impedance to convert common mode voltage into a differential error. A high standalone CMRR specification cannot reject an error that has already become differential.

Include the receiver input network

Trace resistors, capacitors, and protection paths from each input to its reference. Compare their values and frequency dependence. Nominally matched resistors do not guarantee matched impedance when one leg has additional capacitance or a different cable connection.

For a useful thought experiment, apply the same interference voltage to two dividers. If their division ratios differ, their outputs differ. The receiver then sees a real differential voltage even though the original disturbance was common to both source wires.

Measure with a defined imbalance

Use a controlled common mode stimulus within the receiver’s permitted range and a test method appropriate to the interface. Measure the residual output while documenting the impedance in each source leg. Start with matched paths, then introduce a known additional impedance in one leg.

Repeat over the frequency range where interference rejection matters. A low frequency result alone may miss the effect of input capacitance or cable imbalance at higher frequencies. Keep the source and measurement setup from introducing an uncontrolled differential component.

Test intended connection variants

Review balanced sources, supported unbalanced adapters, and any grounding arrangements the product permits. Document the connector wiring for each. A cable that merely fits the connector can present a materially different impedance balance.

Check wanted signal gain and noise after changing series resistance or protection components. Improving one rejection test by increasing resistance can introduce another performance tradeoff that needs evaluation.

Save source models, input networks, adapter wiring, and common mode rejection curves. Diagnose the complete connection before replacing the receiver. A robust interface retains the required rejection with realistic source imbalance; an impressive IC specification measured from an ideal laboratory source does not by itself establish the behavior of the installed audio cable and board.