The single op-amp differential amplifier shown in Figure 1 is often used as an audio balanced line receiver. Obtaining high common-mode rejection is a critical requirement that requires four highly-matched resistors.
Figure 1: The "unity gain" audio line receiver with internal laser-trimmed resistors is a simple differential amplifier.
Audio line receiver ICs, with laser trimmed on-chip resistors (such as the unity-gain THAT1240/INA134 or "-6dB" attenuating THAT1246/INA137) have excellent common-mode rejection, typically 90 dB. Real world audio interconnections are not always balanced and are frequently a combination of unbalanced single-ended, balanced, floating and ground-referred sources.
The simple differential amplifier topology of Figure 1, whether made from op amps using external precision resistors or readily-available line receiver ICs, has two subtle limitations that prevent it from being truly universal.
Simple diff amps have unpredictable input impedance and gain
The first limitation is the unpredictable input "port" impedances to ground at each input. The impedance at the inverting (-) input is voltage-dependent on the non-inverting (+) input. Depending on the type of connections to the inputs, (e.g., unbalanced single-ended, balanced ground-referred, or fully-floating) the apparent impedance at the inverting input can vary over a wide range. An extreme example is when a grounded center-tap transformer drives both inputs with equal but opposite polarities.
In a unity-gain line receiver (where R1=R2=R3=R4) the signal current in the inverting input is three times the non-inverting input current [1,2,3]. Fortunately, the impedance for common-mode signals (both inputs driven equally) are identical at each input port or the circuit in Figure 1 would perform poorly when rejecting hum and interference in the real world.
A second limitation is unequal gain when fed by single-ended sources that have one input connected and the other left floating or "open port." Typical open-port examples are a three--conductor 1/4" (TRS) plug with the "ring" terminal unconnected or an XLR to RCA phono plug adapter cable with either pin 2 or 3 open. In Figures 2a and 2b, a unity-gain line receiver is shown.
Figure 2: Gain variations occur using the circuit of Figure 1 when one input is driven and the opposing input is "open port." Figures 2a and 2b show unity-gain line receivers; 2c and 2d are -6dB examples.
Driving the inverting input while leaving the non-inverting input open provides the expected gain of 0 dB. However, reversing the input connections (feeding the non-inverting input with the inverting input open) provides 6dB attenuation because there is no ground connection for the inverting feedback network to provide 6 dB of gain to negate identical attenuation at the non-inverting input.
In professional audio applications an "impedance-balanced" input having an equal and consistent input impedance to ground regardless of how it's being driven is desirable [1,2,3]. When fed by single-ended sources, a line receiver having predictable gain regardless of which input port is used, and whether the opposing input is open or "closed" (connected), is also a worthwhile property.