Input impedance of audio amplifier, unclear from datasheet
Short answer

Above ±0.3 V the input protection circuit will shunt the signal and drop the impedance.

Long answer

The AN7190 datasheet doesn't show the internal circuit for the chip but we can deduce a few things from the application circuit example.



Figure 1. Application example from datasheet page 5.


The amplifier is designed for in-car entertainment systems. That means single-ended 12 V supply will be used. 
Traditionally in single-ended supplied amplifiers the input is biased to half supply and a decoupling capacitor is used on the input. In the application note we can see that there is no decoupling capacitor and the input voltage can swing positive and negative with respect to ground.








simulate this circuit &ndash; Schematic created using CircuitLab

Figure 2. Input protection circuit guess.


Most likely there is some input protection (like back to back diodes between the input and ground) to limit the maximum input signal. This would limit the DC thump on power-up caused by the previous stage equipment output if, for example, it is biased to half-supply and used a decoupling capacitor on its output. Instead of getting a 6 V thump the protection will probably clamp it at about 0.6 V. This will protect your ears and your equipment. The 47 k\$\Omega\$ input resistors give a discharge path to any external decoupling capacitors.
The imput impedance is specified at 28 k\$\Omega\$ for signals in the range ±0.3 V. Outside that the impedance will fall as the protection shunts the current.
The elimination of the input decoupling capacitors will avoid low-frequency roll-off and so give improved bass. The bridge mode output similarly avoids decoupling capacitors to further improve the performance.