PSRR - Testing Fixture
Your audio bias-T is more difficult than radio-frequency bias-T, because the DC and AC sources contain frequency components not far removed. This situation requires much more careful design of the AC and DC power sources.
Power supplies most often expect that current will always flow one way. Current is expected to flow out to the load, and not backward (into the supply). Backward current flow is a possibility where a 2.4 kHz signal source is added to DC supply and Device-Under-Test (DUT). To absorb any backward-flowing current, a big swamping capacitor might be added to power supply output:






simulate this circuit &ndash; Schematic created using CircuitLab
It is quite possible that DUT contains capacitance between its DC power input and power return. The parallel impedance of DC supply plus swamping capacitor should be very much smaller than DUT internal capacitance. In addition, the AC source must have a low output impedance to drive DUT capacitive load. The AC source might be required to have a low output impedance at much higher frequencies as well, if DUT draws transient currents - a likely possibility for some sample-and-hold circuits. AndyAka's suggestion of a step-down audio transformer is a good solution for a low-impedance AC source. A small transformer can likely cope with DC current flowing through its small number of turns on its low-impedance winding.
If DC plus peak AC current requirements are reasonably low, a power op-amp might be used to combine DC and AC sources. Output impedance can be quite low in the audio frequency range, but some op-amps become unstable when presented with capacitive loads. Try to measure peak current requirements of your DUT before approaching design decisions.