Between the Soundwaves: The APx Analog Generator

An APx515 customer asked, what’s the right way to load the analog generator output of the APx515? Especially:

1. What is the lowest resistive load allowable for the generator output?
2. What is the output impedance of the generator output?

First, the APx515 and all AP analyzers except the APx517 have analog generator outputs which are voltage sources, not current sources. The APx515 has a default source impedance of 50 Ω for the unbalanced BNC outputs and 100 Ω for the balanced XLR outputs, both can also be switched to 600 Ω. Typically the analyzer can source at least 50 mA AC or DC.

Taking into accounts the analyzer’s source impedance and using Ohm’s law we can find the minimum input termination before the analyzer will current limit. For the unbalanced output that works out to 110 Ω at the maximum output of 8 Vrms.

As a practical matter the APx515’s analog generator is intended to drive a high-impedance load, ≥ 600 Ω, but more typically 10K Ω and up. If you truly need to drive a low impedance load consider the APx517. The APx517 has a power amplifier able to drive 35 W into as little as 2 Ω and a stereo headphone amplifier that can drive 100 mW also into 2 Ω.