Hi @matt. I just read your response about measuring amplifiers here: Attenuator for high RMS signal tests
I am wondering if connecting a preamp directly to a the QA400, rather than differentially through the resistor network in your article, allows hum to creep in to the measurement. ( I realize the QA is not differential).
Basically, I get much better results through the preamp–>amp–>QA400, than through preamp–>QA400 directly. Can you shed some light on what might be happening?
In my question on this thread, I mention frequency response is improved when measuring preamp–>amp–>QA400, vs through preamp–>QA400 directly, but I also notice a reduction in 60Hz when inserting the amp between the preamp and the QA400 as well.