I bought a QA451b at the same time I got a QA403 and I haven’t used it quite yet. I’m hoping that you all might be able to answer some questions for me.

I plan on only using external loads when using it, but having it in the line between the amplifier and the QA403, will it add to the distortion or noise?

It provides 12 db of attenuation, making it easier on the QA403. The example used on the product page is that it shifts a 200w (32 dBV, 40 vrms @ 8 ohm) signal down to 20 dBV, so the QA403 basically sees 10 vrms or 12.5 watts. Much further away from the limits of the QA403.

So is my math correct in this? Let’s say I want to measure an amp at 800 watts into 8 ohms. My external load can take it. 800w is 80 vrms into 8 ohms, or 38 dBV. So attenuating by 12 db takes us to 26 dBV or just shy of 20 vrms (50 watts) which is well within the limits of the QA403. With the QA451 I should be able to measure well over 1000 watts into 8 ohms. Is this correct, or am I missing something here? I’m wanting to measure some high power amplifiers and was looking into buying a bunch of smaller value resistors and tapping off of them, but with the QA451 it looks like it isn’t necessary.

Dan