Early rolloff before 20kHz

QA40x
1

I have spent considerable time on the setup of my A403. I am using it with a QA450 that I picked up used. I am seeing the frequency response of every amp I measure roll off early starting around 10kHz, losing about 1.5 dB by the 20kHz point. I have recreated this on my Yamaha M45, a McIntosh MC 2205, and a Harman Kardon Citation 16. All of these amps should not be down consistently 1.5 dB at 20kHz. See attached exported images of the Harman Frequency Response and my loopback test.

I have tried different FFT and sample rates, and I figured out how to increase the measurement range up past 20kHz. I have tried 24kHz up to 80kHz as the upper measurement stop, but there has been little to no change. What am I missing?

Harman frequency response:

HK16 Freqeucy Response
HK16 Freqeucy Response1117×829 20.1 KB

2

Here is the loopback test.

Loopback
Loopback1117×829 18.7 KB

3

The QA450 is spec’d with a 3dB rolloff at 34kHz. I presume to make it usable with class D amps? This information is in the QA450 users manual.

4

Hi Mark, thanks for this. I had read about the filter but didn’t put it together with this rolloff. Will try across my dummy resistors and see if that does it.

5

I defeated the filter on the 450 by removing the 4 shunt caps at the outputs. FR much more useable for what I’m measuring. Thanks!

6

Suggests it might be worth fitting a switch to select the filter (for class D) or not (for wide bandwidth) ?