I have an amp made by crown that is multimode AB & B when testing the amp, I noticed a strange dip in THD+N on the channels. It consistently occurs at the 1.4kHz and recovers at the 3.3kHz. Is this graph normal? It kind of looks like I am hitting a bandpass filter but I’m not certain.
I am using the QA403 and running the output of the amp through a 200W 8Ohm load resistor. There is a Hall effect sensor that is measuring the wattage going to the load resistor and the analyzer probes are connected to the amp output. The software version is 1.215.
Hi @Balahid, it’s always very important to ensure you have a sensical plot manually before you automate. What does your spectrum look like with 1 kHz tone at the same level as your sweep?
If I understand what you are asking, you want me to maintain a 1kHz tone while increasing the dBV. The harmonics look right and the noise floor raises as dBV intensifies which seems to be normal. The amp is alerting me that it is clipping the signal at 7dbv.
Hi @Balahid, no I meant just run a plot like this with your amp. That is, apply the input you think is correct, and measure the output and plot the spectrum. If your graph above is showing poor data, then that is because the spectrum is poor. And you need to diagnose why the spectrum is poor. Is it excessive distortion? Noise?
@Matt I might have a lot of background noise that is interfering my graph looks like this I found a solid bit of additional info on my amp so I will go digging through that and see if I missed something stupid.
Hi @Balahid, yes, the plot explains it all. The measurement of a single tone is really, really poor. Once you ahve this cleanedup up your swept plots will improve. Your amp gain looks a bit low. Are you sure 11 dB is the correct gain? Does this plot have an attenuator in use?
@matt I do not have an attenuator for this plot. I removed it during the troubleshooting stage. I took a closer look and the amp as I turned its dial and the amp doesn’t respond to the rotation past the 11dB gain. It looks like I have multiple problems to sort out. Thank you for the help.
@matt Back again, I found multiple issues with my set up. I swapped out the input wires for some that I made myself and removed the wattmeter entirely. The harmonics are still there the noise is gone. I can also hit the max gain possible for the amp. I tried running the automated test and noticed the dip is nearly gone now. If I am reading it right, the frequency response matches the specs in the manual too.
