I am trying to explain the results
I started at step 1 and followed each step to step 6
looking at your set up, I mainly use the HANN window the RECT Windowing. I prefer dBR to dBV for most measurements just because it is easier to see how far down noise and spurs are. It is possible that the zero bias ckt is not working or that the changes are just to small to see…?
Please explain why I am seeing this result.
So it is not possible to use the QA to probe signals at different locations in the signal path?
Hello YamahaCA,
I don’t think the QA403 is the right solution to “probe” in a circuit like a power amplifier. You should use an oscilloscope for this purpose. Voltage levels inside the amplifier circuit can be too high for the QA40x and oscilloscopes are more convenient to measure signals inside the circuit.
The QA403 is perfect to measure the input to output quality.
The inputs are 100k ohm, so it’s very unlikely they could trip a breaker in your fuse box. What is more likely is that you have a ground connected one place in your circuit via the BNC shell, and then you connect another BNC shell to a non-ground potential. For exmaple, if you have a QA403 output connected to the amp input via RCA, then you have established the “chassis ground” as 0V. Which is normal.
If you then put a scope probe on a QA403 input, and clip the scope lead ground to a power supply rail, then there’s enough current that can flow through the scope probe leads, through the BNC shells, and back to the chassis ground established by the RCA.
There are also certain types of class D amps where both speaker outputs sit and mid rail. For example, on the TPA3255 Class D amp from TI, it’s a push-pull config, and if you connected to 50V, then both speaker outputs idle at 25.000 volts. So, if you put a DVM across the speaker outputs it reads 0V. But you connect an RCA to the input, and then connect a scope probe ground to SPKR- it will fault. The TPA3255 faults very gracefully, so it’s a bit hard to figure out what is going on at first.
Take a DVM and measure from the RCA input shell to the place you connected the scope ground lead. I’ll bet you read a sizable voltage.
RE: THD plot…So this a bit unexpected. When just looking at THD, the left and right track very closely. When looking at THD+N, the right channel is significantly worse. That means you have a lot more non-harmonic energy on the right channel. Up above 1 kHz looks similar, so maybe it’s below 1 kHz?
Where is the difference? It it down around 60 Hz? The region below 1 kHz is obscured in your photos…
PS. is it possible to post screen captures rather than photographs of the screen?
Hi @YamahaCA, ok thanks. Not sure why the right channel is showing so much worse than left in THD+N mode…
Can you please do the following:
- Connect L+ out to L+ in. Connect R+ out to R+ in.
- Short L- in and R- in.
- File->New Settings
- Enable Gen1
- 0 dBV full scale input
- FFT size to 16K
- Enable THD DB measurments, along with N+D dBV and N-D dBV for both channels.
Th device on my desk looks as follows:
I will have to do that in the morning
OK, thanks. I just wanted to make sure the circuit breaker trip event hadn’t caused other damage. You have a bit of 60 Hz being picked up. Probably this is radiated. If you pick the QA40x up, and rotate it around slowly, you’ll probably see the 60 Hz levels change. This is likely due to nearby line-powered equipment that is coupling into the QA40x. But overall things look good.
There is a set of High Tension Power Lines that run about 120’ feet away
What is your summary of thoughts on measuring the ZDR Test Points?
Summary of Harmonic Spikes?
