Release 0.993 will be out today with these plug-ins present. There are two output z plug-ins, one from manual load changes (where you provide two loads, switching when prompted) and also automatic if you have a QA451 or QA450.
The video below is measuring the output impedance of a QA460 without a 1 ohm resistors in the output path. When running the plugin, all that is being measured is amplitude, so you don’t need a huge FFT. As you can see in the video, a smaller FFT was used which offers a great sweep speed. I would make sure to use frequency rounding (right click on Gen1). That makes sure the test tone is centered in a bin.
But it’s a very fast measurement if you have a QA450 or QA451. After the first sweep, note the impedance change on the QA451 app and then the sweep is repeated.
Resistors are not straight wires, They generate distortion all of their own. Resistor distortion is primarily 3rd HD. I am thinking Thermal Coefficient of Resistance. Some of the large braking resistors are notability bad. 1 ppm is 120 dB’s SINAD. Some of these resistors are listed as 100 ppm plus on the datasheet.
Large heavy wirewound resistors tend to test better regardless of TCR ppm. They have larger mass heating elements that do not change temperature as fast or as much with an applied AC voltage.
The resistors discussed here may not be incent.
I tested several types or resistors. So far ARCOL 300 Watt HS300 4R F resistors (they weigh in at 1.4 pounds each) measure best of the large wirewound power resistors.
I just ordered several cheep ebay resistors to test.
Thanks DT
The resistors are on a slow boat. I will post plots next year.
II have had pretty good luck with these resistors so far:
In another thread I posted a link to a video I did for my loads, which I used the above resistors in. I think I have put as much as 700w/8ohms w/out any problems, but at that power level I am not running for more than a few minutes. I have been using this new load setup for several months and have been very happy with it. I never considered how the actual resistor distortion factored in when you look at what a speaker impedance looks like. Here is the video on my loads if you had not seen it before:
I have a much simpler question related to external loads for the QA451. My needs are simple: Measure the output of audio power amplifiers, up to perhaps 250W RMS. I am currently using an 8 ohm, 200W non inductive resistor (one for L, one for R). I’d like to build 10-20dB pad to use ahead of the QA451 to protect it from … shall we say, errors :). While I presume I could use three resistors in series across the 8 Ohm load, that would raise the negative connection to the QA above ground by whatever resistor value is needed. I’d be measuring across the center of the series resistors.
So, assuming I don’t really care about a few dB of added noise (I don’t - this is a power measurement for the customers) what value resistors, and is this approach even viable?
I’m not an EE - just a lowly technician. Any help is appreciated. I do not, at the moment, anticipate extremely high power (above 250W RMS) measurements will be needed in the immediate future.
Scott, thanks so much!! Two questions : I cannot read the text below the binding posts, so if you can hand draw or type it out here I’d sure appreciate it.
And did you use non-inductive resistors? I’ll watch the Youtube video again if I can find it, but if you’ve got a source for the resistors that would help. If not, I can certainly find them.
They will let you test a higher than what you want, but if you want to test more at some point in time, you can. The text at the bottom reads "Plug into desired point (tap) depending on Ohms and attenuation.
