Feel free to post comments or questions on the blog post linked here:
I would think you could use a series-parallel network of modest resistors to get over 1KW continuous dissipation. You would still the the tank of water.
I got some of these to try: https://www.ebay.com/itm/100W-Shell-Power-Aluminum-Housed-Case-Wirewound-Resistor-1-2-4-6-8-10-Ohm/174486964122?hash=item28a03bc79a:g:mnoAAOSw3WZfjovt less than n$3 for 100W (when heat sunk) I checked them. Essentially non-inductive. Less inductance that 3’ of zip cord at 100 KHz. 16 in a series parallel array with some clever patching could get .5 Ohm to 128 Ohm load. I would substitute the QA451 for one of the resistors in the array.
Yes, agree and that style is my preference too.
Given the pigtails on these planer resistors, I’m going to try crimping them together–the same way electricians tie the grounds together in a junction box. I think that will make for very fast assembly of larger arrays. The max 250C rating on the planer resistors is insane, and easy to measure with a point-and-shoot thermometer given the large flat body. There isn’t an IPX rating on these planar, but they are designed for automotive (EVs) and generally if not specified then it means the part is fine to go through an electronics wash (IP67). We’ll see what happens.
Would it be possible to use 2 or more QA451’s in order to increase the power handling capability? Or maybe a separate QA designed box to add on?
Hi @txturbo, I think the simplest and most economical way would be to just add the load resistors externally as outlined in the blog post. That allows you to use ~$25 of resistors and get 2 kW of power handling from the QA451.
Am trying to use high power 4 ohm resistor from Parts Express for higher power amp measurements. When I do, am getting much more noise than with the QA451 itself. I have twisted wires and shortened the connections, but still get noise (even with low input levels).
Am just looking to allow the power vs THD plugin to be run to clipping for a 500 watts amplifier.
Any suggestions?
Hi @rasykora What type of noise? Is the noise there if the amp isn’t present? Can you share a plot of the QA451 output with the loads connected (and the internal loads disabled) and the amp connected and not connected? Thanks, Matt
What is the value of the external load–looks like 4 ohms is that right? And what is the voltage swing across the QA451 inputs?
Yes, they are 4 Ohm and have already determined they are the source of my issue. I have 2 mounted on a nice heat sink and so could use for 2-8 ohm testing. Much lower power resistors are working better. I unmounted one and shortened the wiring and still getting the same results. The plot I supplied was generated under identical conditions except using the internal load vs external.
Am guessing it may be related to their thermal characteristics, but am trying to narrow it down. If you have ideas, please let me know, but no need to troubleshoot the QA config at this time.
Is it possible they are wire wound? Can you send a spectrum plot at 5W and 90W? That will let us see what has degraded.
Will see. Not looking to spend more time analyzing. Am waiting on Parts Express response. Asked if they have a data sheet. Only bought last September, so still under warranty.
Well it took PE over a week to get the data sheets and they had no details on physical composition or electrical behavior of these dummy loads. I did do a quick measurement to see if they were inductive. They do rise in impedance from 4 to 5 ohms between 2 kHz and 20 kHz. Not sure whether this would cause the behavior shown above, but is much more inductive than the inexpensive crossover resistors I have.
I sent my findings back to them and am awaiting a response. Can do a deeper dive once PE determines whether these are defective or not.
My guess is that the parts probably aren’t defective if you are seeing 4-5 ohms up to 20 kHz. If you have time, sending a spectrum plot at 5W and 90W might answer the question.
Thanks Matt! PE decided to issue an RMA and wants them back. I will try to get in a spectral plot before I ship them. While am curious, feel this is clearly something about these resistors that they should investigate.
Thanks for sharing the plots. It definitely looks like the distortion is happening in the external loads. I was kind of thinking the spectrum might be awash in 50/60 Hz as the measurement went south, but they aren’t. But it’s still weird to see a load just go south like that at 6W or so. Can you share the part number for the load?
Here is the webage for the load…
Hi Matt, when will the 451 plugin for the 402 be available?
Hi @DanClark, if it’s the amp output impedance you are referring to, it should be this week.
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