Dummy load for audio amplifier - Page 1
Forum section: electronics / beginners

author: hamster_nz
author_rank: Super Contributor
timestamp: November 04, 2017, 03:55:17 am
content: After readinghttps://www.eevblog.com/forum/beginners/what-makes-a-high-end-audio-amp-'better'-then-a-low-end-unit/and wondering about measuring amplifiers at high power outputs...Is there more sophisticated dummy load for audio amplifiers than than just a high-wattage resistor in a box like this?http://diyaudioprojects.com/Testeq/Dummy-Speaker-Load/You know, something with the same electrical properties as a real speaker?Or are they not worth while as:a) Every speaker is different, so no 'generic' model exists.b) Any measurements you can make are not useful, as they ignore how speaker efficiency can change with frequency.c) Energy in a speaker is in a non-linear electro-mechanical mechanical system, with things like moving masses holding energy, so can't be replicated easily.So is the best dummy load somebody else's cast-off 2nd hand speakers (so you don't mind burning them out), in the garage, with music you & your neighbors don't mind listening to?

author: The Doktor
author_rank: Regular Contributor
timestamp: November 04, 2017, 04:37:00 am
content: No speaker is constant resistance/reactance across the entire (or even a small section of) the audible frequency range. Making a load that accurately matches any given speaker will be difficult/not worth the effort.That doesn't mean a dummy load is useless. A non-inductive resistor of the proper value (usually 2,4,8, or 16 ohms) with sufficient power handling capacity works quite well for most testing. If you need to check for stability under reactive loads, you may need to add inductance/capacitance in series or parallel with that resistance. That's not something a beginner or person doing basic repairs needs to worry about, so just stick to the resistor.As for testing with an actual speaker, imaging checking THD at full output on a 200W amplifier. Would the speaker survive a 200W CW sine wave for a few minutes? What about you ears? Neighbors? I know you said music, but that's not how power & distortion is usually measured.Ed

author: Shock
author_rank: Super Contributor
timestamp: November 04, 2017, 09:42:10 am
content: As mentioned you want a non reactive dummy load, this is the way it's been done for years, you want to measure the performance of the amp not how a speaker loads the amp. High wattage dummy load resistors are fairly expensive but there happens to be a cheap alternative.The US company lowes.com sells item #362342 the Utilitech 240V Universal (water heating) element for ~$9 each, see this video explaining usage. They just happen to be 16 ohms which is perfect to put a 16/8/4 ohm set together depending on how many you have.

author: Ing. Max Koschuh
author_rank: Newbie
timestamp: November 04, 2017, 10:14:57 am
content: The Doktor posted a very qualified and good reply.You can add a little inductor in series to a non-inductive resistor. Simply by taking a large copper wire and manually making a couple of turns, to generate a little coil. Also, you could add a small capacitor (non-polarized, MP type) to simulate the capacitance of the speaker coil.Still, this way you cannot simulate the power, generated by the speaker chassis when acting as a generator. Nor you are able to measure the damping factor of the amp.Note:Due to the mass of the membrane, the stroke is not exactly corresponding to the signal in terms of timing, so is the swing off.Since there is a little delay between swing off of the membrane, while the signal already started to decay, the membrane (and it's coil) act as a generator.After all, the best dummy load is most likely an actual speaker placed in an acoustically dead room.

author: tszaboo
author_rank: Super Contributor
timestamp: November 04, 2017, 04:37:30 pm
content: That curve looks that way, because the crossover circuits built into the speakers.Otherwise, the curve of one speaker is a lot simpler. They have a peak at the resonant frequency and then they go to infinity as the frequency increases.So I guess, if you want to make a more realistic dummy load, buy a crossower (or two) and connect 3 an RL load on the output of those. Or maybe 3x ( RLC series with an L). Or just 3xRBut understanding the crossover is a big step reproducing that curve.