Power Resistor for Audio Measurement


this is Elizma from South Africa. I need your advice on some audio measurement aspects. What type of resistors do you use when making measurements acroos 1ohm ,2ohm ,4ohm and 8ohm constant loads?. Do you always measure across speaker loads or do you use high power resistors? Please help me on this subject. How many Amperes travel through a speaker. Can one use a constant resistor to substitute a speaker? perhaps you can give me the part number or company name for resistors that you know, works well.


the quality of resistors for audio power measurements, it is not so very important. Problems occurs with many types of wire wounded resistors, they change their complex impedance with increasing frequencies. My experience, for DC up to about 10 kHz they remain like an pure ohm's resistor, above they tend to be more and more inductive, much higher they reach their resonance frequency and the capacitive part dominates. When the resistors changes the U/I phase for only some degrees to be an inductor - no problem - the amp see's a higher impedance and the output current decrease a little. The question is: "how precise you want to measure?" Doing a device data sheet specification you'll should use good quality. For test purpose only not so important. If the amps works now with 4 ohms or 4.6 ohms at e.g. 55 kHz and 100 Watts, it doesn't matter. The amp wont reach that power level in practice under that frequency. I use for my 4 ohm measurement many times this power resistor. I guess and hope this type use a bifilar winding (not sure) resulting in being ohmic up to almost 100 kHz. Mounted on a big aluminium block. If you are not sure with your own wire wounded resistors, check them easily with a sine generator and a two channel oscilloscope, connect an additional series resistor and the power resistor as a voltage divider and observe the phase shift with increasing frequency. Do a second test with 1/4 watts film resistors to verify the measurement before, there should be almost no phase shift. Don't use to high ohmic resistors for this test, because - consider the parallel capacitance of the scope probe could also create a lowpass with the series resistor resulting in a small phase shift error at higher frequencies. Sorry I can't recommend a specific manufactorer for a wire wounded power resistor, I haven't test enough different types of them. The one I got was good enough for me and better than my old ones, so I stopped searching.
What would be also a cheap but very good solution?

Build your own resistor!
do a parallel connection of a lot of e.g. 1 Watt or 2 Watt  film resistors until the wished maximum power level reaches. The quality will be very excellent with a much better high frequency behaviour than any wire wounded resistor. No matter if carbon, metal film or metaloxid resistors, to do power measurement it's very ok. Use a two plate construction, a thin copper or brass metal sheet, one for each pole. Do drills in every sheet and solder the resistors close as possible on the metal sheet. The advantage of this construction, the metal sheet acts as very low inductance conductor, like a ground plane. Second, the metal sheets acting as a heat sink. Third, the resitors having only short remaining leads - the heat in the resistors can flow easily in the metal sheets. If you are tired of paralleling so many resistors, why not use a fan to keep the less one at life. Test the frequency response.  Things like temperature cofficient not so important for these tests. I prefer the method of paralleling film resistors to build a cheap but very good load resistor. use an old electric cooker. The electric plates are good loads too (their wires are not so extremly long inside). The load can be easily varied with the rotary swith. And the plates - believe -the amplifier really must be very big to destroy the plates. I used them also for my experiments, but never measured their high frequency characteristics.

What would be a expensive solution?
build uoyour own power resistor e.g. with paralleled expensive bulk foil resistors in a TO-220 package, but for power measurements - nonsense not necessary - OK!, if you have the bugdet and the need for a commercial data sheet specification - why not.

Do I measure many times with a real speaker?
No first, I hate the loud noise of a sine - OK, a wobbling sine sounds a little more interested than a single frequency. Specially at 3 to 4 kHz somebody could get mad about the loud sound, in this frequency range the human ear is very sensitive. Or at high frequencies may be all cats and dogs running away - I don't know.Yes, I could do sine sweeps with a lower amplitude to prevent noise, but in most cases the amplifier copes very good with real speakers - boring.the measurements are not comparable, only with the same type of speaker.the speaker can be damaged, special at high amplitudes - not all speakers are designed for  high continous levels of power. You don't know where are the real limits of your speaker pair. You have only two trials - haha. You don't know the increasing temperature on the coil without checking removing from the chassis.Yes, a nice characteristic of a real speaker is the effect of the inducted voltage by it's own movement, to see the effects on the amplifier. But there are many other tests to see how the amplifier will behaviour with a difficult speaker.

Which other loads are good to test an amplifier?
using a parallel capacitance with a ohmic load. A capacitance on an amplifiers output - that's poison for the amplifier, they don't like that. The phase shift and the increasing currents at high frquencies is a demanding load for the amplifiers control loop. But be careful - the capacitance can cause the amp to oscillate, some "real" nanofarads could be already enough for a audio amplifier. My opinion, a poor amp has also problems to drive a capacitance. Use for that test high quality capacitors (no electroly, tantals or poor foils) good are ceramics, styroflex or small foils in parallel. General, better many small ones parallel than a big one. Start with low amplitude and frequency and increase slowly and always check the heat dissipation in the capacitor - a power amp can kill the capacitor. Special bad capacitors getting hot very fast due their higher ESR.
inductive loads are not a big problem as capacitors.
if an amplifier can cope with the inducted voltage of an speaker? - check the amplifiers bandwidth, slew rate and the dynamic output resistance (damping factor). After measuring them - the results tells you if the inducted voltage will be a problem or not. test one or two channels at the same time, more stress for the power supply and increased heat.
Speaker Impedance and currents?
hard to say in general. Check different speaker manufactorer datasheets for the impedance vs. frequency curves. Adjust a load current same as the low impedance under the specified frequency and check the amplifier for errors and distortion. Get the scenario more worse by increasing power and adding a capacitance and/or inductance. Do such tests with different frequencies and levels (also low amplitudes). It's in your fantasy which additional stress applying to the amp. But please keep in mind, such stress can easily damage the amplifier. You never know the real limit. You don't know if the designer has protected the amp against all bad conditions e.g. short circuit currents, clipping, oscillation, a high applied induced voltage, or over temperature? These tests I hate to do with stranger amplifiers, you never know what happens - not enough stress, the limits can't be seen easily - Too much stress - much work finding and repairing the errors of the property others, destroyed by yourself - no thank you. With your own electronic - oh yeah - it's fun to reach the limits, either easy to repair or knowing no damage occurs because of save circiut desing.