Complex load for power amplifier torture testing
It has been discussed on the forum that we should test power amplifier not only to purely resistive load, but also some kind of the complex load should be used as well to verify amplifier stability and ability to drive complex load that may occur with different loudspeaker types and designs.
This is my take on the possible complex load circuit. It takes into account that some ESL speakers impedance falls deep below 1 ohm at very high frequencies and has capacitive character at middle frequencies.
The circuit is as follows, it is a 4 ohm resistor in parallel with a 33 uF capacitor which is in series wit a small 0.44 ohm resistor.
The real thing looks like this
and this is its impedance measured by LIMP software
We can see that the impedance falls to 0.5 ohm at 20kHz and has 1 ohm and -45° at 5kHz.
I have made some tests with the small linear amplifier described in this thread
https://www.audiosciencereview.com/...vintage-kenwood-ka-5010-amplifier-clone.9902/
This linear class AB amplifier has very good parameters so it was a good candidate to me.
First, I made measurements of responses to square waves with 1kHz and 10kHz repetition frequency, into this complex load. The measurements showed that the amplifier is stable with this complex load. There is a small decrease of impulse top seen in the 10kHz square response, for the reason that the initial impedance is close to 0.44 ohm value. But, no oscillations and overshoots.
1kHz square response
10kHz square response
Next, there is a measurement of THD vs. frequency at output amplitude that corresponds to 5W/4ohm/50Hz power. At 50Hz, the impedance is still resistive.
First plot shows distortion together with output voltage level
Second plot shows distortion vs. frequency in %. We can see that distortion rises above 500Hz, where the impedance starts to fall down quickly.
To compare, I am showing distortion plots of the same amplifier loaded with a purely resistive 4ohm load. We can see that the distortion is quite flat with frequency and stays below 0.007% up to 20kHz.
Distortion in % vs. frequency into resistive 4ohm load
It has been discussed on the forum that we should test power amplifier not only to purely resistive load, but also some kind of the complex load should be used as well to verify amplifier stability and ability to drive complex load that may occur with different loudspeaker types and designs.
This is my take on the possible complex load circuit. It takes into account that some ESL speakers impedance falls deep below 1 ohm at very high frequencies and has capacitive character at middle frequencies.
The circuit is as follows, it is a 4 ohm resistor in parallel with a 33 uF capacitor which is in series wit a small 0.44 ohm resistor.
The real thing looks like this
and this is its impedance measured by LIMP software
We can see that the impedance falls to 0.5 ohm at 20kHz and has 1 ohm and -45° at 5kHz.
I have made some tests with the small linear amplifier described in this thread
https://www.audiosciencereview.com/...vintage-kenwood-ka-5010-amplifier-clone.9902/
This linear class AB amplifier has very good parameters so it was a good candidate to me.
First, I made measurements of responses to square waves with 1kHz and 10kHz repetition frequency, into this complex load. The measurements showed that the amplifier is stable with this complex load. There is a small decrease of impulse top seen in the 10kHz square response, for the reason that the initial impedance is close to 0.44 ohm value. But, no oscillations and overshoots.
1kHz square response
10kHz square response
Next, there is a measurement of THD vs. frequency at output amplitude that corresponds to 5W/4ohm/50Hz power. At 50Hz, the impedance is still resistive.
First plot shows distortion together with output voltage level
Second plot shows distortion vs. frequency in %. We can see that distortion rises above 500Hz, where the impedance starts to fall down quickly.
To compare, I am showing distortion plots of the same amplifier loaded with a purely resistive 4ohm load. We can see that the distortion is quite flat with frequency and stays below 0.007% up to 20kHz.
Distortion in % vs. frequency into resistive 4ohm load