Load limit curve (amplifier voltage and current vs load impedance)

[Interference]

With this thread I would like to introduce an amplifier measurement that has been popularised by F. Montanucci on the Italian magazine AudioReview (and previously, I believe, on Suono).

It is defined as "caratteristica di carico limite" (CCL) that could be translated as "limit load characteristic" or "limit load curve". It consists of testing the maximum amplifier output on different load impedances between 2 and 16 Ohm. This is tested both for "peak power" ("regime impulsivo") and "continous power" ("regime continuo").

Below you can find two examples (source).

The more vertical the curve, the better the capability of the amplifier to provide a load-independent voltage.

I find this measurement very revealing, if not fundamental when comparing amplifiers of similar SINAD. The declared specifications of power vs impedance are often lacking or inconsistent. I would love to see this kind of measurement in Amir's reviews, but I understand it is not easy to set up in a consistent manner.

I would like to hear from you folks your opinion on this kind of testing.

Best.

P.S.: I would like to add that Montanucci has been also regularly conducting transient intermodulation measurements (see article above) with a protocol similar to the one proposed by Otala, although the corresponding plots are not much readable from a quantitative point of view.

 

[NTK]

I really like these curves. They make it much easier to calculate what sound pressure level (SPL) you would get from a pairing of speaker and amplifier (when assuming the SPL output is amplifier limited, not speaker diaphragm excursion limited).

Speakers are voltage sensitive instead of power sensitive. That means, if the FR of the speaker is sufficiently flat, a sine wave input of 2.83 Vrms (for example) at any frequency within the flat FR range of the speaker will output the same SPL. However, since the speaker impedance (R) is usually not constant across the frequency range, and power = V^2 / R, the power drawn is not constant and vary with frequency.

With these curves, you can see easily what the maximum output voltage the amp can deliver at the speaker's minimum impedance, and therefore easily determine the maximum SPL output you'll get with the speaker/amplifier pairing.

The downside is significantly more work and expenditure on dummy loads. You'll need quite a few tests with quite a few sets of different impedance loads to trace a detailed curve. (From the charts, it looks like the tests were ran with loads of 2.0, 2.3, 2.7, 3.2, 4.0, 5.3, 8.0, and 16.0 ohms.)

 

[audio2design]

"Peak" of course highly variable w.r.t. peak duration.

 

[Head_Unit]

The more vertical the curve, the better the capability of the amplifier to provide a load-independent voltage.

I think this kind of test is great-I used to do that for a now-killed car magazine. The only difference was I used 8-4-2-1 ohms, and used a simpler scale (power versus impedance) for one single line which you could see how much it "drooped"

The next big value is using partially capacitive and inductive test loads, like the Power Cube https://www.audiograph.se/ however this is more work of course to create the test loads, and more tests to run. And at some point, to paraphrase what someone once posted, instead of worrying about if one amp clips somewhat sooner than another, "you just need a MUCH more powerful amp and don't worry about it"

 

[Interference]

The downside is significantly more work and expenditure on dummy loads. You'll need quite a few tests with quite a few sets of different impedance loads to trace a detailed curve. (From the charts, it looks like the tests were ran with loads of 2.0, 2.3, 2.7, 3.2, 4.0, 5.3, 8.0, and 16.0 ohms.)

I wonder if this could be achieved with an electronic load (although I never owned one so I am not sure if they can simulate a "fixed impedance" mode).

 

[DonH56]

Been discussed quite a bit previously, and Amir looked in to the PowerCube, but it was expensive and would not handle the higher-powered amps.

Building A Reactive Load for Amplifier Testing

So interest has been expressed in more comprehensive power testing of amplifiers going beyond just resistive load. Audiograph makes such a box for automated testing but the retail is north of $25000. Way too much money for the limited use we will get out of it. But you do get cool graphs out...

www.audiosciencereview.com

https://www.audiograph.se/wp-content/uploads/2018/10/PowerCube_12p_brochure_complete.pdf

 

[Head_Unit]

I wonder if this could be achieved with an electronic load (although I never owned one so I am not sure if they can simulate a "fixed impedance" mode).

Audiograph has I guess made something like that-I imagine it is like a programmed amplifier to provide a series of counter-voltages? But I'm not certain.

 

[DonH56]

Audiograph has I guess made something like that-I imagine it is like a programmed amplifier to provide a series of counter-voltages? But I'm not certain.

You could look at the link in my post right above yours...

 

[Head_Unit]

You could look at the link in my post right above yours...

I *could* but apparently *did not*
Time for another eye doctor appointment...

 

[amirm]

Audiograph has I guess made something like that-I imagine it is like a programmed amplifier to provide a series of counter-voltages? But I'm not certain.

It presents a fixed set of impedances and at a number of reactances. Of note, it only works for 1 kHz. So it is not programmable in any sense. DC loads is what you are thinking about but they are not suitable for this application because they generate a lot of distortion/noise.

 

[restorer-john]

The bar graph comparisons for power amplifiers into 8/6/4/2/1R like Yamaha presented in their sales reference manuals back in the day were simple, but conveyed a lot of information in one graphic.

 

[charleski]

It looks like he’s using 1%THD as the measurement criterion. Italian AudioReview doesn’t put its material on the web, but I found this, which seems to indicate such is the case if you look at the different graphs. This is fairly standard, but it should really be clearly specified (especially since some measurements fudge things and relax the criterion to 3% or even more for tube amps).

 

[Head_Unit]

conveyed a lot of information in one graphic.

Actually I now like neither the Italian one nor Yamaha nor my own-really the graph should be in dBW (at like 16/8/4/2/1 ohms, but the dBW referred to 2.83V/8ohms). THAT would be more meaningful. Put the watt values somewhere in smaller print, with a boilerplate explanation that a 60% difference in power only equals like one small volume click

 

[restorer-john]

Actually I now like neither the Italian one nor Yamaha nor my own-really the graph should be in dBW (at like 16/8/4/2/1 ohms, but the dBW referred to 2.83V/8ohms). THAT would be more meaningful. Put the watt values somewhere in smaller print, with a boilerplate explanation that a 60% difference in power only equals like one small volume click

The trouble with dBW is it doesn't sell amplifiers and people like "power" output numbers, just like kW and Nm for car specs.

I agree it conveys good information, but it never really got off the ground.

 

[DonH56]

...And going from 100 W to 200 W sounds a lot better than just adding 3 dBW... 100 is a lot bigger than 3, so it must be better! And all those receivers that go from 100 W/ch to 105 W/ch for the next model up only adding 0.2 dBW, well, Marketing would go bollox over that.

 

[valerianf]

Looking at the 2 curves from the first post the continuous current is very different between the 2 amplifiers.
For sure , if I have the information, I will chose the amplifier able to provide continuously the highest current.
The speaker driver has an Electrical motor and more amps is better.
Let us not forget basic knownledge (in the old time some AVR got a toroidal transformer).