ASR dummy load configuration

[dualazmak]

Hello,

JBL uses a fixed L-Pad in the tweeter crossover for the M2 Monitor speaker.

An 11R 10 Watt resistor is used in parallel with the tweeter and another 13R 30 Watt resistor is in series with the parallel tweeter and 11R resistor.

What this does is operates the amplifier at a higher power output where the amplifier operates at a higher SNR. Also the amplifier operates into a flatter, more resistive load. I use this fixed L-Pad configuration in my tri-amp system. See page 14 https://rephase.org/projects/JBL_M2_crossover.pdf .

In terms of Class AB operation most AB amplifiers overate in Class A for the first few watts. In terms of Sound Quality try it and let us know what you think.

Thanks DT

Hello DualTriode,

You wrote;
What this does is operates the amplifier at a higher power output where the amplifier operates at a higher SNR. Also the amplifier operates into a flatter, more resistive load.

and I believe this general feature of amplifiers would properly explain and justify my use of Jantzen Audio's 22.00 Ω 10W Audio-Grade Non-Inductive Wirewound SuperRes Resistor for fine tuning of midrange drivers, tweeters and super tweeters in my multichannel multi-amplifier configuration.

I am now very much happy and relieved that my way of eliminating attenuators (ref. here and here) was good and alright!

Thank you indeed for your suggestive notes.

 

[DualTriode]

Conclusion
My ARCOL NHS300 (Non-inductive, 300W) 4Ω 1% power resistors are innocent and have been released by the parole board with a work permit on my bench on account of good behavior with secondary duties as a boat anchor.

I'm convinced that amp characterization using this resistor series (such as those published by ASR) can be trusted. I also have come to believe that even though I've found papers dating from the '50's on VCR and TCR distortion in resistors, this problem is still ignored in much of the amp testing you'll find online and largely explains why Amir's distortion results are often lower.

Summary
Today's tests used the AHB2 amplifier, which employs much less feedback than the NC400 to achieve its world-class distortion specs. As expected, this resulted in reduced distortion suppression of the poorly performing test load constructed from paralleled water heater elements.

Measurements
2 second continuous sweeps at 96k/24bit were used. Power levels were 1W, 5W, 10W, 100W and 200W. The (3) test loads were; (1) of the 4Ω ARCOL resistors, (4) of the 4Ω ARCOL resistors in series/parallel for a net 4Ω load, and a parallel array of (3) water heater elements with a net 4.2Ω load.

Heat
Neither of the ARCOL resistor setups showed any detectable warming. They remained as cool to the touch after the measurements as before. The water heater elements (rated at 4,500W each submerged) remained on the bench air-cooled. I did notice a very slight warmth to the touch after the 200W sweep.

Step One
Again, the major distortion component was the 3rd harmonic, and while the 2nd harmonic was again insignificant in most cases, it did start to rise at the 100W and 200W levels with the water heater elements. The measurements performed are the same as on the NC400; (5) sweeps using each of the (3) 4Ω load configurations. Again, higher orders than the 3rd harmonic were so far down they are had no material affect on THD.

2nd Order Harmonics
View attachment 155272

Step Two
Now we compare the single 4Ω ARCOL resistor to the quad series/parallel array that also nets to 4Ω. All (5) power levels effectively overlap. If some level of VCR (voltage coefficient) or TCR (temperature coefficient) distortion were within the range of my measurements, the quad array would have shown significantly lower distortion. The fact that the distortion traces overlap tells me that the resistors are innocent.

Level vs. THD
View attachment 155267

3rd Order Harmonics
View attachment 155271

Step Three
Now we compare the single 4Ω ARCOL resistor to the paralleled water heater elements. These traces do NOT overlap. The paralleled elements show significantly more distortion at each power level.

Level vs. THD
View attachment 155268

3rd Order Harmonics
View attachment 155270

God bless you and your precious family - Langston

Hello All,

Back from the beach this afternoon and saw all the colorful plots.

Here is one of my own making.

There is something odd going on with the AHB2 amplifier. Perhaps some THX feed-forward or feed-back kind of stuff.

Power output is near 12 Watts. The dogleg happens with all of the 4 Ohm resistors we have been testing. The dogleg does not show up with the APx1701 amplifier.

Thanks DT

 

[restorer-john]

Hello All,

Back from the beach this afternoon and saw all the colorful plots.

Here is one of my own making.

There is something odd going on with the AHB2 amplifier. Perhaps some THX feed-forward or feed-back kind of stuff.

Power output is near 12 Watts. The dogleg happens with all of the 4 Ohm resistors we have been testing. The dogleg does not show up with the APx1701 amplifier.

Thanks DT

View attachment 155432View attachment 155433

Your upper plot is marked +117dBV. You no doubt mean +17dBV, which translates to ~12.5W@4R.

 

[DualTriode]

Hello DualTriode,

You wrote;
What this does is operates the amplifier at a higher power output where the amplifier operates at a higher SNR. Also the amplifier operates into a flatter, more resistive load.

and I believe this general feature of the amplifier would properly explain and justify my use of Jantzen Audio's 22.00 Ω 10W Audio-Grade Non-Inductive Wirewound SuperRes Resistor for fine tuning of midrange drivers, tweeters and super tweeters in my multichannel multi-amplifier configuration.

I am now very much happy and relieved that my way of eliminating attenuators (ref. here and here) was good and alright!

Thank you indeed for your suggestive notes.

Hello,

My impression is that you are half way there.

You have removed an l-pad and only replaced it with one resistor in parallel with the tweeter. With only a resistor in parallel with the tweeter this lowers the impedance the amplifier operates into. Not a good thing to do, IMO.

Take a look at page 14 of the link in my previous post to see the placement of the second resistor.

To select the resistor values you may want to try an assortment of resistors or use the original l-pad and measure resistor values from that.

Thanks DT

 

[DualTriode]

Your upper plot is marked +117dBV. You no doubt mean +17dBV, which translates to ~12.5W@4R.

Typo,
Yes +17dBV.
+17dBV is an estimate of the placement of the line on the plot.
Perseveration on the keys.
DT

 

[dualazmak]

Sorry, but I deleted content of my post here made just before which was not suitable in this thread.

 

[DualTriode]

Hello All,

Now I have a couple quality power amp test resistors I am going to go back to the NAD THX 216 amplifier.

The first time around I measured in the high 90's SINAD using High distortion 500 Watt elevator braking resistors.

Anyone taking any bets that the SINAD breaks 100 with the high quality wire wound RH Vishay power resistors?

Thanks DT

 

[Langston Holland]

The first time around I measured in the high 90's SINAD

That's pretty impressive already. The service manual indicates 0.03% THD (doesn't seem to include noise) at rated output for 20Hz, 1kHz and 20kHz. 20log(1/0.03%) = SINAD of 70.5dB.

Betcha a nickel it doesn't clear 100dB (that's the highest bet allowed in my family). : )

 

[DualTriode]

ASR does the SINAD measurement at 5 Watts output. The Manufacturer's specification is at full power, perhaps that is a difference.

 

[Jbrunwa]

In ham radio we used dummy loads that are suspended in a mineral oil filled paint can to handle up to 1500 watts for several minutes. Might this scheme work for audio with appropriate resistors?

Dummy Load Construction

Building a Dummy Load and measuring power accurately

www.k4eaa.com

included to show construction, not circuitry

https://www.dxengineering.com/parts/ame-adl-1500x

 

[Langston Holland]

In ham radio we used dummy loads that are suspended in a mineral oil

Radio tech is often transferrable to and usually precedes audio "discoveries" by decades. : )

Pat Brown of Syn-Aud-Con (also called Pro Sound Training) made a fancy one in 2011. Ham radio transmission power can be huge just like the output power of the largest professional audio amps, thus the need for cooling to keep the load resistance down to a reasonable size and weight.

The primary issue of this thread for me was insuring the distortion from the resistive load remained at least an order of magnitude below a state-of-the-art audio amplifier so that the test didn't interfere with the results. I have no idea if these oil bath load setups are low distortion or not, I do know that electric water heater elements are not low distortion.

God bless you and your precious family - Langston

Here's a picture not shown in the article that includes an add-on to quadruple power handling

 

[Cbdb2]

I don't understand the need for perfect test resistors when speakers have orders of mag more voltage coefficient (non linearities). The perfect amplifier doesn't care if the vcr is 1000% it still puts out exactly Vin x A. Just like it doesn't care that the impedance of a speaker goes from 40ohms to 4ohms.

So I would rather see how an amp behaves with a variable load ( high vcr load) than a perfect one. Its a better indicator of real life performance.

If your just trying to get the lowest numbers, your right.

 

[iNetRunner]

Stereophile has used a simulated speaker load for amplifier measurements since 1995. JA described the setup (and the schematic) in his article Real-Life Measurements. And later in July 2007 Keith Howard further outlined the reasons in his article Heavy Load: How Loudspeakers Torture Amplifiers.

The Finnish HiFi magazines have also used simulated speaker loads for amplifier measurements (the previous magazine “Hifi-Lehti”, and the current magazine “Hifimaailma“). They described their speake load simulator in the March issue in 2009. Sadly there isn’t any online versions of that article (and it was in Finnish). If I remember anything about it, I think it was a slightly modified or somewhat similar version to Stereophile’s circuit.

Anyway, just mentioning this as a further point for maybe trying something similar for discovering misbehaving amplifiers (e.g. rising distortion, less driving capability, worst case results being instability and safety features engagin etc.) when faced with real world speaker.

 

[Cbdb2]

Ideally measurements should be made with a few different loads. 8, 4 and 2 ohm resistors and at least one terrible speaker impedance.

 

[DualTriode]

I have been taking a similar look at test load resistors for much lower output headphone amplifiers.

Caddock TO 220 footprint resistors MP930 30 Watt are, testing the Topping A30 Pro HPA down to 121 SINAD.

Caddock makes a big brother MP9100 100 Watt resistor. A couple of these in series or in parallel would get us up to 200Watts perhaps at much lower distortion than these Vishay and Arcol wire wound resistors are capabible of.

I will try a couple of 8R Caddock MP9100's in parallel and report back.

Thanks DT

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Order today, ships today. MP9100-4.00-1% – 4 Ohms ±1% 100W Through Hole Resistor TO-247-2 Moisture Resistant, Non-Inductive Thick Film from Caddock Electronics Inc.. Pricing and Availability on millions of electronic components from Digi-Key Electronics.

www.digikey.com

 

[kongwee]

All these load only look at one or few dimension. My ideal test at least rip 2 a way driver loudspeaker and test the point from vocal coil from both driver. The closest on how amplifier look at it's loads.

 

[Scgorg]

All these load only look at one or few dimension. My ideal test at least rip 2 a way driver loudspeaker and test the point from vocal coil from both driver. The closest on how amplifier look at it's loads.

The problem with doing this is that you'll also be measuring non-linearities from the coil's back EMF, which is not a part of the amplifier's distortion. I believe SBAF had a whole thing where they measured two headphone amplifiers that performed similarly on a dummy load on a "real" load by using this method, and the result was quite simply that the amplifier with lower output impedance (and hence less affected by back EMF from the driver) showed less distortion, especially at low frequencies. This distortion did not stem from the amplifier driving the load, it stemmed from non-linearities in the coil itself.

 

[kongwee]

The problem with doing this is that you'll also be measuring non-linearities from the coil's back EMF, which is not a part of the amplifier's distortion. I believe SBAF had a whole thing where they measured two headphone amplifiers that performed similarly on a dummy load on a "real" load by using this method, and the result was quite simply that the amplifier with lower output impedance (and hence less affected by back EMF from the driver) showed less distortion, especially at low frequencies. This distortion did not stem from the amplifier driving the load, it stemmed from non-linearities in the coil itself.

Ok take a step back. Real world crossover with at least two inductor to stimulate tweeter and woofer at least.

 

[restorer-john]

I have been taking a similar look at test load resistors for much lower output headphone amplifiers.

Caddock TO 220 footprint resistors MP930 30 Watt are, testing the Topping A30 Pro HPA down to 121 SINAD.

Caddock makes a big brother MP9100 100 Watt resistor. A couple of these in series or in parallel would get us up to 200Watts perhaps at much lower distortion than these Vishay and Arcol wire wound resistors are capabible of.

I will try a couple of 8R Caddock MP9100's in parallel and report back.

Thanks DT

MP9100-4.00-1% Caddock Electronics Inc. | Resistors | DigiKey

Order today, ships today. MP9100-4.00-1% – 4 Ohms ±1% 100W Through Hole Resistor TO-247-2 Moisture Resistant, Non-Inductive Thick Film from Caddock Electronics Inc.. Pricing and Availability on millions of electronic components from Digi-Key Electronics.

www.digikey.com

Dualtriode, I will interested to see your results with the 100W Caddocks. If you look a their derating curves, I am concerned they will self destruct at high powers. We all know how much abuse wirewound resistors can take and how hot they can get without damage.

Keeping a TO-247 at 100degrees C is hard enough and they are already derated 50% at that power.

 

[DualTriode]

Dualtriode, I will interested to see your results with the 100W Caddocks. If you look a their derating curves, I am concerned they will self destruct at high powers. We all know how much abuse wirewound resistors can take and how hot they can get without damage.

View attachment 185985

Keeping a TO-247 at 100degrees C is hard enough and they are already derated 50% at that power.

Hello @restorer-john,

I am with you and I share your concern.

100 Watts may be more of a nominal value rather the limit to approach.

I am still a bit leery about their distortion performance as the temperature increases.

The heat sinks may end up weighing several pounds. The little Aavid heat sinks I have in hand will start getting too hot to touch with 3 or 4 watts dissipation.

The first thing that I want to look at is performance down at 5 Watts, you know, the dashboard level of performance where we measure SINAD and plot the FFT.

One thing that is interesting to me is the internal resistance to heat flow between the heating resistive element and the resistor case (Thermal Resistance (RθJC)) is much lower in the 100 Watt resistor. With a larger heat sink the resistive heating element temperature will be more stable. Less temperature swing to cause distortion. At 1000hZ with a large warm heat sink the temperature will not rise as fast or as much or cool as fast or as much as the voltage / power input varies with test input frequency.

We will see.

Thanks DT