Why do mobile amplifier manufacturers and test reviewers rarely show distortion measurements at low and high power vs frequency?

[Mark185]

I have noticed with the exception of Amir, no one seems to publish a full set of measurements that show distortion vs power and frequency. I recently purchased a used ARC Audio 1000.6 amp rated at 175 watts per channel. This is considered a very high end Class D amp and I think it does measure well at 5 watts. I understand why marketing concerns would keep manufacturers from showing the whole truth but it is still somewhat disappointing to see what I am measuring.

I have a Quant Asylum analyzer and have been experimenting with measurements on the ARC amp and some others I own. I will caveat this by saying that I am not using a high pass filter like Amir uses with Audio Precision equipment to filter out the switching frequency of Class D amps. I am building one from plans I found on the internet but it is not finished. I also have problems with radiated power line noise and an FM transmitter at a nearby fire station. I use good shielded cables and the differential inputs on the QA403 but still have some noise getting through so my measurements are probably a bit lower than these amps are capable of. The value to me is that my test conditions are the same for all so I can still compare. Having said all this, here are some plots that tell a story.

At ~5600 Hz and 106 watts the measurements are quite different:

Going down to 125 Hz at 5 watts, this is the picture and its not bad.

Going to higher power at 125 Hz and here is the result:

 

[amirm]

The closest I have seen is from Hypex/Purifi where they show sweeps at a couple of frequencies from what I recall. Other than that, you are right that it is not an industry practice (not that measuring anything, is!).

 

[EERecordist]

I would guess they never got into the habit. Some friends have a mobile audio business and their founder is an industry leader.

The sales process is not measurement-based. If someone comes in with a 6 figure car, it is brand, what they carry, how they can fit it in, and price - the higher the better. Some systems have DSP for eq tuning.

Do you have a relatively calibrated sound level meter, like an iPhone with the NIOSH app? Measure your car noise floor. Then measure your listening level.

My guess that distortion is inaudible because of the noise floor.

I don't think even at the highest end car they get the noise floor down to a level you could hear distortion. Don Norman has written about user experience design for cars drivers.

The driver does a complex integration of the feel of the steering, visuals, the sound of the car, G-force, accelerator, brake, and vibrations. If you make the car silent that creates a safety problem. You can look him up and his writings on the subject.

Of course when we get fully self-driving taxis, the operator sound integration is not needed.

Given all of that, I'm not sure Dolby ATMOS in vehicles is needed. It will add to the price though.

 

[Mark185]

I do understand that the noise floor will mask the distortion I am able to measure. I am afflicted with a need to measure. . My first job ~ 35 years ago was as an instrumentation engineer in a facility that tested gas turbine aircraft engines and helicopter drivetrains. I have suffered with this affliction ever since. I do also enjoy listening to music. My school was a Path Train ride from Greenwich Village and I was lucky enough to hear some of the greatest jazz musicians in the world at the Village Vanguard and other small clubs as a young man. That was part of what motivated me to study electrical engineering.

My career and family life took me away from my enjoyment of tinkering with audio gear and setting up good listening environments in my cars and homes for many years. I recently got back into it, bought some decent measurement instruments, and started pulling an old collection of amplifiers out of storage to play with. The ARC 1000.6 is an amp I was able to buy used for a very good price.

Looking at your history signature, it probably won't surprise you to know my favorite home audio equipment was and still is Apt Holman Preamp and Amplifier One. I know there is better equipment but they suit me well. I will look up Don Norman's writing. Thanks for the tip!

 

[Mark185]

I should also give credit to the RAWCAt YouTube channel for motivating me to get back into mobile audio. I find most of his videos to be both entertaining and informative. He also publishes distortion and noise measurements and has tuning tutorials that were great for an old enthusiast getting back into this.

 

[Mark185]

A follow up on my ARC 1000.6 amplifier. I tried doing some frequency sweeps from 20-20,000 Hz at different power outputs and observed the output on my scope. Nothing unusual there but at powers above ~60 watts and frequencies above ~6 kHz, something in the amplifier started to 'sing' and I could hear the frequencies emanating from it. I know I could isolate it using a mechanic's stethoscope if I open the amp up but I decided to call ARC first. They immediately put me in touch with tech support who then forwarded my call to Brad. If I am not mistaken, Brad is one of their senior designers and President of the company. He said it is possible that the power supply caps were what were 'singing' but the output inductors would only do that if there was physical damage. He suggested I send it back in to be tested and cost would be somewhere between $50-175 depending on what they find. He told me that there had been multiple updates to this amp since its manufacture date in 2018 and they would perform all the updates before sending it back.

This is amazing customer service and I cannot think of another mobile audio manufacturer that treats its customers like this. Remember I bought this used. Most manufacturers would not even respond to a customer buying a used product.