Measuring THD on Denon RCD-M41DAB

26.01.2019 21:53

Around a month ago my old micro Hi-Fi system wouldn't turn on. I got a Sony CMT-CP11 as a gift from my parents way back in 2000 and it had served me well. I really liked the fact that I had a service manual for it with complete schematic, and over the years it accumulated a few hacks and fixes. It did start to show its age though. For example, the original remote no longer works because its phenolic paper-based PCB had deteriorated.

Unfortunately, it seems that now the mains transformer had died as well. I spent some time searching around the web for a replacement, but couldn't find any. I contemplated rewinding it, but doing that on a mains transformer seemed too risky. Finally, I gave up and just bought a new system (however, if anyone knows a source for a Sony part 1-435-386-11 or 1-435-386-21 I would still be interested in buying one).

So anyway, now I'm an owner of a shiny new Denon RCD-M41DAB. In its manual, it states that the output amplifier is rated as:

Denon RCD-M41DAB audio amplifier rating.

Image by D&M Holdings Inc.

This a bit less than CMT-CP11, which was rated 35 W at the same conditions. Not that I care too much about that. I doubt that I ever cranked the Sony all the way to full volume and I'm not very sensitive to music quality. Most of my music collection is in lossy compressed files anyway. However, I was still curious if Denon meets these specs.

Unfortunately I didn't have big enough 6 Ω resistors at hand to use as dummy loads. I improvised with a pair of 8 Ω, 60 W banks I borrowed from my father. I connected these across the speaker outputs of the Denon and connected a scope probe over the left one.

Setup for measuring THD, with resistors instead of speakers.

To provide the input signal I used the Bluetooth functionality of the RCD-M41DAB. I paired my phone with it and used the Keuwlsoft function generator app to feed in a sine wave at 1 kHz. I set the app to 100% amplitude, 100% volume and also set 100% media volume in the Android settings. I then set the volume by turning the volume knob on the RCD-M41DAB.

The highest RCD-M41DAB volume setting before visible distortion was 33. This produced a peak-to-peak signal of 37.6 V and a power of around 22 W on the 8 Ω load:

Output signal at maximum level before distortion.

Using the FFT function of the oscilloscope it was possible to estimate total harmonic distortion at these settings:

FFT of the signal at maximum level before distortion.

U_1 = 12.9 \mathrm{V}\qquad[22.2 \mathrm{dBV}]
U_3 = 41 \mathrm{mV}\qquad[-27.8 \mathrm{dBV}]
THD = \frac{U_3}{U_1} \cdot 100\% = 0.3\%

For comparison, I also measured the THD at the unloaded line output of a cheap Bluetooth receiver. I used the same app and measurement method and that came at 0.07% THD.

The next higher volume setting on RCD-M41, 34, was visibly clipped at around 39 V peak-to-peak:

Output signal at one setting past maximum level.

I achieved the rated 30 W (albeit at 8 Ω, not 6 Ω) at volume setting 36. At that point the signal was badly clipped, producing many visible harmonics on the FFT display:

FFT of the signal at 30 W output.

Calculated THD at this output level (including up to 7th harmonic) was 12.9%

So what can I conclude from these measurements? First of all, I was measuring the complete signal path, from DAC onward, not only the output stage. Before saturating the output I measured 0.3 % THD at 22 W, which I think is excellent. According to this article, 1% THD is around the level detectable by an untrained human ear. I couldn't achieve 30 W at 10% THD. However, I wasn't measuring at the specified 6 Ω load.

If I assume that the output stage would saturate at the same peak-to-peak voltage at 6 Ω load as it did at 8 Ω, then it would output 32 W at a similar distortion. This would put it well below the specified 10% THD. Whether this is a fair assumption is debatable. I think the sharply clipped waveforms I saw suggest that most of the distortion happens when the output stage supply voltage is reached and this is mostly independent of the load.

That said, the volume setting I found comfortable for listening to music is around 10, so I'm pretty sure I won't ever be reaching the levels where distortion becomes significant.

Posted by Tomaž | Categories: Analog | Comments »


19.01.2019 20:07

And so another year rushed by. In the past twelve months I've published 19 blog posts, written around 600 notebook pages and read 13 books.

Perhaps the largest change last year was that I left my position at the Department of communication systems at the Jožef Stefan Institute. After 7 years behind the same desk I really needed a change in the environment and I already switched to working only part-time the previous fall. This year I only planned to handle closing work on an EU project that was spinning down. I found it hard to do meaningful research work while working on other things most of the week anyway.

"Kein Jammern" poster.

Even though I led my project work to official (and successful) completion I feel like I left a lot of things undone there. There are interesting results left unpublished, hardware forgotten, and not the least my PhD work which got kind of derailed over the course of the last project and was left in some deep limbo with no clear way of getting out. I thought that by stepping away of it all for a few months I will get a clearer perspective on what exactly I want to do with all of this. I miss research work, I don't miss the politics and I have yet to come to any conclusion if and how to proceed.

As a kind of ironic twist, I also unexpectedly got first authorship of a scientific paper last year. According to git log, it took almost exactly 4 years and around 350 commits and it tells a story quite unlike what I initially had in my mind. After countless rejections from various journals I basically gave up on the last submission going through. It was accepted for publication pending more experimental work, which caused a crazy month of hunting down all the same equipment from years ago, spending weekends and nights in the lab and writing up the new results.

A history of the number of journal pages written per month.

I spent most of the rest of my work days at Klevio wearing an electrical engineer's hat. Going from a well equipped institute back to a growing start-up brought new challenges. I was doing some programming and a lot of electronics design and design for manufacture, a field I barely touched with my electronics work at the Institute. In contrast to my previous RF work, here I brushed up on my analog audio knowledge and acoustics. I discovered the joy of meeting endless electromagnetic compatibility requirements for consumer devices.

Not surprisingly, after this I was not doing a lot of electronics in my spare time. I have several hardware projects in a half-finished state still left over from a year ago. I wish to work more on them this year and hopefully also write up some interesting blog posts. Similarly, I was not doing a lot of open source work, short of some low-effort maintenance of my old projects. Giving my talk about developing GIMP plug-ins was an absolute pleasure and definitely the most fun presentation to prepare last year.


Drawing has remained my favorite pass-time and a way to fight anxiety, although I sometimes feel conflicted about it. I did countless sketches and looking back I'm happy to see my drawing has improved. I made my first half-way presentable animated short. It was nice to do such a semi-long project from start to completion, although it sometimes started to feel too much like a yet another afternoon job. I have some more ideas and with everything I learned last year I think it would be fun to try my hand at animating something more original, if only I could manage a more relaxed schedule for it.

All in all, looking back at my notes suggests it wasn't such a bad year. Except maybe December, which tends to be the most depressing month for me anyway. As last time, I'm not making any big plans for this year. I'm sure it will be again too short to clear out my personal backlog of interesting things to do and everything else that will want to happen before 2020. I only hope to waste less of it on various time-sinks like Hacker News and other addictive brain candy web sites. These seem to be starting to eat up my days despite my trying to keep my distance.

Posted by Tomaž | Categories: Life | Comments »