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

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