siliconchip.com.au Australia’s electronics magazine May 2019 41
AmplifierTHD vs Frequency,1kHz, 1W 21/12/18 20:12:070.0110.020.050.10.20.5Total Harmonic Distortion ( )%20 50 100 200 5001 k2k5k 10k20k
Frequency ()HzAmplifierLeft/RightChannelCrosstalk 21/12/18 18:3 32 :720 50 100 200 500 1k 2k 5k 10k 20k
Frequency ()Hz-60+2 0-50-40-30-02-01+0+1 0Relative Amplitude (dBr)+3 0+4 0+5 0+6 0
Left channel (undriven)
Right channel (driven)Fig.5: crosstalk figures for this amplifier are not
particularly great, with less than 20dB separation
between channels. This is probably due to the close
proximity of the output filter inductors for each channel.
This generally isn’t a problem when playing regular
music recordings, but if it bothers you, you have the
option of using two separate modules, one for each stereo
channel.Fig.4: the measured distortion performance of the left/right
channels on our sample module (after fixing the output filters),
into an 8resistive load. While not quite as good as the amplifier
designs we publish, it’s below 0.1% THD+N up to about 3.5kHz
(with a 20kHz bandwidth) which is not too bad. It certainly
sounds acceptable. We must use a 20kHz filter to remove the
switching residuals, hence the drop-off in readings above about
6kHz, above which the main harmonics are filtered out.
Yuanjing Class-D amplifier
features and specifications- Inputs: 3 separate channels (left, right,
subwoofer) - Outputs: 5 x 50W RMS into 4Ω (21V
DC supply) or 5 x 30W RMS into 8Ω
(24V DC supply) - Supply voltage: 4.5-27V DC
- THD+N: typically around 0.05% at
1kHz, 1W - Frequency response: 20Hz-20kHz, ±1dB
- Efficiency: up to 90% (comes with
small heatsinks fitted) - Switching frequency: 400kHz ±3kHz
- Self protection circuits: over-voltage,
under-voltage, over-temperature, DC
offset, over-current and short-circuit
protection. - Input connectors: 3-way pin header or
Bluetooth wireless - Output connectors: 5 x 2-way termi-
nal blocks - Power connector: solder pads
- Module size: 165 x 115 x 25mm
capacitors, it shows a slight lift at
20kHz, continuing to rise to 30kHz,
then dropping sharply to -60dB at
1MHz.
Naturally, after doing that, the unit
sounded much better, with an excel-
lent high-frequency response; very
different from our first listening test!
The subwoofer response is also
shown in Fig.2. It has a peak at 28Hz
and is -20dB at 250Hz, which is close
to ideal.
The subwoofer amplifier can put
out significant power and the IC is
supposed to handle 2Ω speakers, but
we found that 4Ω is the minimum for
this particular module. You won’t find
many 2Ω drivers (outside of cars), an-
yway.
By the way, you may notice that af-
ter this modification, the module has
a slight (2dB) rise at the low-frequency
end, close to 20Hz.
This is probably due to crosstalk with
the subwoofer section and the design of
the PCB, but it should not be a problem
because most loudspeakers will not re-
spond to such low frequencies.
A small amount of low-end boost
will generally improve the response of
most loudspeakers anyway.AM radio frequency avoidance
The TPA3116D2 has advanced os-
cillator/PLL circuitry which employs
multiple switching frequency options
to avoid AM interference.
These options cover 15 different
frequencies, ranging from 376kHz to
1278kHz, so it can be set to avoid the
AM band in most countries.
Our module was pre-set at 400kHz
(403.5kHz measured) so that only the
first harmonic will fall into our local
AM band.
We also checked the output with aspectrum analyser and found that the
first harmonic (807kHz) was 57dB low-
er than the audio output signal level, so
there should be very little interference
with AM radio receivers (see Fig.3).
If you are going to use the module
in other places where 400kHz radia-
tion could be a problem, you could
modify the unit according to the data
sheet, but that would be quite tricky.
So we suggest that you instead try
to keep the speaker leads short – less
than 1m if possible – so they make for
poor transmitting aerials.
The spectrum from 500Hz to 40MHz
is otherwise very clean.Distortion and noise (THD+N)
The unit is quoted as having a
THD+N figure of 0.1% at 1kHz with a
25W output. We decided to verify this
with some measurements.
The maximum power into an 8Ω
load is 40W RMS and the THD+N read-
ing was 1% when clipping started to
be noticeable at this level. The high
THD+N at very low power levels is
merely noise. As expected, the mod-
ule will deliver 50W into 4Ω loads.
Fig.5 shows a plot of THD+N vs
frequency for the module. These fig-
ures are the best that we were able to
achieve after changing the output in-
ductors. The distortion above 10kHz
may be higher than indicated because
we used a 20kHz “brick wall” filter