42 Silicon chip Australia’s electronics magazine siliconchip.com.au
Yuanjing Amplifier Frequency Response 22/12/18 12:27:15-60+2 0-50-40-30-20-10+0+1 020 50 100 200 500 1k 2k 5k 10k20k
Frequency ()HzRelative Amplitude (dBr)Subwoofer output
Left/right outputsYuanjingTHD vs Frequency1, kHz, 1W 22/12/18 13:39:530.0110.020.050.10.20.5Total Harmonic Distortion ( )%20 50 100 200 500 1k 2k 5k 10k20k
Frequency ()HzLine in
BluetoothFig.6: the self-protection features of the TPA3116D2 IC.
to eliminate subharmonics from the
400kHz switching frequency, which
otherwise would have affected the
measurements.
The 80kHz bandwidth measure-
ments we usually take with linear am-
plifiers cannot be made with Class-D
amplifiers. Therefore, we took some in-
termodulation distortion (IMD) meas-
urements to clarify the level of distor-
tion at higher frequencies.
The IMD measurements were taken
by injecting the SMPTE-standard fre-
quencies of 500Hz & 2kHz (2:1) and
the resultant spectrum shows accept-
ably low noise up to 24kHz. The aver-
age level is 0.11% which verifies the
THD+N measurements; this is not bad
for a Class-D amplifier.
Crosstalk
We checked out the crosstalk of the
amplifier module (Fig.5) and the re-
Fig.7: the frequency response of the Yuanjing-brand 4.1
channel amp is fine out-of-the-box, unlike the other one
we tried. Note how its subwoofer low-pass filter is far
less aggressive than the other board’s, with significant
amounts of low bass making it through, up to a few
hundred hertz.
Fig.8: distortion performance is similar to the cheaper one;
slightly worse at lower frequencies (probably due to the
use of less-linear coupling capacitors), and slightly better
at higher frequencies. Its performance is significantly
better when using the line input pin header compared to
Bluetooth, likely to due to digital artefacts and noise in the
output of the Bluetooth module.sults were as not as good as specified,
probably because of the design of the
PCB and the interaction of the output
inductors, which cause feedback into
the opposite stereo channel.
There is not a lot you can do about
this; it may be possible to re-locate the
inductors or substitute ferrite beads,
but if you want really good crosstalk
performance, given its low cost, you
could simply use a separate module
for the left and right channels.
While we were working on this ar-
ticle, similar modules have appeared
on eBay for around $5. So it’s hardly
worth arguing about!Protection features
The TPA3116D2 is a well-protected
device and has self-protection for over-
voltage and under-voltage conditions
as well as an output DC fault, short-
circuit, overload and over-temperatureconditions. When an over-current,
short-circuit, over-temperature or DC
offset fault is detected, the module
switches itself off and you need to cy-
cle power to restore its function.No point changing the op amps
As mentioned earlier, the unit we
obtained came with two NE5532 op
amps in sockets. Most dual op amps in
DIP-8 packages have the same pinout,
so it’s easy to swap them – but there
isn’t much point!
Firstly, while the NE5532 is an old
design, it has stood the test of time and
even by today’s standards still has out-
standing performance.
And secondly, the distortion and
noise in this amplifier is dominated
by the amplifier ICs themselves and
not the op amp-based preamplifiers.
We tried replacing the NE5532 with
newer OPA1642s (soldered to SOIC-