40 Silicon chip Australia’s electronics magazine siliconchip.com.au
XD172700 Class-D amplifier
features and specifications- 3 x 50W RMS into 4Ω (21V DC supply)
- 3 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,
+3,-0dB (after modifications) - Efficiency: up to 90% (only needs a
small heatsink) - Switching frequency: 400kHz ±3kHz
- Self protection circuits: over-voltage,
under-voltage, over-temperature, DC
offset, over-current and short-circuit
protection. - Input connectors: 3.5mm stereo jack
socket or 3-pin JST header - Output connectors: 3 x 2-way termi-
nal blocks - Power connectors: 2-way terminal
block and DC barrel socket - Module size: 100 x 70 x 30mm
01020304050607080901000510 15 20 25 30 35 40 45 50
Output Power (W)Power Efficiency (%)PVCC= 6V
PVCC= 12V
PVCC= 24VGain = 26dBT
RA= 25°C
L= 4ΩG018
Power Efficiency (BTL) vs Output PowerAmplifier Frequency Response 17/12/18 15:39:09-30+1 0-25-20-15-10-5+0+520 50 100 200 500 1k 2k 5k 10k 20k
Frequency ()HzRelative Amplitude (dBr)Subwoofer output
Left/right pre mods
Left/right post modsA glance at the Texas Instruments
data sheet (www.ti.com/lit/ds/sym-
link/tpa3116d2.pdf) indicates that
when properly implemented, the IC’s
frequency response should be almost
ruler flat to about 40kHz.
The data sheet also recommends
that the LC filter after the output stage,
if fitted, should have a 10μH induc-
tor and 680nF capacitor on each out-
put pin.
However, we measured the supplied
LC filter at 55μH and 1μF, which would
explain the drastic reduction in high-
frequency response. We tried reducing
the output inductor values to 10μH,
which considerably flattened the fre-
quency response.
As per the data sheet, high-current
ferrite beads can be used in place of
the inductors, if the capacitors are also
changed to 1nF.
This will not be as effective at re-
ducing radiated emissions, however,
and doing this will require quite a bit
of soldering which may damage the
dual-layer PCB.
Changing the inductor values has
another benefit besides flattening the
frequency response; we found that
they got hot during use because the
wire used is too thin.
Audio inductors should be air-core
types to avoid non-linearity in the
core material.
We published instructions for wind-
ing 10μH inductors using 30.5 turns
of 1mm diameter enamelled copper
wire on standard bobbins available
from Jaycar and Altronics.
This was in the August 2011 is-
sue, on page 67 (siliconchip.com.au/
Article/1129). It was intended for use
in the Ultra-LD Mk.3 amplifier mod-
ule but is certainly applicable to this
one, too.
You then just need to remove the
existing inductors and solder the im-
proved ones into place. Keep them as
close to the PCB as possible and mountthem all with the same orientation to
reduce problematic magnetic field in-
teractions.
While you should ideally replace
the 1μF capacitors with 680nF capaci-
tors as per the data sheet, in practice,
it doesn’t make that much difference.
You can see the revised frequency
respone (after changing the inductor
values) as the blue trace in Fig.2
With the 10μH inductors and 1μFFig.1: sample efficiency curves from
the Texas Instruments TPA3116D2
data sheet. Efficiency is higher with
lower supply voltage but of course,
maximum power is also lower in those
cases. Efficiency also increases with
output power; in other words, device
dissipation does not increase much as
the output power rises.
Fig.2: frequency response of the 2+1 channel amplifier module
before and after we modified it. The mauve curve shows the
subwoofer output, which purposefully rolls off at around
100Hz, the left/right response as supplied is in red, and
post-mods is in blue. It’s now much flatter above 1kHz, and it
sounds a lot less muffled!
Fig.3: spectrum analysis of the output waveform shows that the
main peak at 403kHz, representing what’s left of the switching
waveform after filtering, is 40dB below the audio signal while its
first harmonic at 806kHz (in the AM broadcast band) is at -57dB,
so the amplifier should not cause too much AM interference.
Still, we’d keep the speaker leads as short as possible!