inexpensive, too, at only
a little over $10.
On my first run
through testing this
module, I was so
surprised by the
performance I measured
that I reached out to Jill
(my contact at Parts
Express) to get some
more information. It
showing no less than
9.1% THD+N at low
power, rising very
quickly to more than
60% THD+N. That can't be
right! Jill put me in touch
with Rory, the Product Line
Manager for these modules
over at Parts Express, who sent me a note about the
test strategy for this type of device. He grabbed one
off the shelf and took some of his own
measurements to confirm my findings.
Rory wrote, "Most stand-alone class D amps
provide onboard reconstruction filtering, but amp
boards designed to be built into a device will
sometimes omit it because they intend for the driver
VC inductance to take care of the needed filtering.
The PAM8610 board we sell is one of this type."
That's true. There is no filter on the output of this
module. Rory and I had a short phone conversation
where he explained his own test setup, and
recommended I limit the bandwidth of my THD
analyzer to keep residual switching noise from
impacting the measurements. He also suggested
adding a series inductor in each leg of the speaker leads
to simulate the inductance of a voice coil (which wasn't
provided) into a resistive dummy load. He checked a few
speaker datasheets, and we determined that 50 uH in
each leg should accurately simulate a speaker driver load.
Armed with this information and several low DCR chokes,
I went to try again; refer to Photo 11.
The second time measuring this module, I switched
to my other audio analysis system which has better filter
options (a Tektronix AA 501A Mod WQ distortion
analyzer with SG 505 ultra-low distortion oscillator) and
re-ran the numbers with a 30 kHz low pass enabled,
along with the inductors in each leg of the speaker leads
(Photo 12). This seemed to do the trick since
performance improved drastically! It no longer seemed to
March 2015 49
PHOTO 10. Craig A.
Lindley's electric guitar
project (Nuts & Volts,
A class D amplifier — or switching amplifier — differs
from most other types of amplifiers in that instead of directly
amplifying a signal, it uses high speed pulse width modulation
(PWM) followed by a low pass filter to recover the analog
waveform from the pulse train. These amplifiers are very
efficient, but can be challenging to design and implement.
Class D amplifiers commonly use a switching frequency
around 10x the highest frequency of the signal to be
amplified, such as the PAM8610 with a 250 kHz switching
frequency to deliver full audio bandwidth. A "class T" amplifier
is the name for Tripath's proprietary implementation of class
D technology, which uses an advanced control scheme and
an extremely high switching frequency ( 50 MHz or greater) to
deliver even higher efficiency and performance.
PHOTO 11. The PAM8610 next to the 50 uH low DC resistance
inductors to simulate the speaker load.
PHOTO 12. Tektronix TM504A mainframe housing the AA
501A audio analysis system.