All,
A Ham called the lab and said he was having very bad RFI and had DF'ed it to a
new housing development about 1800 feet away. After working with the builder he
got access and found the source of the noise was not one but 3 of these duplex
outlet BlueTooth audio amplifiers. The house had 3 units powered by a common
12V power supply in the basement. Typically they were connected using 30 feet
of speaker wire, and about the same length of wire to the power supply in the
basement.
The Ham sent us one of the actual unit for testing. The unit fits into a
duplex wall box and looks a lot light a light dimmer with single knob on the
front.
The one I tested used a "Filter Free" TPA3110D2 15-W Filter-Free Stereo Class-D
Audio Power Amplifier made by TI.
They claim this "filter Free: device will pass FCC Class B on the data sheet.
Here is a quote from the datasheet
"The TPA3110D2 has been tested with a simple ferrite bead filter for a variety
of applications including long
speaker wires up to 125 cm and high power. The TPA3110D2 EVM passes FCC Class B
specifications under
these conditions using twisted speaker wires. The size and type of ferrite bead
can be selected to meet
application requirements. Also, the filter capacitor can be increased if
necessary with some impact on efficiency.
There may be a few circuit instances where it is necessary to add a complete LC
reconstruction filter. These
circumstances might occur if there are nearby circuits which are sensitive to
noise. In these cases a classic
second order Butterworth filter similar to those shown in the figures below can
be used.
Some systems have little power supply decoupling from the AC line but are also
subject to line conducted
interference (LCI) regulations. These include systems powered by "wall warts"
and "power bricks." In these
cases, it LC reconstruction filters can be the lowest cost means to pass LCI
tests. Common mode chokes using
low frequency ferrite material can also be effective at preventing line
conducted interference."
The reality is that with 10 foot speaker leads the conducted emissions was 18
to 24 dB above part 15B irrespective of the type of power supply used.
Did I mention that this housing development was going to 40 houses total with
about 3 of these units on average???
The main oscillator in this case was running at approx.. 250 KHZ, as expected
harmonic energy was observed at 250 Khz intervals.
The data sheet also talks about using a 2 pole LPF for better for better
harmonic suppression. I built a filter with a corner freq of 27Khz on the
bench.
Lo and behold. Passed part 15b with headroom to spare.
One more interesting fact. With the amplifier installed at W1AW I asked Joe
Carcia NJ1Q to play some music using the system.
I went back to the lab, about 175 feet away. Not only could I see significant
carriers at 250 KHZ intervals... but at 10 MHz... I could use FM detection and
hear the audio Joe was playing on the unit under test, actually with reasonable
quality. The load driving the speakers was enough to vary the oscillator so
that slope detection of audio was possible and FM detection was listenable.
The device, made by MCM Audio is no longer made but there are others on the
market. A unit made Pyle costing twice as much, had the LC filter built in...
and was extremely quite using all the variations I tried with speaker lead
lengths and power supply lead lengths.
73,
Paul Cianciolo, W1VLF
ARRL EMC Engineer
ARRL - The national association for Amateur RadioT
225 Main Street, Newington, CT 06111-1400 USA
Telephone: (860) 594-0392
FAX: (860) 594-0259
Email: w1vlf@arrl.org
World Wide Web: www.arrl.org
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