Bert,
Not really even 3 feet of wire failed muster.
With a resistor as a load it did fine.
Which was how I originally tested it.
Another interesting fact was you could "tune" where you wanted the harmonics to
be the strongest by varying the length of the speaker/power supply leads.
73,
Paul Cianciolo, W1VLF
ARRL EMC Engineer
ARRL - The national association for Amateur Radio™
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
-----Original Message-----
From: RFI <rfi-bounces+pcianciolo=arrl.org@contesting.com> On Behalf Of Bert
Sent: Monday, October 5, 2020 9:57 AM
To: rfi@contesting.com
Subject: Re: [RFI] Speaking of audio amplifiers... an interesting case
Hi all,
Just an observation - it says in the claim text
"including long speaker wires up to 125 cm"
which is the same as 4 ft. In the test 10 ft of wire was used. Maybe
that makes a difference? Bert VE3NR
On 2020-10-05 9:44, Cianciolo, Paul, W1VLF wrote:
> 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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