[RFI] Speaking of audio amplifiers... an interesting case

[Bert]

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
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