SMSL A100 class D amplifier test

[pma]

To me, it is enough that internet DSL modem stops working if the amp is on and with speakers connected by 3m wires. It is also enough to see that the amp manufacturer did not follow Infineon recommendations on output filtering vs. speaker wire length. It is also enough to hear FM radio interferences. It is enough to see that the added LC filter fixes the problem. The filter made according to Infineon datasheets. I do not have instruments to measure according to FCC. But I want the amp to be compatible with other devices. Show me the protocol that it passed FCC test with 3m speaker wires as is, filterless, without additional filters. Not the protocol with 40 cm speaker wires.
BTW, I measure behind the speaker cable. So, the mere cable inductance does not help.

 

[pma]

I see measurements connected to outputs. Where is the single loop antenna at 1, 3, and 10m distance while amp is connected to speakers to support claims it emits compliance-breaking levels of radio power? It is useless to speculate on CFR Title 47 Chapter 1 Subchapter A Part 15 compliance if you do not measure it in the manner specified by the FCC. Which is at 3m, non-contact, single loop antenna.

I do not have instrumentation to provide valid FCC EMI test. However, I have prepared a test with a single-loop antenna with 25cm diameter, placed 1m from the resistive load. Same speaker cable of 3m length, once used "as is" connected to SMSL-A100 output, then with the LC filter at the amp output (2x33uH, 2x470nF). Below the measurements of loop induced voltage. From the frequency and amplitude it is easy to calculate B and H of the EM field. The amp was working at idle.

1. Filterless operation as is normal with SMSL-A100

2. With LC filter

3. Maths

From firsthand experience with wifi and MA12070 (I still run 2x PA3s as biamp for stereo speakers)... signal strength never changes. And since my teardowns of the PA3s PCBs, I have added a wifi video doorbell (Reolink) which has both amps and speakers flanking the transmission path to either side.

Now, you are speaking about PA3s, I do speak about SMSL-A100. Do they have same circuit schematics? Especially output circuit? Do they share same modulation scheme of MA12070? I do not know. And, the amp operation (without added LC and with long speaker wires) paralyzes my ZXHN H267A VDSL modem/router. We probably speak about two absolutely different configurations, right?

 

[MaxwellsEq]

Bench tests with dummy loads often use short cables, so it's possible that this unacceptable behaviour doesn't get spotted. Perhaps bench tests should always use a realistic length for speaker cables, e.g. 3m of 12AWG.

 

[Mike123]

I bought this amplifier for a non-standard application. It had to have a single-polar power supply and minimum consumption/maximum efficiency. In view of the peculiarities of living with a long power outage, the amplifier is must be powered by a battery and should work from this battery as long as possible, but with the highest available quality. The MA12070 chip has a very original design and suits these requirements best of all. Unfortunately, not bad Chinese companies like Topping or SMSL often release raw products. Therefore, the amplifier has undergone the following upgrades:

1. The amplifier has too much gain. Since the signal source for the amplifier is Topping DX3PRO with an output voltage of 2V, the gain of the input buffer was reduced. I do not remember what the gain was initially, but not less than 10, and maybe even 100. The gain was reduced to 1. This gave an acceptable volume level and reduced noise and distortion.

2. By default, the MA12070 microcircuit starts up in the most economical, but the lowest-quality power profile 0. But the SMSL company did not consider it necessary to install a microcontroller to change these modes by the user. Therefore, a board with a microcontroller was soldered to the I2C microcircuit, which switches the amplifier to the highest quality power profile 2. This eliminated high-frequency interference and reduced distortion with increasing frequency. The graph shows that the THD does not exceed 0.1% at a frequency of 10 kHz. Moreover, this is on a 4 Ohm resistor load, which is quite heavy for the MA12070. On real acoustics with a nominal resistance of 6 Ohm, everything is even better, despite the presence of inductances and capacitors.

3. At the amplifier output, 10 μH / 1 μF LC filters were installed to eliminate high-frequency interference.

4. A 24 V DC power connector was installed.

As a result, the amplifier became suitable for voicing a home PC and voicing a smartphone via Bluetooth DX3Pro. Its own Bluetooth and USB input are very poorly made and are no good.

I don't regret buying this amplifier. After all, SMSL is a fairly high-quality Chinese brand. The case is quite good, the microcircuit is pressed to the aluminum bottom. The layout of the PCB is also done quite professionally. Well, I know how to eliminate the shortcomings and it is not difficult for me .