SMSL A100 class D amplifier test

[pma]

SMSL A100 amplifier review

This test review was performed to comply with FTC: 16 CFR Part 432 (July 5, 2024) - Power Output Claims for Amplifiers Utilized in Home Entertainment Products
The SMSL A100 is a small, affordable, 2-channel Class D amplifier. It embeds an Infeon MA-12070 amplifier chip and it has a built-in power supply (AC 100V-240V input), which is a nice feature. The inputs are in stereo RCA, USB-A and Bluetooth. I bought it from Amazon.de at price of 80.37 EUR with free shipping, so it is a bargain.

The amplifier works in the BTL mode and on any of the speaker binding posts (+ or -) there is +12.25Vdc against the signal common, which is at the body of RCA connectors. So be careful when measuring the amp, use well rated coupling capacitors and differential input mode of the ADC. Another note - the amplifier is a class I instrument and its signal common is connected to the PE (middle pin of the mains plug). This may worsen S/N when connected to the class I signal source like class I DAC.

1. Manufacturer's specifications
Manufacturer's specifications are listed below

Frankly speaking, the specified output power seems quite high related to the chip used, so I rather attach specifications from the MA-12070 datasheet. The SMSL A100 implementation cannot have better power rating than the chip used, itself.

MA-12070 datasheet specifications

This is something I might believe so I will consider the continuous rated power to be 2 x 30W/8ohm/1kHz.

2. Test conditions
The amplifier was tested with the built-in power supply, 230Vac, and 8ohm load in both L and R channel. Testing with 4ohm load will be done later just as an orientation measurement, as the rated power to 4ohm is unknown.

3. Pre-conditioning
First of all, the amplifier was pre-conditioned at about 1/8 of the rated power, 2 x 4.2W/8ohm, for one hour, as requested by the FCT document. At the end of the pre-conditioning period, the amplifier case was only slightly warmer than in the beginning. The distortion in the beginning and in the end of the pre-conditioning was unchanged.

4. Frequency response
Frequency response was measured at 5W output power into 8ohm. The amplifier has filterless design, without output LC filter, so the frequency response is independent of load impedance.

5. Power frequency response measured at 27W/8ohm, to 40kHz

Power frequency response was estimated as 20Hz - 20kHz. Behaviour above 20kHz is a bit erratic.

6. Distortion was measured at volume pot set to maximum, G = 40dB

6.1. Distortion at 5W/8ohm

6.2. Distortion at 21W/8ohm

6.3. THD vs. stepped output voltage into 8ohm load at 1kHz with measurement bandwidth 45kHz and measuring bursts of 0.8s length and 0.5s intervals

Maximum power is 37W/1kHz/8ohm with THD = 10.5%

The amplifier meets the specs at 30W/1kHz/8ohm with THD<1%

6.4. THD vs. frequency at 5W/8ohm

There is a steep rise of THD above 1.5kHz and THD > 1% above 13kHz with measuring BW45kHz.

6.5. 5 minutes test at rated (derated) power 2 x 27W/8ohm/1kHz

The amplifier has passed 5 minute test at 27W/8ohm/1kHz, being only mildly warm at the end of the test.

Further increasing of power resulted in distortion > 1% in the 5 minute test. But not to the shut down.

7. Conclusion
The amplifier is a mixture of good and bad points. Built quality is quite good, concerning the bargain price. It did not get hot during tests and never shut down itself. Frequency response does not have the infamous dependence on load impedance and response peaking above 10kHz, as it is a filterless design. However, this brings troubles in the field of electromagnetic interference. My wifi staying near was almost paralysed. The DAC feeding the amp with test signals was unable to work without using USB-ISO isolator. For the 5 minute rated power test I had to use analog generator, as the DAC was freezing at amplifier high power due to emitted interference.
Manufacturer's specification of power is vague and unusable. So I rather used the specs from MA-12070 chip datasheet, that declares rated continuous power 30W/8ohm/1kHz per channel. This was almost fulfilled in the 5 minute test, the amplifier was able to do it at 27W with THD << 1%.

8. Listening
I have connected the SMSL A100 with my Topping DX5 (class I) which gets the signal from S/PDIF coax, and to my surprise, there is no issue with hum, buzz or whistles. The sound is “normal”, nothing to complain. We shall see.

Pavel Macura 12/6/2024

 

[pma]

Interference - it is a kind of issue. When my small AM radio is put near the speaker cable, it can demodulate and play the same music that goes from speakers. Tuned somewhere between 600 - 700 kHz . “FCC compliant”.

 

[JustJones]

I don't get these cheap amps. Hope they make someone happy.

 

[Zek]

It embeds an Infeon MA-12070 amplifier chip

It would be interesting to test an amplifier with a slightly "stronger" Merus MA5332 chip.

 

[Ropeburn]

Interference - it is a kind of issue. When my small AM radio is put near the speaker cable, it can demodulate and play the same music that goes from speakers. Tuned somewhere between 600 - 700 kHz . “FCC compliant”.

Quite a cheap AM transmitter!

 

[restorer-john]

The HF distortion at just 5W is intolerable and would likely be audible, letalone at higher powers.

I appreciate the time spent doing this, but seriously, this LoFi cheap stuff doesn't deserve the effort.

 

[kyuu]

On the contrary, I think this "LoFi" stuff is the only stuff worth testing. We don't need to know if a huge, overbuilt amplifier that costs several thousand dollars can pass the testing. It probably will, but the vast majority of people aren't looking to spend that kind of money on an amp. And I don't think gatekeeping this hobby to only those that can afford to blow large wads of cash is a useful position.

Even though I question the usefulness of the FTC spec for actual use, I do appreciate you taking the time to do this testing and sharing the results with us @pma. I'm curious at what radius from the amp you had issues with RF noise, if you happened to note that? Would isolating the amp a few feet from DACs and any equipment utilizing wifi be enough, or is everything in the room kind of hosed?

 

[restorer-john]

On the contrary, I think this "LoFi" stuff is the only stuff worth testing.

Was anyone testing portable radio cassette players, transistor radios, walkmans or clock radios in the 1980s? Nope. That's what this stuff is. It's disposable consumer rubbish masquerading as HiFi. It's not. It's not even close. The output amplifier stages in most car audio stock head units will outperform this easily.

I could pull out a dozen random 1970s BOTL ancient integrated amplifiers that will have lower distortion, higher power, actual features and functionality than this.

@pma Do you want to do an ITU IMD test on this SMSL, both at 5W and 21W?

 

[pma]

I can do it. But please remember that power of the composite twin-tone with 2 components of the same amplitude is half of that of the sine wave of the same peak amplitude as the composite signal has. So, at 2.5W and 10.5W. 21W CCIF would send the amp to clipping.

 

[Sir Sanders Zingmore]

There is a steep rise of THD above 1.5kHz and THD > 1% above 13kHz with measuring BW45kHz.

If THD > 1% for 13kHz at 5w doesn't that mean the amplifier can't be rated at more powerful than 5w?
I clearly don't understand the FTC rule (perhaps someone can explain to me what I've missed).

 

[pma]

Yes, with respect to audio band distortion. But they (manufacturers and reviewers) will measure it with 20kHz BW and filters and then the amp meets the requirement, though 20kHz BW makes no sense for THD and THD+N measurements above 10kHz. Again, no simple answers. I cannot recommend this amp, though it behaves quite well regarding long term power and thermal management. The EMI issue of the filterless design is too strong.

MA12070 datasheet, to explain effect of limited measuring BW, see the distortion plots. So usual.

https://www.infineon.com/dgdl/Infineon-MA12070-DS-v01_00-EN.pdf?fileId=5546d46264a8de7e0164b750002861a5

 

[Sir Sanders Zingmore]

Yes, with respect to audio band distortion. But they (manufacturers and reviewers) will measure it with 20kHz BW and filters and then the amp meets the requirement, though 20kHz BW makes no sense for THD and THD+N measurements above 10kHz. Again, no simple answers. I cannot recommend this amp, though it behaves quite well regarding long term power and thermal management. The EMI issue of the filterless design is too strong.

MA12070 datasheet, to explain effect of limited measuring BW, see the distortion plots. So usual.

https://www.infineon.com/dgdl/Infineon-MA12070-DS-v01_00-EN.pdf?fileId=5546d46264a8de7e0164b750002861a5

Thank you, that makes sense.
I have very little technical knowledge but really appreciate the work you are doing and the information you are sharing with us. Thank you!

 

[Joe Smith]

I don't get these cheap amps. Hope they make someone happy.

They do! They work just fine!

 

[Joe Smith]

I have a similar SMSL amp, the AO100, which I believe is discontinued. A nice performing unit for non-critical listening. Has a single input + bluetooth. Nice digital volume chip and remote. I don't use for louder or critial listening, but for a "dining room" system, these can be very useful little amps with a minimal footprint. The SMSL amps benefit from having internal power rather than relying on separate power bricks like Fosi or Aiyima.

 

[pma]

Even though I question the usefulness of the FTC spec for actual use, I do appreciate you taking the time to do this testing and sharing the results with us @pma. I'm curious at what radius from the amp you had issues with RF noise, if you happened to note that? Would isolating the amp a few feet from DACs and any equipment utilizing wifi be enough, or is everything in the room kind of hosed?

The distance helps a lot, of course. As the amp works 1m from the Wi-Fi router during tests, it affected everything going through the Wi-Fi, like internet for mobile phones and internet radio. Several meters of distance fixed the issue. For the DAC, it is OK when it is supplied by signal from S/PDIF either optic fiber or coax. The USB is always an issue, only reduced with the USB isolator and unusable without it. I have not tried yet the direct connection of the amp with PC through the USB A-A cable, but I will.
I am just curious, that is why I bought this amp and why I am testing it.

 

[pma]

@pma Do you want to do an ITU IMD test on this SMSL, both at 5W and 21W?

I can do it. But please remember that power of the composite twin-tone with 2 components of the same amplitude is half of that of the sine wave of the same peak amplitude as the composite signal has. So, at 2.5W and 10.5W. 21W CCIF would send the amp to clipping.

It is probably much much better than you would have expected. Why? Because all the mess is above 20kHz with elevated noise floor. Below the plots in W and dBc at 10.34W.

 

[pma]

The HF distortion at just 5W is intolerable and would likely be audible, letalone at higher powers.

This may be questionable again, because the HF distortion components are all only above 20kHz. Below is the measurement of THD vs. frequency at rated power 27W/8ohm with measuring BW 24kHz. The distortion is gone and for 20Hz-12kHz the THD<1%. BTW, have you tried too hear a difference between level matched 5kHz sine and 5kHz square? Try it, it is more than interesting. It must be the low distortion source, of course. All we get is an ultrasonic distortion from that A100. 19+20kHz in the audio band is OK. It is a typical class D, of course worse than the good ones, but sonically there are no issues. The main issue is the EMI. @kyuu might be interested in this plot as well and I will add it to the review. We have to be fair.

 

[pma]

Now, this is the reason why one should really worry! And why my wifi and USB collapsed.

Below is the scope plot of the output voltage with the 10kHz sine and also the HF spectrum of this signal.

@amirm , do not you think you should include such measurements with the cheap class D boxes?

WARNING! This amplifier is a source of considerable EMI interference emitted from speaker cables!

 

[Vladimir Filevski]

it is a filterless design. However, this brings troubles in the field of electromagnetic interference. My wifi staying near was almost paralysed.

This is a major shortcoming.
I thought filterless class D amplifier usage was limited to small bluetooth portable loudspeakers, where cable length from amplifier to speaker is limited to a inch or two. Conventional stand-alone hi-fi class D amplifier should incorporate low-pass filter at the output.
But, as I know, substantial number of the low cost class D amplifiers do have low-pass filter at the output. Can you please test such class D amplifier, with respect to electromagnetic interference?

 

[restorer-john]

The IMD is much better than I expected for sure. Thank you for testing it, Pavel.

You are using your custom LPF up front of the ADC I presume? I don't use one myself (with my QA-403), as most of the gear I restore, repair, rebuild is traditional Class A/AB with no weird surprises out of band.