The curious case of the Infineon MA12070

[BobVonBob]

My family gifted my younger brother a turntable and speaker setup for Christmas, and this is the story of how that caused my spiral into madness.

(TL;DR for those who don't care: The Infineon MA12070 amp chip is incompatible with some speakers. What?!)

Upgrading from a Victrola suitcase player, we got him Polk Monitor XT20 speakers and an SMSL SA300 amplifier (and, irrelevant to the story, a U-Turn Orbit Basic turntable).

My family gets together at my maternal grandparents' house to celebrate Christmas. It turns out they've still got some of their records lying around and would like to hear them, so my brother and I get to setting the whole thing up, we drop the tonearm. Nothing. Shit. Try the Bluetooth connection on the amp. Still nothing. Shit shit. Try the turntable plugged into my grandparents' soundbar. It works! Christmas is saved and my grandparents play their old LPs for the first time in some 35 years. There's still the question of whether the problem is with the amp or the speakers. I take them both home at the end of the day since I'm the only one with another amp and speaker set to test these.

The next day I start testing them. Plug the SMSL SA300 into my home theater speakers, it works perfectly. Great, so the problem must be the Polk XT20s? Plug them into the SA300 amp like yesterday and I hear a quiet repeated popping sound that I didn't notice before, probably just too quiet to hear over people talking. Plug the speakers into my Loxjie A30 and I hear the same repeated pops. Case closed, must be the speakers. Right?

I fire off an email to Polk support, and if I were most anyone else that would be where the story ends, but I'm the curious sort, I want to know what this popping is all about. Two hours of internet searches later and I've gotten nowhere, but something I saw in passing on one of those awful AI-generated "blog" sites sticks in my head, that the popping could be the sound of protection circuitry triggering.

It's at this point I realize both of these amps use Infineon MA12070 amp chips, so I start wandering down that road. Half an hour later I stumble across Voll Audio, specifically the page for their P44 bookshelf speakers:

MA12070 Amplifier Compatibility: Unfortunately we have received reports from customers of some incompatibility with select MA12070 based amplifiers. If you already have an MA12070 based amplifier we would recommend against purchasing our P44s or emailing us first to confirm. If you are purchasing our P44s and don’t have an amplifier we recommend against any amplifier that features the MA12070 amplifier chip. Not all 12070 based amplifiers exhibit this issue and we haven’t received any reports of any other amplifier chip having issues. The Loxjie A30, SMSL AO100, SMSL SA300 and SMSL AO200 are all confirmed to exhibit this incompatibility.

What?!

I had to test this, so the next day I visit a friend, he's got a receiver with class AB amplification, and sure enough the speakers work great. It's not the amp, it's not the speakers, it's both of them together.

No idea what causes this, if you want wild speculation from an electronics hobbyist with almost no experience, perhaps the crossover in the Polk XT20s acts like a dead short for an instant when first energized, and the MA12070's protection reacts to that faster than other amplifiers, causing it to never proceed past that phase? I don't know if that's even a reasonable hypothesis, but it's all I've got. Feel free to speculate, but I probably won't have any way to test your ideas.



Hopefully someone finds this fascinating, and in light of this I'm all ears if you have suggestions for replacement speakers which will work with the SMSL SA300, or a replacement amp with Bluetooth (and ideally a remote control) which specifically does not use the MA12070.

 

[kemmler3D]

Interesting story, I'll be keen to see discussion from the EE-type folks around here ... It seems very odd that a protection circuit could be engaged with any input at all, no matter how low... I imagine it's a design flaw, speaker + amp "compatibility" is rarely like this, from what I've seen.

 

[SlowJim]

I ran into this as well with the XT20's and RSL ia255. When I called tech support for both companies they acted like I was crazy and stupid . Thanks for posting this and confirming I am neither (in this scenario at least)

 

[DVDdoug]

Strange! Or as Vizzini says, "Inconceivable!"

But class D is pretty complicated so it's probably not too hard to screw it up, even if there's nothing wrong with the actual chip. Or infineon's recommended circuit may not be ideal and some manufacturer's may have discovered the issue and found a work-around.

 

[xanalog]

Erin's review of the Polk XT20 speaker has this graph:

Magnitude never seems to dip below ~4R across the spectrum.
At least, that is off-the-table!

 

[RandomEar]

I've also had problems with an MA12070-based amp (AO200) and my specific speakers. There was a constant buzzing sound, but it did play music. The solution was to add a 22 μH choke in line with the speakers. Details here.

It would be interesting to see if a choke also solves your problem, but I guess returning the amp would be the safer solution.

 

[xanalog]

I've also had problems with an MA12070-based amp (AO200) and my specific speakers.

I run an SMSL AO200MkII, feeding a pair of Infinity Primus P153, as a part of my desktop rig.
The AO200 never gets turned off and never makes unwanted peeps or squeaks.
But @BobVonBob post did get my ears perked up!

 

[DVDdoug]

Magnitude never seems to dip below ~4R across the spectrum.

I'd guess it's something beyond the "audio spectrum" in the ultrasonic range. The MA12070 datasheet shows a switching frequency of 600MHz or more and I suspect that's where then problem lies, especially since the problems seem to show-up with little or no actual audio signal.

An inductor ("coil" or "choke") will increase the impedance at higher frequencies, resulting in less current from any switching signal leaking-out. There should already be a filter but some implementations may not have enough filtering for low-impedance or capacitive loads. (I only scanned-through the datasheet but it does "mention" 22uH as an "EMC filter")

 

[BobVonBob]

If it helps (or hurts) anyone's theories, I shoved a 2.2 ohm resistor in series with the speaker and they started playing. If they were mine I'd get a pair of 25W resistors and be done with it, but alas they aren't, and I'm not going to gift that kind of jank to my brother.

 

[AnalogSteph]

Then I bet the chokes as suggested earlier would help. You don't need an awful lot of inductance to get to 2.2 ohms well in the ultrasonic range. You could pretty much take the end of the speaker cable, wind like a dozen turns of the + conductor onto a pencil or screwdriver shaft, try it and add some more if needed, and stabilize the windings with hot glue or epoxy or something. Shorten the - conductor as needed. Repeat for the other speaker. That's assuming the cable isn't like 4 mm² or something. Find a piece of something thinner in that case, maybe 18-20 AWG.

 

[Joe Smith]

Weird. I have a SMSL AO100 amp in my dining room, and no problem whatsoever with the older KEF Coda 7 speakers in that system.

 

[Joe Smith]

So would the issues with the chip cause any harm to other speakers, even if they seem to play well without any popping or noise?

 

[Corvuscorone]

I have two SMSL AO200 amplifiers and have used them with several speakers without any problems. They have been connected to, among others, Polk R200 and Elac DBR62 speakers. I also had no problems with the SMSL A100 I previously owned. The problems you describe sound strange, although the low-power A100 amplifier seemed to distort very easily in my use.

 

[mcdn]

Infineon has a whole application note on implementation of output filters for the MA12070: https://www.infineon.com/assets/row...ilter-recommendations-applicationnotes-en.pdf

The crux of the issue is that they sell it as a "filterless" amp, making for cheap implementations using just a ferrite bead at the output. This is only recommended for very short (<60cm) speaker cables, and relatively low power outputs, and is documented in the app note to have issues with speaker designs that have low impedance at high frequencies.

For integrated solutions like active speakers it's not a problem, the designer can match things up correctly. But for MA12070 amps intended for general use the designer really must use a suitable output filter with a properly sized output inductor of 6.8uH. From Loxjie's own teardown video of the A30, nothing like that seems to be present. (http://www.loxjie-audio.com/newsview.asp?id=70)

 

[BobVonBob]

Then I bet the chokes as suggested earlier would help. You don't need an awful lot of inductance to get to 2.2 ohms well in the ultrasonic range. You could pretty much take the end of the speaker cable, wind like a dozen turns of the + conductor onto a pencil or screwdriver shaft, try it and add some more if needed, and stabilize the windings with hot glue or epoxy or something. Shorten the - conductor as needed. Repeat for the other speaker. That's assuming the cable isn't like 4 mm² or something. Find a piece of something thinner in that case, maybe 18-20 AWG.

Lmao. Absolutely floored when this worked.

 

[mhardy6647]

Lmao. Absolutely floored when this worked.

View attachment 500439

Wonder when the AudioQuest version is hittin' the market?

 

[moonlight rainbow dream]

I experienced this interaction with the Loxjie A30 and a pair of C-notes. At the time, I thought the amp was defective. I figured it out a bit later, and gifted it to somebody else with a different pair of speakers.

 

[BobVonBob]

I'm surprised how difficult it was to find information on this phenomenon given unfiltered MA12070 amps seem fairly popular, and three people with similar issues popped up just in this thread. If nothing else I've added to the pile, perhaps search engines will pick this one up and I can spare the next poor sap a headache.

 

[mcdn]

It's the top hit for "MA12070 speaker compatibility" and "MA12070 speaker problems" on Google already, thanks to ASR's insanely high ranking. If you edit the title to refer to the SA300 it'll probably do the same for "SA300 speaker problems" as well.