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Class D mini amps

Posted by madisonears (A) on November 19, 2018 at 19:40:17

Just went shopping after my 15-year-old LM3886 chip amp quit in the kitchen system. I want small, only medium power, decent sound, reliability would be nice; nothing ultimate.

The array of suppliers and units is bewildering at first, but can be sorted into a half-dozen or so types of amp based on the chips they employ. Tripath is out of business, but many "T" amps are still available, built either with NOS chips or counterfeits. Then there are several major TI Class D chips, with new versions being released every few years. Another recent player is STMicroelectronics with their TDA series. Each family of chips is available with different amounts of power output. Some of the claims are quite extraordinary for amps you can hold in the palm of your hand.

In fact, it is mostly bullshit, and they all do it. If you are interested in any of these amps, you must not rely on the seller's performance data. They cherry pick the best case numbers based upon a very limited area of performance. The chipmakers are complicit in this, as they provide specifications derived from testing conditions that do not typically occur in real world applications. One must dig a bit deeper, and understand a bit of Class D architecture, before you can realize what is truly going on with these amps. Carefully READ THE DATA SHEETS of the chips themselves, closely examine the graphics. It is worth your time if you want to make an informed buying decision. The seller's specs are almost meaningless.

There are reasons that the same chip with even slightly different implementations will have different sound, especially when attached to a vast population of different speakers. They are extremely sensitive to the load presented, even with the patchy output filters typically employed. No speaker provides the type of load under which these chips are measured, and none of these small amps will actually perform to spec.

Those who believe Class D amplification, at least as employed in these tiny amps, can equal the performance of equipment with beefy power supplies and heavy duty output devices, are fooling themselves. The typical review of these amps usually focuses on the power output, which is the primary criterion by which they differ from each other. But all are outmatched by any decent AB amp, and there are reasons why that can't be overcome.

The design philosophy around these chips is to maximize efficiency and minimize real estate. Miniaturization and low power consumption are the real objectives, and if audio quality comes along for the ride, well, okay. But it usually doesn't, and an examination of their REAL performance specifications will readily reveal that.

After a full 12 hours of research spread over a couple days, I ended up purchasing an Indeed TDA7498E MkII on eBay. From the garbled description and a few pics of the guts, it seems to be the best implementation of the latest TI Class D chip marvel. It costs about twice as much as the same chip in the cheapest amp, but there are reasons for that, and it might be worth it. Even this design conceals a basic flaw that probably means it will not meet spec, but I might be able to rectify that myself. I contacted the builder and he cannot, or will not. I do not harbor any delusions that it will meet the published specs in a real world application, but it might sound better than the other available versions.

When I have more time, I might try to build my own miniature LM3886 amp (most available are bigger and more expensive than I want). THAT is a chip with superb specifications based on real world applications, and I know it sounds excellent if decently implemented.

I'll report here on the Indeed after it arrives in a few long weeks.

Peace,
Tom E


berate is 8 and benign is 9

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