Recently, I was in the market for a Class-D amplifier to replace an aging model. This article reviews the very compact Fosi Audio TSA7498E 2 channel amplifier I bought from a China based business, via Amazon. tl;dr: Can't recommend, use something better.
I needed a small and efficient amplifier for connecting a pair of old 4 Ohm monitor boxes (JBL Control One) to a computer. Class-D amplifiers are known to be much more power efficient than traditional Class-AB ones. For example, I measured an old Class-AB Pioneer A-109 stereo amplifier at 33 W, during idle. That means that energy is wasted in the form of heat, which might be especially annoying in the summer. Whereas a comparable Class-D amplifier just consumes a few watts, and likely completely shuts down during idle operation.
Unfortunately, Class-D amplifiers often aren't clearly marketed as such and thus kind of hard to select for when browsing Hi-Fi vendor's product pages. Also, some Hi-Fi vendors just use Class-D for very large amplifiers, where Class-AB simply gets impractical and only have Class-AB for living room amplifiers.
The best bet is to look at the power consumption ratings, if available.
When searching for Class-D amplifiers on Amazon the results are flooded with Chinese products, i.e. distributed by some no-name small companies, which probably are renamed often, whose wares are shipped from Amazon warehouses, without much quality control. Judging from the product images, many products from different vendors look quite similar to each other, i.e. it seems different vendors are just reselling wares from the same manufacturer.
I bought the Fosi Audio TDA7498E 2 channel amplifier because its design is minimal and compact and it isn't too expensive. Also its reviews are quite positive, however nowadays Amazon is also flooded with paid and low-quality reviews, so this has to be taken with a grain of salt.
The Fosi Audio TDA7498E indeed is very compact and has a low power consumption. On idle I measured 0 W or so and during real use I never measured above 1 W power.
The (mostly) Good¶
Except for the volume knob, the design is nice, i.e. it comes in a full metal case and with dual-use speaker connectors, i.e. one can connect speakers with banana plugs or raw cables. However, the volume knob isn't marked and way to easy to turn, i.e. it's very easy to turn it by accident to some high level. This is especially problematic because the useful range for living room volume levels is tiny, i.e. a few millimeter turn already leads to a high volume. Last, it comes with a real power switch.
At least when connecting 4 Ohm speakers, the amplifier creates a too high amount of background noise. Probably all amplifier speaker combinations produce some background noise, but usually it's only audible if you place one ear just a few centimeters from the speaker (while playing silence and turning the volume knob up). With the Fosi Audio TDA7498E amplifier the background noise is still audible in a few meters distance, which makes it unsuitable for office usage.
There is a channel imbalance at living room volumes, i.e. below 30 dB or so, of plus 6 dB on the left, when using audio sources with standard line-out levels. That means one either has to compensate that amount by adjusting the line-out balance settings of the connected audio source (if even possible). Alternatively, one might reduce the audio source's line-out level to 72 % or so and turn up Fosi Audio TDA7498E's volume accordingly.
Both workarounds are ridiculous, i.e. one simply doesn't artificially decrease the quality of the input signal to work around channel imbalance in the amplifier.
Last, the speaker polarity on both channels is reversed, i.e. black is positive and red is negative. Granted, it would be worse if just one channel was reversed (cf. phase shift), but still, this speaks volumes regarding the quality of the product.
My hearing is better than I thought. That means I immediately noticed the channel imbalance and background noise. Furthermore, the compensation levels and range (where the imbalance occurs) I subjectively determined by changing mixer knobs until I didn't notice any imbalance anymore, first while listening to normal music and then to pink noise, matched a later verification with a sound-level meter pretty well.
The alsa-utils package contains the
speaker-test command which
is quite convenient for testing speakers. Example:
FWIW, I also was able to detect a defect speaker with speaker-test, where its deficiencies were more obvious.
In case no dedicated sound-level meter is available, depending on your smartphone, a sound-level measuring app might be good enough for speaker balance testing (see also). FWIW, I tested the Google Pixel 6a with the 'Noise Meter' Android app and observed reasonable performance. However, as of the end of 2022, the 'Noise Meter' app seems to have vanished from the Google play store, as well as the company behind it (JINASYS) seems to have vanished from the internet.
As mentioned previously, the Amazon Reviews for the Fosi Audio TDA7498E amplifier are quite positive, i.e. as of 2022-12-31 it has an average rating of 4.3 on a five star scale with 63 % 5 star and 19 % 4 star ratings.
Especially in the last years, Amazon is plagued more and more with fake reviews and reportedly doesn't police enough against them. Thus, some of those positive reviews might be fake, either generated by some 'AI' bots or by some incentivised persons.
However, many of the positive Amazon reviews may very well still be genuine. This can be explained by various factors such as:
- The seller sells various batches and revisions, also perhaps sourced quite heterogeneously, via the same product page.
- Some reviewers simply have kind of bad hearing.
- Some reviewers are simply used to listening at a frankly too high volume where the channels aren't imbalanced. Either some of this group already have bad hearing or they are on a good way to damage their hearing capabilities.
- Some reviewers are biased, e.g. in order to reduce their cognitive dissonance, i.e. to avoid recognizing having made a bad purchase (which would require some effort to get a refund and get a replacement). Or due to some kind of groupthink, where a reviewer already bought into the narrative of some enthusiastic reviews (because of a good story such as: simple underdog product is as good or better than one from an established Hi-Fi company and you have discovered and saved a good penny on it).
Mini or micro stereo systems are an alternative to a dedicated amplifier, because they are widely available and tested by reputable independent entities. For example, in Germany there is the independent Stiftung Warentest consumer organisation which tests this product group periodically. Their last tests include power ratings, besides the usual feature comparisons, such that selecting systems with Class-D amplifiers (and other required features) is much simpler than having to research all such details on ones own.
I settled on an ultra compact stereo system from an established vendor: the Panasonic SA-PMX94 (a.k.a. SC-PMX94 or SC-PMX94EG-S or SC-PMX94EG-K). It comes with a pair of 3 Ohm speakers which are quite decent.
It might be even possible to connect third-party speakers to that system, possibly even 4 Ohm ones, as the conventional wisdom is that speakers that exceed the amplifiers impedance rating (within reason) should work while connecting lower Ohm speakers is considered dangerous, as they might draw too much power such that the amplifier operates outside of its safe operating area and might get damaged. However, I'm not an electronics engineer and haven't tested it, so YMMV.
That stereo system comes with some features I don't really need, but are still kind of nice to have, e.g. a DAB+ receiver and integrated CD player.
The best thing about it, besides good sound quality (i.e. no excessive background noise and balanced channels!) and a reasonable volume knob, is its connectivity. That means besides Bluetooth, optical S/PDIF (TOSLINK), standard analog-in (Cinch/RCA) it also integrates a standard USB sound device!
In that way it is able to also replace an existing USB sound card and possibly a separate external DAC device. That means one can simply connect the stereo system via USB to a computer and it should be detected like an external USB sound card. I'm using it under Linux where it worked out of the box without any extra drivers.