Rehearsals Refined The Fosi Audio BT20A Pro Amplifier

Manufacturer: Fosi Audio Rehearsals Refined The Fosi Audio BT20A Pro Amplifier

Fosi Audio reached out to us again, with an offer to review their new BT20A Pro amplifierAfter the pleasant surprise of the TB10D amplifier there was no question we would pass up a chance to review another model.   This amp is an upgrade to the already available BT20A; don’t let that price fool you when it comes to quality.

The exterior of the BT20A Pro may look almost the exact same as the original TB10D model but the insides are very different.   It is a two channel amp which now incorporates Bluetooth 5.0, a fully functional 3.5mm pre-out port and just like it’s predecessors it can supply 300W to both channels.  There are some other interesting inclusions to check out as well, though not if you are looking for Dolby or aptX support.

Product Specifications
  • Model Number: TB10D TPA3255
  • Compatibility: Banana plug or bare wire
  • Output Power:
    • 300 Watts x2@4Ω
    • Terminating Impedance: 2Ω – 8Ω
  • Input Modes:
    • RCA
    • Bluetooth 5.0 – transmission distance up to 50′
  • Output Modes:
    • Passive Speakers
    • 3.5mm PRE-OUT
  • Audio Specifications:
    • Frequency Range: 20Hz – 20kHz (±1.2dB)
    • THD: 0.005%
    • SNR: ≥108dB
    • Signal: Analog
  • Internal Hardware:
    • TPA3255 Class-D
    • Four NE5532 Op-Amps, two of which are replaceable
  • Dimensions: 150 x 110 x 40 mm / 5.9 x 4.3 x 1.6 inches
  • Overall Weight: 1.05kg
  • Power: 32V 5A External Power Supply

USD list, use code 20APROMKT20 for 20% off for the rest of the month.  

Manufacturer Description
“Fosi Audio BT20A PRO is an upgraded Bluetooth 5.0 HiFi stereo amplifier with PRE-OUT port, which can be used as a preamplifier. The BT20A PRO supports RCA/Bluetooth input, suitable for most passive speakers as well as active ones. It adopts TI TPA3255 high-performance Class-D power amplifier chip enabling true premium sound quality and allows ultra low distortion across the audio band.”

Putting The Pro Into A Novel Amplifier

We first learned about Fosi, a Chinese based company who specialize in “Hi-Fi products, DIY kits and DIY guidance for audiophiles” when they sent us the original amp for review. They offer a variety of products at decent prices and if the quality of the TB10D is anything to go off of, they are higher quality than you might expect.  The BT20A Pro is an upgraded version of their base BT20A, with improvements to both total harmonic distortion and signal to noise ratio.  It is also rocking a copper volume knob which is a little detail, but quite aesthetically pleasing.

The amp is roughly the size of a normal paperback book, though it certainly weighs a little more.  It will fit in just about any space you have available and the combined weight of the power brick and amp is still light enough you could carry this with you.  The speakers on the other hand could be a problem, so get someone else to bring those!  It can handle a basic two speaker setup, straight through to the unholy mix of five pairs of 8-Ω speakers it is currently powering here.

Inside The BT20A Pro

The antenna is the most obvious upgrade to this model compared to the original TB10D amplifier, though it is not new to the non-Pro BT20A.  The pairing is as easy and reliable as you would expect and there is visual confirmation of pairing via the power light which switches from red to blue.  The 50′ mentioned in the specifications is not an understatement as the BT20A Pro could not only handle a line of sight signal of that distance it maintained a connection with my upstairs neighbours phone after he left.  It was quite a surprise to hear the music his phone was streaming when the amp was powered back on to do more testing.   Thankfully you can reset all wireless connections by simply depressing the volume knob for five seconds.

Inside the BT20A Pro you will find a Texas Instruments TPA3255 Class-D amplifier chip and four TI NE5532 Op-Amp chips.  While two of the four chips are soldered onto the board, the other two can be replaced.  Fosi lists the compatible replacements, which include the LME49720HA, MUSES02 120-140, OPA26041P, OPA2134PA 25-35, LM4562 and NJM4556AD.  That gives you a huge variety of customization, if you so choose.

In addition to the unpowered analog output to your speakers, Fosi also included a proper 3.5mm PRE-OUT on the BT20A Pro.  You can use it to connect a powered subwoofer or speaker but you can also use it as a preamp and attach it to a power amplifier.  That lets you use the volume, treble and bass to control the audio, passing on all the work to the power amp.  With 300W available already you may never need to make use of it.  Seeing it on an amp of this cost is a very nice bonus.

The other internals include Sumida advanced high-power inductors, an Omron relay, and two high-quality 50V, 2200uF electrolytic capacitors.

Hitting The High Notes

Fosi presented themselves with a bit of a challenge, as the BT20A Pro needs to beat the already impressive TB10D.  Once again Fosi proved surprisingly capable at providing HiFi performance at a LoFi cost.   It was immediately noticeable that the left and right channels had far superior separation, and not just by a little.  The surround effect was far more immersive than on the TB10D during movies and music.

In addition to the separation improvements, the audio picked up nuances the original amp did not and the overall sound was fuller.  One might scoff at having only treble and bass knobs; that same person may also have a smiley face curve on their equalizer.  The adjustments these make to your audio have also been vastly improved, to the point you go from almost eliminating the highs or lows completely to bringing them well out in front.   The potentiometers behind the knobs have a true midpoint as well, if you are seeking a neutral sound stage.

The lovely copper volume knob should be used carefully!  The BT20A Pro’s 300W rating is not kidding around, and fully capable of blowing out speakers so make sure you don’t crank it up to 50% and then power up the amp.  It could well be the start of a bad day for your stereo equipment and your eardrums.   At high volumes the SNR remains very low and no distortion is noticeable, at least not before the speakers used to test started to complain!

Cue The Dénouement

Audio reviews are far more qualitative than most of the ones you will find on PCPer, as audio preferences are at least as personal as cuisine.  There was nothing tested which the Fosi BT20A Pro did poorly at, and the improvement in quality and features compared to the original amp they sent was every bit of a pleasant surprise as discovering them for the first time.  However, those seeking a 7.1 surround experience or who consider low latency Bluetooth a necessity and not a nice extra may feel differently.  You simply aren’t going to get those features for less than $100, instead you get two very well designed channels and superb bass and treble control.

The quality of audio input does certainly have an effect,  low quality YouTube audio will sound significantly different than a master track played from Tidal or a properly mastered movie.  The Fosi picks up the nuances of your source, for good or bad, and makes them quite audible.  You won’t have any complaints about the levels, no matter the source.

The impressive SNR and THD of the BT20A have had an interesting side effect, they are now used to do a proper level check on musical compositions.   My upstairs neighbour, also featured in the previous review, now prefers to check the levels of his music on the Fosi as opposed to the monitors and professional amp he has upstairs.  If that doesn’t give you an idea of the level of quality Fosi offers you for the price then not much will.  The fact that Fosi has given a 20% off code for readers of this review for the rest of the month, 20APROMKT20, means you can try out the BT20A Pro for $80.  

The idea of a low cost, high quality two channel amp which is small enough to be easily portable may seem odd but it does make a lot of sense, especially for homes that don’t have the space for a large amp.  The inclusion of Bluetooth 5.0 makes it even more useful as you can avoid a lot of extra wiring, or hook it up outside to enjoy music with your barbeque.  Set up and tear down is a laugh, and won’t take you any time at all.

We would definitely like to thank Fosi for sending the BT20A over for testing, and for proving that they are still finding ways to improve their products without raising the price as so many others are doing these days.


Review Disclosures

This is what we consider the responsible disclosure of our review policies and procedures.

How Product Was Obtained

The product is on loan from Fosi Audio for the purpose of this review.

What Happens To Product After Review

The product remains the property of Fosi Audio but is on extended loan for future testing and product comparisons.

Company Involvement

Fosi Audio had no control over the content of the review and was not consulted prior to publication.

PC Perspective Compensation

Neither PC Perspective nor any of its staff were paid or compensated in any way by Fosi Audio for this review.

Advertising Disclosure

Fosi Audio has not purchased advertising at PC Perspective during the past twelve months.

Affiliate Links

This article contains affiliate links to online retailers. PC Perspective may receive compensation for purchases through those links.

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About The Author

Jeremy Hellstrom

Call it,, or PC Perspective, Jeremy has been hanging out and then working with the gang here for years. Apart from the front page you might find him on the BOINC Forums or possibly the Fraggin' Frogs if he has the time.


  1. Bob

    Getting 2x 300W from a 160W power supply is pretty impressive! This amp’s ~400% efficient?

    • Jeremy Hellstrom

      The chip –

      I could have gone into the whole spiel about upgrading the power supply and peak power versus average continuous power but I really didn’t think it would be that interesting to read for most and that those that know amps would already be familiar with what the rating meant.

      • Operandi

        I really wish FOSI and others didn’t even quote those power specs. Aside from the fact its impossible to achieve those numbers and 300 watts per channel would never be usable not matter what it makes them look like the car audio section at Walmart which is too bad cause they are legit good amps.

        • Jeremy Hellstrom

          That’s pretty much how the entire industry runs, be it stage amps or home audio models. Whatever wattage you see will be about twice what you can actually get, if you can feed it enough power. A power amp connected via the pre-out could certainly change things. The original TB10D lists 32V 6A and 28V 8A power supplies as working, which gets you just short of 200W. If you wanted to push things, you could try a higher wattage power supply to see what happens.

          • Operandi

            Its not though. Only cheap shit plays the sketchy fantasy wattage numbers game. Any decent class AB amp will hit or get within 10% of their specs, and or even sometimes exceed them. The same goes for bette class D moduels from the likes of IcePower or Hypex.

            Point being Fosi is good enough as is; they don’t need to be making shit up.

            • Sebastian Peak

              I agree that it’s a bad idea to quote maximum power as the nominal number. I don’t know of any major Hi-Fi amplifier brand that does that. It is interesting to see exactly what momentary power is, as dynamic sound comes from the ability to handle transients, and 300W, even if it’s just for a second, can make all the difference. A tiny amp like that driving speakers at even 75W per channel with low distortion would be impressive enough.
              Not having heard the TI chip I won’t claim that Hypex or ICEpower are “better”. I have read about the superiority of Hypex over ICEpower, for example, and ICEpower modules can be found in my dual-mono Calyx CTI integrated, which sounds fantastic – but the implementation matters as much as the chip.

              • Operandi

                Its only worse in this case because Fosi is quoting what the chip is capable of, not what they are selling you as a product.

                I mean I have no doubt its perfectly up to powering anything a similar class AB with similar (real world) wattage would so its fine, its just annoying marketing that they don’t need to be doing.

                IcePower should be better, its in another class entirely but I can’t claim that personally either. I may buy one of these just to see and compare it to my DIY IcePower amp and give the Fosi to a good home. Cheap enough experiment to see what it can do.

  2. Dvon E,

    Thank you, Jeremy, for providing a USEFUL review and commentary on the live stream. If Amazon doesn’t screw up I will own one of these small miracles before noon tomorrow, with free Prime shipping and an additional $8.00 off of the already discounted $79.99 (no-coupon) price. I’m stunned; it would have been great at the full price!

    Guys, click those links! This Canadian knows the best deals.

    • Dvon E.

      Aaand, I screwed up. Clicked the wrong line in the review, didn’t notice the cute copper knob was missing and ordered the 100W version. That’s actually enough for me, but I’m trying to cancel and get the better one now. Most of the previous comment still applies.

      • Dvon E.

        Well, Mr. H, your message appears to have been effective. Amazon is sold out of the sexy version until May. I’m now expecting delivery on Star Wars day. Such is life.

        • Sebastian Peak

          He did go on a bit about the knob, knob feel, the appearance of the knob, and various other knob-related content until all I could think about was Jeremy playing with his knob.


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