Fosi Audio TB10D Mini Class D Amplifier Reviewed

Have you ever typed “amplifier” in to Amazon or eBay? Do so, and you’ll find a plethora of miniature amplifiers, many with outrageous power output claims and with various features. Some give you Bluetooth, SD card and USB playback and even an FM radio. Some have an output for a passive subwoofer (good luck finding one of those) while some are as basic as it gets, with a single input, minimal controls (including discrete bass and treble controls if you’re lucky) and a tiny enclosure. They all look very similar – and that’s because, in essence, they are. Most are nothing more than a generic amplifier board fitted into a project box and packaged with a power supply. They all likely originate from one of a few Chinese factories, but are sold under countless brands at prices ranging from £20 on up.

But are they any good? Well, that depends. Some of them are terrible, and some downright dangerous. Unfused mains plugs and power supplies of dubious quality with fake safety ratings are abundant. Get a decent one though, usually denoted by a seller with a good returns policy and a longer than average warranty, and they can actually perform very well. My workshop system consists of one of the more fully-featured units, and I use the FM radio and Bluetooth functions all the time. It’s only 15W per channel, and none of those watts are what you’d call high fidelity, but it can make a lot of noise through the cheap Pioneer car speakers mounted in the wall.

But what if you want something better? There are a few Chinese brands that stand out with their own designs and a quality of product that is clearly a departure from the mass-produced rebadged norm. One of them is Fosi Audio, who expressed interest in having their products reviewed and sent over their TB10D amplifier. It fits firmly into the minimalist camp and I question the 300W x2 power rating, but it does pack some nice tech into its diminutive enclosure of unibody aluminium with a solid aluminium fascia and aluminium knobs. Classy.

The amp has a single RCA analogue input, and outputs on 4 mm binding posts that will also accept banana plugs. The front hosts bass, treble and volume controls, with a physical power switch. That’s your lot, besides the included 32V, 5A power supply with universal voltage compatibility and a power cable supplied based on the country of purchase. I was pleased to see the UK power cable supplied was appropriately fused, and the power supply does appear to be honestly specified with genuine ratings and its weight and build quality are typical of a good power brick.

Inside is a class D amplifier based around the TPA3255 chipset from Texas Instruments, data sheet Here for anyone interested. The TPA3255 is a high-power, 4-channel amplifier that can be run in a stereo BTL mode as it is here for higher current delivery and power output.

It also integrates pop-free soft start, overcurrent, over temperature, clipping, short circuit and DC voltage protection and proprietary high-speed gate driver error correction (PurePath Ultra-HD), which aims to reduce distortion across a Wideband frequency response up to 100kHz.

The TPA3255 specifies a mere 0.006% total harmonic distortion plus noise (THD+N) at 1W into a 4Ω load, rising to just 0.1% at the onset of clipping. Power supply ripple rejection is >65dB at 1kHz in BTL mode, so the chip isn’t especially sensitive to a quality power supply. The chip is 90% efficient at 4Ω 

The amp is backed up by 4 2200uF, 50V electrolytic filter capacitors and four large inductors to make up the LC output filter. The preamplifier is based around the NE5532 low-noise op-amp, and is implemented almost entirely in surface mounted components. I thought the implementation of the circuit design, and the quality of the board itself were excellent, though there are no branded components other than the TI chip itself.

The chip is covered by a large heatsink which should be perfectly adequate, and they even used thermal compound and height spacer washers to ensure proper thermal coupling between the surface of the chip and the underside of the heatsink. You don’t see this very often in amps like this. I also noted that the speaker terminals are wired to the amp board with cable of an appropriate gauge to carry the high-current output signal, which again is not always the case. Overall an excellent implementation. But how does it perform?

I’ll start with the prefix that, with a 5 amp power supply at 32 volts, you’re limited to about 160W. Idle loss of this chip is about 5W, so you can expect to get a maximum of about 155W out of the power supply assuming it can safely operate at its ratings. This means you won’t get anywhere near the 300W per channel that this chip can deliver into a 4 ohm load – but as that rating is at 10% distortion, you wouldn’t want to. At a more reasonable yet still high 1% distortion the chip is specified to deliver 260W into 4Ω and 150W into 8Ω. In theory you could step the power supply up to the full 48V (closer to the maximum of 53V that the chip supports), but that’s within 5% of the maximum voltage rating of the filter capacitors inside the amp so doing so will significantly reduce their longevity. You may also push the heat output beyond the capability of the heatsink and enclosure.

Given the power supply, and considering a more reasonable distortion level of 0.1% or less, I would expect to get around 70W per channel out of this amp into a 4Ω load. Testing a class D amplifier is more difficult due to the output filter network, which filters out the switching frequency but can skew traditional means of testing using a fixed resistive load.

I also wouldn’t want to push the amp to deliver its maximum power (at the onset of clipping) for a long time. In most domestic settings though, where an amp like this is likely to be used, you won’t need anywhere near that amount of power unless your speakers are unusually inefficient. It’s safe to assume that this amp will produce more than enough output for its intended use, and I certainly found this to be the case. I ran the amp with a pair of Tannoy precision 6.2 floorstanding loudspeakers. They are 89dB efficient (measured at the industry standard 1W / 1M) and are a relatively easy load, though do benefit from plenty of current to really get them going.

The amp powers up without drama and crucially without and loud pops or bangs as promised, with one caveat. If you switch on the power supply with the amp’s power switch set to the on (up) position, you will hear a pair of loud thumps in the speakers as the power supply stabilises. It is recommended therefore not to control the power to this amplifier at the wall socket and instead always use the front switch as intended.

When idling there is a very faint hiss from the speakers, but it’s nothing of concern. Commercial amps at more than 10 times the price produce produce more noise than this. The tone controls have a wide range of adjustment and the volume control is a logarithmic type that requires very little adjustment to make a lot of noise.

They work well, though they’re small and cramped and difficult to adjust without accidentally nudging adjacent controls. This is par for the course – though digital controls with buttons might have been a better design. The tone controls also have no central detent, so there’s no way of knowing when you’re at the flat position unless you’re familiar with the loudspeakers and can set them by ear.

It’s because of this that describing any subjective qualities of the TB10D is not an easy undertaking. This isn’t hi-fi in the true sense, though it’s not supposed to be. If there were a means to bypass the tone circuit, or a reference to adjust them to a flat response, the amp is quite capable of a relatively neutral performance. But this was never designed to be the heart of a hi-fi – it’s designed to be used on a desktop, in a workshop or in an environment where it will be adjusted subjectively to fit the environment, and where objective performance is, for the most part, wholly unimportant.

That aside, the amp has plenty of power to fill a large room. It doesn’t have the current delivery to produce earth-shaking bass; the small form factor doesn’t allow for the massive reservoir capacitors, substantial power supply and output filters that a class D amp requires to produce bags of current. It has a sweet top end though with plenty of detail, and it’s nice and clean through the midband. It’s a pleasant listen and doesn’t draw attention to itself, especially given the context of its price (roughly £65 in the UK) and its tiny stature.

If you’re looking for an amp that will fit in the tightest of spaces and is stripped back to the bear essentials, the Fosi Audio TB-10D is an obvious contender. It’s tiny, provides useful tone controls to tailor the sound to your liking, and has only a single input. Its size isn’t without compromise – it won’t rattle the windows with its bass output, and the cramped controls are a little frustrating, but it’s difficult to criticise given its price and the fact that its intended use case is outside of the context of a typical hi-fi stack. It therefore earns my recommendation.

If you purchase anything using our Amazon links, we’ll earn a small commission through the Amazon affiliate program at no extra cost to you. You can check out the TB10D on amazon by clicking here, or at the Fosi Audio website by clicking here. If you’d like to see some objective measurements, check out the review on Audio Science Review.