I had the opportunity to check out a Fosi V3 recently. There's been some talk about this amp on other forums like Audio Science Review, but I didn't see anything on AK, so thought I'd post some thoughts here.

After spending time and comparing the V3 to some other amplification options, a few things were pretty clear to me.

First, it's a well thought out component that does several things better than others making essentially the same amplifier (it uses the TI TPA3255 Class D IC like a lot of other budget manufacturers). So if you're Fosi, it's probably pretty difficult to differentiate and set your product apart.

It seems they've chosen to cater to the part of the market that doesn’t necessarily prioritize the lowest price, but values a product that’s already been upgraded to a premium standard. For the V3, this means Wima film capacitors and Japanese name brand capacitors (Nippon Chemi-Con, Elna) and inductors (Sumida) - things you'd think about upgrading on your own if you wanted to get the most out of the platform. So you don't have to wonder if it'll be reliable or whether sound could be improved, since high quality name brand components were used from the start instead of the cheapest generic ones.

In the case of op amps, some of us like “rolling” op amps to optimize sound, similar to trying different vintages/brands of tubes in tube gear. Just like with tubes though, there’s no one brand/part# of op amp that's going to satisfy everyone, so they (wisely IMO) just put in a basic TI op amp, but made the rolling process easier by installing DIP sockets. So it’s trivial to swap op amps without having to desolder/solder or put in your own sockets. That’s pretty smart.

Fosi also put in a logarithmic tapered volume control, which we know is traditionally what audio gear has used. Somewhere along the way, a lot budget manufacturers have substituted in linear taper pots and reduced the amount of usable volume control (especially at lower levels). The build quality is also well above what you typically see at this price point. There’s nothing cheap or DIY-looking about the V3. Many times, products at this price point will use a generic chassis similar to what you can buy from Mouser/Digikey that’s functional, but not very attractive. The V3 uses a very professional looking chassis with various sized ventilation holes placed in an attractive pattern that serves both a cooling and aesthetic function.

The standard configuration ships with a 35V/5A power supply (that has a form factor like a laptop power supply) that only provides a portion of the full power capability of the amp. A 48V/5A power supply is available as an upgraded option, and I’ve read that a 48V/10A supply would provide even more power. Power output varies based on supply specs, but also with impedance of your speaker. 4ohm speakers seems to be the most ideal match, enabling the most power for a given supply. Once you get to 8ohms, power output is reduced fairly significantly - but whether you actually need that power depends on the size of your room, efficiency of your speakers, and how loud you like to go.

The ASR site has a bunch of test measurements and it tests well and is recommended there based on measurements and specs. But what really matters is how the amp sounds. Personally, I found the V3 to be very a smooth sounding, musical amp that reminds me of the Mcintosh and tube gear I’ve owned. It doesn’t provide the fastest, most dynamic/detailed sound or pinpoint holographic placement of musical elements, but it’s also never fatiguing and helps to make even subpar recordings pleasant to listen to.

I first tried the V3 in a budget level setup: Infinity RS3 bookshelves speakers (8ohm nominal/89dB sensitivity), Kimber Kable 8PR speaker cables, and a 80’s vintage Sony Car Walkman CD player - and compared it to a budget level TPA3116 50W Class D amp that I bought as a board and installed in a cigar box. Even driving 8ohm speakers, I only had to turn up the V3 about halfway to get all the volume I needed. I tried a couple of tracks off Rumours to get a sense of both amps ability to drive deeper bass (Dreams) and make acoustic guitar sound real/live (Never Going Back Again).

I thought the V3 gave a smooth and slightly rounded presentation. The detail was there, but some aspects of the music (such as the hi-hat on the right channel in Dreams) didn’t stand out as much as with the TPA3116 based amp. Both were pleasant to listen to, but tonally they were slightly different.

I then switched to a mid-fi setup using Monitor Audio Gold Reference 20 tower speakers (6ohm nominal/89dB sensitivity), Kimber Kable 8TC speaker cables, and a Sony DVP-NC685V multi-format disc player. The amplifier used as a reference point this time was a home theater receiver (Yamaha RX-V667 in bi-wire and pure-direct mode) and I played a variety of different SACD tracks (Gaucho and Kind of Blue) and Garbage’s 2.0 CD for something louder/busier. The V3 is resolving enough to tell the difference between speaker cables too. I tried some lower end Monster Cable to start with in this setup, and it all sounded fairly flat/dull, but switching to the Kimber 8TC’s was a big improvement.

Similar to the differences using the RS3’s, the difference between the V3 and Yamaha receiver was also a matter of presentation and emphasis. The V3 was very cohesive and musical, but didn’t provide the same level of detail, sparkle, and resolution as the Yamaha. BTW, the Yamaha was driving the Gold 20’s in bi-wire mode using two sets of 8TC's, while the V3 was using a single run of 8TC, but I wouldn’t necessarily attribute the sound differences to single wire vs bi-wire configurations.

I said earlier that the V3 reminded me of some of the Mcintosh pieces I’ve owned - one in particular. I’ve owned 10-15 different vintage receivers through the years, mostly 60’s-70’s vintage, from the usual suspects (Sansui, Yamaha, Sony, Concept, Kenwood, etc). The one receiver that sounded notably different from the others was the Mac 1900. It set itself apart by sounding more fluid, musical, and everything sounded good on it.

When doing a direct comparison against other receivers, you’d notice that it wasn’t quite as detailed and fast, and all the elements in the music were present, but more part of a cohesive whole rather than clearly distinct as separate components in the soundstage. So you might not get the same wow factor when some impressive flourish burst into a song, but a subpar recording sounded more even and less ragged. You could listen for hours without fatigue and everything sounded consistently good.

Based on what I've heard so far, the Fosi Audio V3 within the budget amp world is pretty similar to what the Mac 1900 is within the vintage receiver world. Aesthetically pleasing, extremely well built, very musical and pleasant to listen to, but not the ideal choice if your primary goal is to listen with your eyes closed to every subtle nuance of a song, or get the highest level of resolution in comparing whether you prefer an early pressing of a record to a recent reissue. In tube terms, it's more Scott LK-72 than Fisher KX-100.  Which really went against my preconceived impression of Class D amps being colder and more sterile sounding - I had always steered away from Class D for this reason, but this amp made me realize that Class D doesn't mean less musical.

If using 4ohm speakers (I don’t own any at the moment), some of these characteristics may change as I’ve read higher impedance speakers de-emphasize some of the higher frequencies in Class D amps such as the V3. Maybe with a longer break-in period, some of what I've observed may change a bit as well. I’ll make a point to compare again after I’ve put a few hundred hours on it. But the value, build-quality, and sound is already very very good.

Other thoughts: Banana plugs are best with this amp. I tried using spade terminated speaker cables, but the speaker terminals are too closely spaced to use full sized spades without worrying that there might be an accidental short created with some shifting of wires. This isn’t a construction or build quality issue, but just a consequence of having such a compact sized form factor. It's also pretty lightweight, so if you had stiff, garden hose sized speaker cables, the amp isn't going to keep those weighed down in place.

The 3.5mm "pre-out" connection also really isn’t a preamp out as you'd normally think of one, where the output is modulated by the volume control. It’s a fixed output and while it might be useful to daisy chain your RCA input to a second V3, to my way of thinking, it can’t really be used as a pre-amp output or a subwoofer input (where you’d need the output level to change with the volume of the main amp). This output seems simply misnamed and something most users are unlikely to use. Having a true preamp out on a budget amp isn’t common anyway.

To sum things up, the Fosi V3 is a very well built, basic functionality, small form factor amp with a very musical, cohesive presentation that’ll have you enjoying the music whether you’re listening to an audiophile recording or a bootleg concert. At least with 6 and 8ohm speakers, it won’t provide pinpoint placement of instruments or provide the highest degree of resolution that more analytical leaning amps do. But it provides excellent value in an attractive package and is the best executed budget amp I've run across so far.

Some pics:

There's actually a cool anodized orange knob you can buy for $10 to replace the black one. Adds a bit of bling to the amp. The on/off switch is integrated with the volume knob.

Size compared to the 3116 amp I installed in a cigar box:

Since I likened it to a Mac 1900 (and I happened to have one still!), a shot together:

My messy comparison setup (you can see the Yamaha receiver on the lower right). Can't see the speakers, but if I zoom out, it gets exponentially messier looking, so I spared you.