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Product: Fosi V3 Mono - power amplifier
Manufacturer: Fosi Audio - China
Approx. price: V3 with 48V/5A PSU: €160 each
Reviewer: Lucio Cadeddu - TNT-Audio Italy
Reviewed: October, 2024

Foreword

A closer look

The V3 Mono is therefore a mono'ed V3 with no volume knob, with an unbalanced RCA and a balanced XLR input, and a rear selector that allows you to adjust the gain (+25 or +31 dB, but +20dB on the XLR input) i.e. how much the amplifier boosts the input signal. Technically, it is very similar to the original V3, with the addition of a circuit, called PFFB (Post-Filter Feedback Technology, here is the white paper by Texas Instruments), which should make the amplifier more insensitive to the load, a problem that plagues the high frequency response of many Class D circuits. In essence, it is a sort of feedback circuit, as you can see in the diagram below.

If you prefer, you can purchase a single 48V/10A power supply that supplies both mono amplifiers, a convenience which saves space and flying extra cables. My pair of V3 Mono, kindly supplied by a TNT-Audio reader, was equipped with two separate 48V/5A power supplies.

The technical specs, in relation to the output power, speak of 120 watt RMS on 8Ω (and 240 on 4Ω) with a THD of 1%. Let's say that, as usual, accepting a lower distortion rate, the power output is closer to 100 watt. However, these are impressive values for the cost and size of the amps, especially considering that the peak power on 2Ω is close to 400 watt.

The three opamps that equip the V3 Mono are user-replaceable, and a pair of MUSES02 operational amplifiers is supplied for you to experiment. In a short video Fosi Audio explains how to proceed with the replacement. I kept the default NE5532 opamps. There are also videos on YouTube that compare the sound of the V3s using both the MUSES02 and the Sparkos SS3602. It is a sort of modern tube rolling which alters - in one way or another - the sound of the amps. If desired, other compatible opamps can be installed, such as OPA2604, OPA2134PA, OP249, AD827, RM4559, LM4562, LME49720HA etc.

The first production batch of V3 Mono power amps was phase-inverting, you need to contact the company with the serial number to understand whether yours are phase-inverting or not. Those purchased from August 2, 2024 onwards are non-inverting. In any case, any phase test track is sufficient to understand if the phase is correct or inverted. For example, you can use the free Focal CD I reviewed some time ago. If your amps are phase-inverting, just invert the polarity at the speakers' end.

I've tested the mono V3s in comparison with a pair of ZA3s, also from Fosi, and with the usual ZeroZone IRS2092 monoblocks. I used the unbalanced RCA inputs, with the gain set to +31dB. I didn't notice any sound quality difference with the gain set to +25dB.

Double Trouble

The V3 integrated amplifier had impressed me, and I still consider it the best compact Class D integrated amplifier among those I have listened to so far, certainly superior to its “bigger” (in terms of overall dimensions) brother ZA3. The V3 Mono power amps confirm the goodness of the design and emerge victorious from a head-to-head comparison, under equal test conditions (same power supply), with the ZA3 brothers.

The differences are more or less the same as those found in the comparison between the V3 and ZA3 integrated amplifiers: the timbre setting is substantially identical, and it couldn't be otherwise given that the circuits are very similar, but the structure of the layout of the V3 seems to give something more: the sound is globally more balanced, coherent and refined. The first difference that catches your ear is the low range: deep, powerful and articulated on the V3 Mono, with the Z3A it is less deep and even slightly out of sync; the pace of the bass range seems wrong when compared to the rest of the audio spectrum. As usual, the bass range turns out to be the area where the amplifiers suffer the most, because of the larger energy (current) demand. I don't have an explanation why the V3 performs better, given that the chipset and power supply are identical, but the fact remains that the difference is easily noticeable.

In the mid-high range the sound of the ZA3 is slightly more artificial, it has something electronic in it, something almost completely absent in the V3. This touch of artificiality can be heard on voices and on certain percussions, which appear quite poor in harmonic structure. The high range, at times, seems less precise and extended with the ZA3, while with the V3 Mono it is precise, rich in details and, overall, more natural. This can be attributed to the presence of the PFFB circuit in the V3, which should guarantee a more extended high range response, regardless of the load. However, the same difference could be heard when using the V3 integrated amp, which did not use the PFFB feedback circuit.

From a dynamic point of view, the performance of the two contenders is substantially similar, with a small advantage for the V3 in the mid-high range. However, the difference in terms of sound-stage is evident: with the V3 Mono the sound is large, extended in 3 spatial dimensions, while with the ZA3 it remains a bit confined to the space between the speakers. Even with the integrated amps the same strange phenomenon occurred.

In conclusion, and I imagine this is the question many of you are asking, how do the V3 Monos perform compared to the ZeroZone monoblocks? I expected the difference to be smaller, because the sound of the mono V3s is very good overall, despite the price, but once again, the ZeroZone monoblocks prove they're vastly better, both in terms of timbre, imaging or dynamics. While with V3 Mono you don't feel the urge to raise the volume, with the ZeroZone's you can't help but feel the need for higher SPL. Why? simply because the sound seems to flow effortlessly, with no compression or distortion of any sort. I don't think it's a question relating to the difference in power output, because I haven't pushed the mono V3s to their natural limit...simply, as soon as you reach a comfortably high listening level - I'm talking about peaks of 100dB at the listening point, just to be clear - you don't need to up the ante any further, while with the ZeroZone monoblocks you just can't get enough, until you reach insanely high SPL.

Remarks & complaints

1. It's not a bad idea to make the gain adjustable between +25 and +31 dB on the RCA input (+20dB on the balanced XLR), in order to adapt to different situations and settings. At both +25 and +31dB, during my tests, the V3 Mono warmed up quite a lot, which is normal considering the high power enclosed in such a tiny cabinet. What is strange is that the V3 integrated amp didn't heat up so much; the ZA3 runs much cooler, thanks to the larger cabinet, I assume.
2. I still believe the balanced connection is pure nonsense, because no one uses balanced sources at this price tag.
3. Two power supplies, two power amplifiers, lots of cables! I wonder if it's really like that complicated to put everything inside a single cabinet, to make a pure and simple stereo amplifier, without XLR inputs. Or, if you wish to optimize space, why not create a single cabinet for the power amplifiers and a single external power supply?
4. From a sound quality point of view, the two competitors (V3 Mono and ZA3) are close. However, the V3 Mono win across the board. There is no area where the ZA3 are to be preferred.

Availability

The V3 Mono amps are available in the usual online stores, both on Amazon (at the moment they appear as “not available”, though they are on both the UK and US sites), on Aliexpress and on the official Fosi Audio store. On Aliexpress you can sometimes get a lower price, but there is the risk of having to pay customs.

Conclusions

Finally Fosi Audio has understood that two integrated amplifiers (the ZA3) to be used as mono amplifiers is pure nonsense, so here are these V3 Monoblocks! They're simple to use and sound terrifically good. For around 300€ you take home two powerful, dynamic and transparent monoblocks, which you would never expect for such a low price. Let's be clear: for 300/350€ there is no pair of monophonic amps that comes close, not even a stereo power amp that can even remotely compete, also because, at this price no one is able to produce similar products, apart from ZeroZone, which however belong to a category of its own. So...well done Fosi, after two sad disappointments (P3 and ZA3) a new product that sounds damn good.

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Copyright 2024 Lucio Cadeddu - direttore@tnt-audio.com - www.tnt-audio.com

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