Fosi Audio V3 Mono Amplifier Review

[DeToxin]

Should have high gain ??? If you don't have low impedance speakers and want to maximise the signal sent to them from a 1v source then sure, set the gain high.

but

If it is loud enough for you already then don't change the setup for something that will have no value to you and instead keep the gain at the lower setting and your input signal high so you can enjoy lower noise levels for the same listening volume.

I was under the impression that the avr pre-out performance scaled with the increased voltage up until it hits it's best performance at about 1v, at which point it degrades.
And that the power amp performance also scaled with input voltage.
So I thought the avr's best performance should be at around the 75-80 volume level.
Have I misunderstood?

 

[gwing]

I was under the impression that the avr pre-out performance scaled with the increased voltage up until it hits it's best performance at about 1v, at which point it degrades.
And that the power amp performance also scaled with input voltage.
So I thought the avr's best performance should be at around the 75-80 volume level.
Have I misunderstood?

I think so. To explain a little:
Every piece of audio equipment has a fixed noise level generated from its own electronics and hum etc., it will output to the next component your variable level music signal plus that fixed noise. This means that to get the best performance (highest signal to noise ratio) you want to send the highest level signal you can rather than attenuating the signal at the source (obviously you shouldn't send a signal voltage higher than the next component can accept but that isn't a problem for you as you have a maximum 1V output and the V3s can accept 2 or 4 volts).
Now the amplifier receiving the signal needs to apply sufficient gain to generate the listening volume you require. If the low gain setting can do that then use the lower gain. If you choose a higher gain it will be louder and exceed the maximum volume you want so you will have to go back to your source and reduce the volume there - but then you are amplifying more both the signal and the noise so that you get more noise (i.e. a lower quality signal) for the same volume. Only if you can't reach the volume you want at low gain would you set your V3 to higher gain.

 

[DeToxin]

I think so. To explain a little:
Every piece of audio equipment has a fixed noise level generated from its own electronics and hum etc., it will output to the next component your variable level music signal plus that fixed noise. This means that to get the best performance (highest signal to noise ratio) you want to send the highest level signal you can rather than attenuating the signal at the source (obviously you shouldn't send a signal voltage higher than the next component can accept but that isn't a problem for you as you have a maximum 1V output and the V3s can accept 2 or 4 volts).
Now the amplifier receiving the signal needs to apply sufficient gain to generate the listening volume you require. If the low gain setting can do that then use the lower gain. If you choose a higher gain it will be louder and exceed the maximum volume you want so you will have to go back to your source and reduce the volume there - but then you are amplifying more both the signal and the noise so that you get more noise (i.e. a lower quality signal) for the same volume. Only if you can't reach the volume you want at low gain would you set your V3 to higher gain.

Thanks for the thorough explanation, I think I understood the need to not amplify unnecessary noise from the source and thus preference low gain provided that the listening volume is sufficient.. I've done a few more searches and it seems that 75 on the volume dial is indeed 1v preout and past that is the point at which signal begin to degrade.
I'll do some proper listening tests with low and high gain this weekend.

@Thomas_A reported that best performance comes with using an external DAC connected to the analogue in on the avr (https://www.audiosciencereview.com/...ac-pre-out-to-portable-mac-direct-png.110202/) so I might look into my options there too, I can probably live without audyssey and sub out when listening to music.

 

[Pulkass]

Did you notice any difference in performance?

I have a pair of v3's powering some kef q concerto meta's and I'm not sure if I am imagining it, but the bass feels a little thin compared to when I demoed them at the store, and the bass driver has almost no excursion compared to the store demo, at similar volumes (not matched exactly, because I forgot to take a measurement).

Is it possible the v3 is struggling to drive the speakers around 80-100hz range? And would swapping the power bricks to 2* 48v10a offer any kind of improvement?

Impedance graph from Erin's audio corner attached.

The brighter led amp, now it s not working, tried all pre dac connections and combinations, to no avail, have requested return to Ebay, seller was Fosi audio, so I guess no problem, will send just one of the two.

 

[Audionaut]

Hello @Fosi Audio

When will v4 and v5 be on the market? How do they differ from v3?

 

[antcollinet]

Hello @Fosi Audio

View attachment 537181

When will v4 and v5 be on the market? How do they differ from v3?

The V5 looks like a game console. Not to my taste at all.

 

[gwing]

The V5 looks like a game console. Not to my taste at all.

Nor mine. My V3s look positively pretty compared to either of these new designs.

 

[Aimarketingbot]

Nor mine. My V3s look positively pretty compared to either of these new designs.

Right one looks like an external 5.25" HDD caddy and the left one some cyber punk sandwich press xD
I get it, they want to try new styling but they should definitely approach the community for feedback before release.

 

[Bleib]

V5 has a built in power supply (confirmed with pictures on the backside)

 

[Kali-]

Hello @Fosi Audio



When will v4 and v5 be on the market? How do they differ from v3?

for what i seen:

• hypex instead of texas instruments
• no op-amp swap
• integrated power supply

 

[Pulkass]

Arrived two monos. One is powered by a 48 V 10 Amperes. The others one by a 48 V 5 Amperes. The led of the more powered is brighter (!).

I am requesting the return. Bought on eBay from their fosi audio shop. The brighter power on led amp is muted !!!! Not working.

 

[Guddu]

Fosi V4 & V5 Mono - Fosi goes Hypex

News - The new comers from Fosi for Yr2026 Fosi V4 Mono - Hypex 250 watts - approx $500 - 600 Fosi V5 Mono - Hypex 550 watts - Touch display - Most likely a remote - approx $1000

www.audiosciencereview.com

 

[Pulkass]

Yes. There have been a noticeable number of user reports of Fosi Audio V3 Mono amplifiers arriving dead or failing very early, although it's impossible to know the true failure rate from forum posts alone.
Some examples reported by users include:
Units with no sound after only a few hours or days of use despite the power LED operating normally. �
Fosi Audio Global Community
Multiple owners reporting one amp in a pair failing while the other continues working, with symptoms such as relay clicks but no output or a burnt smell. �
Fosi Audio Global Community +1
More recent posts from 2025 describe units dying within months and owners seeking warranty replacements. �
Fosi Audio Global Community +1
That said, the V3 Mono is also a very popular budget amplifier, so online reports are naturally skewed toward people with problems rather than the many owners whose units work fine.
Your situation is interesting because:
You bought two separate V3 Mono amps,
from different sellers,
and both are not working.
Statistically, that makes me wonder whether the issue could also be external, such as:
the 48 V power supply (if shared),
incorrect trigger/auto-on behavior,
input selection (RCA vs XLR),
faulty source or cables,
speaker wiring,
or a gain/connection issue.
Can you describe exactly what "not working" means?
Does the orange LED light up?
Do you hear the relay click after power-on?
Is there absolutely no sound, or is it distorted?
Are you using the supplied power supplies, or a single shared 48 V supply?
Are you feeding them by RCA or XLR?
Those details would make it easier to tell whether you've been unlucky with two defective units or whether there's a common setup issue.

I am sending back the second one, faulty, another one is working at home. Had l known before, !!!!! Za3 is much better. And l have one

 

[PacketofCrisps]

Would there be any benefits to grabbing a pair of these for my Yamaha A4A receiver? I'm using B&W 706S3 speakers that dip down to 3.7Ω.

 

[ThsM]

Would there be any benefits to grabbing a pair of these for my Yamaha A4A receiver? I'm using B&W 706S3 speakers that dip down to 3.7Ω.

What do you expect to gain by adding these?

 

[Pulkass]

What do you expect to gain by adding these?

They are nothing more than toys, extremely warm, and had to send back two because not working at all-the Fosi audio amp za3 is better!!!!!-I bought an Audiophonic equipped with Hypex 500 x---more serious stuff.

 

[Pulkass]

Three have been sent back, not working , l bought the 5th one today, hope to gather 2 items-stereo- working! Must have fetched the wrong supply line, and unlucky me. (!)

 

[PacketofCrisps]

What do you expect to gain by adding these?

The presumption is powering the speaker better at low ohms over that of the A4A? 'traditional' hifi advice seems to say you need a high amperage amp capable of being stable down to 2 ohms to power the B&W 706 speakers due to the 3.7 ohm dip, I'm not saying that is true but years of that of advice gets ingrained, "high power amp can better control your speakers" etc.

 

[ThsM]

The presumption is powering the speaker better at low ohms over that of the A4A? 'traditional' hifi advice seems to say you need a high amperage amp capable of being stable down to 2 ohms to power the B&W 706 speakers due to the 3.7 ohm dip, I'm not saying that is true but years of that of advice gets ingrained, "high power amp can better control your speakers" etc.

A friend of mine, also has the A4A, powering big towers (Nubert nuVero 110) which dip down to 3.5 ohm. He is very happy with the setup and has no intention to add any extra Amp. But thats him. If you feel the need to add one, go for it. But tbh, I would not expect any improvement by adding these V3 Monos.

 

[antcollinet]

The presumption is powering the speaker better at low ohms over that of the A4A? 'traditional' hifi advice seems to say you need a high amperage amp capable of being stable down to 2 ohms to power the B&W 706 speakers due to the 3.7 ohm dip, I'm not saying that is true but years of that of advice gets ingrained, "high power amp can better control your speakers" etc.

If you are not getting audible distortion when playing loud then you don't have a problem - and any "upgrade" will make no difference.