Fosi Audio V3 Mono Amplifier Review

[mike70]

Excellent points, I was about to say "No, a class AB will sound different from class D", then I saw "blind listening"... and I didn't say anything, as I am simply not sure at all I would be able to tell one from the other.

If you're curious, you can do your own blind listening experiment. The level of audio engineering needed to surpass the human ear limits was achieved decades ago.

Class D distortion in high frequencies was something in the first generations (as always happens with new tech). Then the first use of class D was in subwoofer amps, but, since some time ... that's a thing of the past.

Today, classical brands like NAD or Marantz, or even premium, like Jeff Rowland, have their costlier amps implemented with class D.

 

[telemike]

I think simply given 120 watts into 8 Ohm, their best use case is NOT the main full-sized stereo system.

Excellent (imo) use cases:

1) Desktop audio: a decent DAC/Pre + 2 Fosi v3 Mono + a pair of nice bookshelves

2) Additional channels in mutichannel HT setup (in my case + 4 Dolby Atmos heights)

I guess you don’t remember that in the 70’s 100 watts was high dollar. Most common stereo systems were usually 15 to 75 watts.

 

[NorthGraves]

I guess you don’t remember that in the 70’s 100 watts was high dollar. Most common stereo systems were usually 15 to 75 watts.

True, many things have changed since then.
Wired phones were also popular in the 70's. Not so much in 2025.

So, I think it's OK to enjoy the latest advancements in technology, like 400W balanced mono into 8 Ohm from Schiit Vidar 2F at $799 USD.

 

[gwing]

I think simply given 120 watts into 8 Ohm, their best use case is NOT the main full-sized stereo system.

Excellent (imo) use cases:

1) Desktop audio: a decent DAC/Pre + 2 Fosi v3 Mono + a pair of nice bookshelves

2) Additional channels in mutichannel HT setup (in my case + 4 Dolby Atmos heights)

It isn't just the amplifier that is important for building a full sized stereo system.

Back in the day I had a good 10W per channel amp with good dynamics that played louder than my ears could take in a pretty large room but admittedly the speakers back then were more sensitive and fewer watts were needed.

But even today I run a main system with a pair of the V3 and normal sensitivity standmounters and they can play effortlessly louder than my neighbours or other members of the household will tolerate, even when they are in different rooms or the garden

 

[NorthGraves]

It isn't just the amplifier that is important for building a full sized stereo system.

Back in the day I had a good 10W per channel amp with good dynamics that played louder than my ears could take in a pretty large room but admittedly the speakers back then were more sensitive and fewer watts were needed.

But even today I run a main system with a pair of the V3 and normal sensitivity standmounters and they can play effortlessly louder than my neighbours or other members of the household will tolerate, even when they are in different rooms or the garden

You don’t buy higher wattage to play anything “louder”, to start with.

It’s the control, authority, effortlessness, distortion % at _the same_ volume etc.

But yes, 120W can be sufficient in a number of use cases.

My three other amps are 240x2 AB, 250x2 AB, 250x5 D.

 

[gwing]

You don’t buy higher wattage to play anything “louder”, to start with.

It’s the control, authority, effortlessness, distortion % at _the same_ volume etc.

Well sort of. But they are linked and, provided you are not playing so loud that you drive your amp into clipping, extra watts get you none of those things.

 

[Dure16]

This amp gets full power with 2.8 volt input vs 4 volts that is available.

New guy here and I apologize if this is a basic question, but does this mean my X3800H cannot push the Fosi to output full power? I believe the X3800H pre outs are rated for 1.4V.

 

[NTK]

New guy here and I apologize if this is a basic question, but does this mean my X3800H cannot push the Fosi to output full power? I believe the X3800H pre outs are rated for 1.4V.

Welcome to ASR!

The Fosi V3 Mono has a measured voltage gain of 25.6 dB (= 19 ×). Given a 1.4 V input, it will output 26.7 V. When convert to power output into 4 Ω, it is 26.7²/4 = 178 W. Amir's measured max power was 192 W, therefore, you can drive the Fosi V3 Mono almost to its max output with 1.4 V input.

Since the X3800H can output to 2 V with performance degradation, it would most likely be fine as it should be very rare that you'll need full output.

[Edit]
Corrected errors. Measurements from the V3 Stereo were incorrectly used.

 

[jmc06]

V3 mono, minidsp and polk really offer a cost-effective combination for those of us who are short of money

 

[Dure16]

Welcome to ASR!

The Fosi V3 Mono has a measured voltage gain of 25.6 dB (= 19 ×). Given a 1.4 V input, it will output 26.7 V. When convert to power output into 4 Ω, it is 26.7²/4 = 178 W. Amir's measured max power was 192 W, therefore, you can drive the Fosi V3 Mono almost to its max output with 1.4 V input.

Since the X3800H can output to 2 V with performance degradation, it would most likely be fine as it should be very rare that you'll need full output.

[Edit]
Corrected errors. Measurements from the V3 Stereo were incorrectly used.

Really helpful response. Thank you.

 

[jmc06]

If you don't have any ground noise issues from the cables you have, there is no benefit to changing. There is a switch on the V3 Monos for gain on the back of each. Just make sure to set that according to your input source.

I tried both balanced and RCA cables from a DMP-A6 with the V3 monos and there was no noticeable difference driving Klipsch Heresy IVs.

May I ask if the output of minidsp seems to be 2vrms? Should the gain setting at the back of the device be 19db or 25db?

 

[jmc06]

请问minidsp2x4hd（2vrms）

Welcome to ASR!

The Fosi V3 Mono has a measured voltage gain of 25.6 dB (= 19 ×). Given a 1.4 V input, it will output 26.7 V. When convert to power output into 4 Ω, it is 26.7²/4 = 178 W. Amir's measured max power was 192 W, therefore, you can drive the Fosi V3 Mono almost to its max output with 1.4 V input.

Since the X3800H can output to 2 V with performance degradation, it would most likely be fine as it should be very rare that you'll need full output.

[Edit]
Corrected errors. Measurements from the V3 Stereo were incorrectly used.

May I ask which rca gain19db.25db setting should be selected for minidsp2x4hd (2vrms)?

 

[gwing]

请问minidsp2x4hd（2vrms）

May I ask which rca gain19db.25db setting should be selected for minidsp2x4hd (2vrms)?

My preference would, if using RAC inputs, be to always use the lower gain setting irrespective of the pre-amps output level as that would give a better overall signal to noise result than using higher gain and reducing pre-amp volume settings to compensate. Only if the monos are not being driven to full output *and* cannot achieve the required sound levels would higher gain be useful.

 

[Elvom]

Welcome to ASR!

The Fosi V3 Mono has a measured voltage gain of 25.6 dB (= 19 ×). Given a 1.4 V input, it will output 26.7 V. When convert to power output into 4 Ω, it is 26.7²/4 = 178 W. Amir's measured max power was 192 W, therefore, you can drive the Fosi V3 Mono almost to its max output with 1.4 V input.

Since the X3800H can output to 2 V with performance degradation, it would most likely be fine as it should be very rare that you'll need full output.

[Edit]
Corrected errors. Measurements from the V3 Stereo were incorrectly used.

Im on a Denon x3800h using v3 mono with preout on Front and left channels(kef r3), using the fosi on +31 gain on RCA still gets me about -4db lower volume than my center and surrounds(kef R2C,Q350) powered by the x3800h, does this matters? Should i set the fosi to +25 gain and not worry about the calibration sets even bigger volume differences?

I dont really know how this works, thanks for any input.

 

[NTK]

Im on a Denon x3800h using v3 mono with preout on Front and left channels(kef r3), using the fosi on +31 gain on RCA still gets me about -4db lower volume than my center and surrounds(kef R2C,Q350) powered by the x3800h, does this matters? Should i set the fosi to +25 gain and not worry about the calibration even bigger volume differences?

I dont really know how this works, thanks for any input.

Technically speaking, performance is better with the lowest gain for the power amplifier (as long as the gain is sufficiently high to gives you loud enough sound). The reason is that it maximizes the signal output from the upstream devices (in your case, the Denon X3800H), which usually perform best when its output is closer to its rated output.

Having said that, if you don't have any idle noise problem with the +31 dB gain, then switching to a lower gain doesn't give you any realizable benefit in the sound. My guess is that you won't be able to hear a difference (when channel levels are calibrated properly) and you can just choose the gain setting you feel more comfortable with (theoretically better performance but higher level difference between the V3 powered channels and the X3800H powered channels, vs lower level differences).

 

[Elvom]

Technically speaking, performance is better with the lowest gain for the power amplifier (as long as the gain is sufficiently high to gives you loud enough sound). The reason is that it maximizes the signal output from the upstream devices (in your case, the Denon X3800H), which usually perform best when its output is closer to its rated output.

Having said that, if you don't have any idle noise problem with the +31 dB gain, then switching to a lower gain doesn't give you any realizable benefit in the sound. My guess is that you won't be able to hear a difference (when channel levels are calibrated properly) and you can just choose the gain setting you feel more comfortable with (theoretically better performance but higher level difference between the V3 powered channels and the X3800H powered channels, vs lower level differences).

Zero noise on +31, Thanks for the anwser, it solves everything!

 

[Brian Hall]

May I ask if the output of minidsp seems to be 2vrms? Should the gain setting at the back of the device be 19db or 25db?

I don't have a minidsp so I don't know the answer.

 

[antcollinet]

On Archimagos blogspot you can see the 32 sample RCA measuring. It is much worse than XLR. Amir only measured XLR. So XLR is/should be the only option for the Fosi.
View attachment 467492

There is really no reason for the RCA to be 30dB worse than the XLR - unless there is ground loop noise in the eystem. I suspect that Archimago was not able to eliinate the ground noise properly. If so, this is not a failure of the RCA input of the amp, any more than it is a failure of any RCA input on any other amp. It is a system problem.

 

[rsqt]

There is really no reason for the RCA to be 30dB worse than the XLR - unless there is ground loop noise in the eystem. I suspect that Archimago was not able to eliinate the ground noise properly. If so, this is not a failure of the RCA input of the amp, any more than it is a failure of any RCA input on any other amp. It is a system problem.

If I plug an "open" RCA cable into the V3 mono (other end not connected to player), I get lots of hum/noise. If I do the same with the 3E Audio it is quiet. Same cable.

 

[antcollinet]

If I plug an "open" RCA cable into the V3 mono (other end not connected to player), I get lots of hum/noise. If I do the same with the 3E Audio it is quiet. Same cable.

Right - but now. you are connecting an antenna to an open RCA input which has high input impedance. It is not representative of real world listening, and has no bearing on what the noise of the input is like.

Short the other end of the RCA cable - and the noise will go away - that is what a low impedance output of a source looks like.