Fosi Audio V3 Mono Amplifier Review

[antcollinet]

The pair of amps I have each have their own 48v 5amp power supply. Although each having their own 48v 10amp PS would not appreciably increase the wattage, would it allow for the amp to draw more energy when needed?

More energy would be increasing the wattage - so no - if the amp can't deliver more watts, it can't deliver more energy.

 

[SoySalsaPicante]

More energy would be increasing the wattage - so no - if the amp can't deliver more watts, it can't deliver more energy.

Just to confirm, using a 32V PSU instead of 48V will result in lower thermals (and of course maximum power output), but using an equivalent voltage yet higher (maximum) current one will not increase thermals, right?

 

[antcollinet]

Just to confirm, using a 32V PSU instead of 48V will result in lower thermals (and of course maximum power output), but using an equivalent voltage yet higher (maximum) current one will not increase thermals, right?

Depends - first what do you mean by equivalent voltage? Equivalent to 48V, or 32V, then increasing current from what to what?

In the end it will depend if your speaker impedance dips low enough to need the extra current available - and if the amp doesn't hit a current limit itself.

 

[SoySalsaPicante]

Depends - first what do you mean by equivalent voltage? Equivalent to 48V, or 32V, then increasing current from what to what?

In the end it will depend if your speaker impedance dips low enough to need the extra current available - and if the amp doesn't hit a current limit itself.

For the exact song played back at the exact same volume, comparing, for example, 48V 5A vs 48V 10A, should I expect the same thermals? Same for comparing, say, 32V 5A vs 32V 10A?

If I’m not mistaken, the same exact song played at the same exact volume on a PSU of 48V 5A will heat the amp more than playing with a 32V 5A PSU, is that right?

 

[antcollinet]

For the exact song played back at the exact same volume, comparing, for example, 48V 5A vs 48V 10A, should I expect the same thermals? Same for comparing, say, 32V 5A vs 32V 10A?

If I’m not mistaken, the same exact song played at the same exact volume on a PSU of 48V 5A will heat the amp more than playing with a 32V 5A PSU, is that right?

Same song at same volume with same voltage supply, regardless of current capability in the supply - identical thermals. - assuming the volume level chosen fits within the capability of the lower current supply.

Same song, same volume with different voltage supply, then the higher voltage will get hotter. The amps are switching devices. it costs more energy (heat loss) to switch 48V than 32V regardless of load.

 

[SoySalsaPicante]

Okay, thanks. So let’s say we allow a 50% efficiency (insanely poor, and much worse than reality with any halfway decent switching PSU), we took the amount of wattage needed for maximum peak expected volume at the MLP with the known efficiency of the attached speakers, added an extra 30% headroom there as well (again, in the interest of conservativeness), and, as a hypothetical example, got 120 watts. We’d be better off using a 12V 10A PSU than a 24V 5A PSU from a thermal standpoint.

 

[TurtlePaul]

Okay, thanks. So let’s say we allow a 50% efficiency (insanely poor, and much worse than reality with any halfway decent switching PSU), we took the amount of wattage needed for maximum peak expected volume at the MLP with the known efficiency of the attached speakers, added an extra 30% headroom there as well (again, in the interest of conservativeness), and, as a hypothetical example, got 120 watts. We’d be better off using a 12V 10A PSU than a 24V 5A PSU from a thermal standpoint.

You need sufficient voltage to get to the power you want. A 32 volt power supply can only do about 35 watts per channel into 8 ohms and 70 watts into 4 ohms, even if the PSU can deliver infinite amps.

Also, audio is sine waves, not direct current. A 24V 10A direct current is 240 watts, but it is 24V supply peak to peak can only do a 17V RMS sine wave.

 

[mike70]

I understand that voltage is constant, when impedance gets low, the amp needs more current to reach the same power as before.

So, if the amp doubles power from 8 to 4 ohms, it have the current needed (whatever amount is needed).

Or I'm wrong? (I'm not an electronic engineer or anything close to that )

 

[antcollinet]

A 32 volt power supply can only do about 35 watts per channel into 8 ohms and 70 watts into 4 ohms,

Where do you get those figures from. I make it 64W and 128W (more with little flat top clipping) when the amps are run in BTL - as these chip amps always are.

(V/root(2)^2)/R



The V3 stereo is measured at 62.3W at the knee of clipping, and 85W with 1%THD with the 32V supply.

 

[antcollinet]

when impedance gets low, the amp needs more current to reach the same power as before.

Not quite - at the same voltage - when the impedance gets low, the current increases - and therefore the power also increases.

So if the impedance halves, the power doubles. Not the same as before.


And an amp only needs enough current to meet it's specification - if it is specified as having the same power at 4 and 8 ohms, then it doesn't need to double the current at four ohms, only increase it by Root(2) times - at the correspondingly lower voltage.

 

[Sokel]

Okay, thanks. So let’s say we allow a 50% efficiency (insanely poor, and much worse than reality with any halfway decent switching PSU), we took the amount of wattage needed for maximum peak expected volume at the MLP with the known efficiency of the attached speakers, added an extra 30% headroom there as well (again, in the interest of conservativeness), and, as a hypothetical example, got 120 watts. We’d be better off using a 12V 10A PSU than a 24V 5A PSU from a thermal standpoint.

If you worry about thermals that's the chart you need for TPA3255 at BTL config:

link

Doesn't have any numbers for 2 Ohm demands (cause that's the key, the speaker asks and the amp must deliver, it's not the other way around) but it's safe to assume that thermals needs would skyrocket (assuming a perfect PSU that can deliver whatever) .

In practice, if temps get over 60° C which is defined for home use one probably needs a better built amp.

 

[mike70]

Not quite - at the same voltage - when the impedance gets low, the current increases - and therefore the power also increases.

So if the impedance halves, the power doubles. Not the same as before.


And an amp only needs enough current to meet it's specification - if it is specified as having the same power at 4 and 8 ohms, then it doesn't need to double the current at four ohms, only increase it by Root(2) times - at the correspondingly lower voltage.

So, the "current question" is another audiophile dream ... If the speaker have 4hms and the amp have the power you need in 4 ohms (listening position distance, speaker, sensitivity , enough headroom, etc) you're done.
If the power supply is 5A, 10A, 20A, whatever ... is irrelevant.

 

[TurtlePaul]

Where do you get those figures from. I make it 64W and 128W (more with little flat top clipping) when the amps are run in BTL - as these chip amps always are.

(V/root(2)^2)/R



The V3 stereo is measured at 62.3W at the knee of clipping, and 85W with 1%THD with the 32V supply.

62.3 is into 4 ohms, which I rounded up to 70. I said the 8 ohm and 4 ohm power with 32V. The difficulty of the math in the real world is that these chips can't operate rail-to-rail.

 

[CraigVMN]

OK, so now I have two questions.

1. Would swapping out the 48v 5 amp PS on the Fosi V3 mono to a 48v 10amp PS increase the heat of the components inside the amp, just due to the higher current figure?

2. Would the 10amp PS be capable of more headroom, if the load (speaker) called for it?

 

[jEDGEc0m]

I just got a pair of V3 monos.

I posted a bunch of things, some of it related to the above, on the user impressions thread.

Post in thread 'Fosi v3 Mono - User Impressions | Owner's Thread'

Sep 9, 2025

TLDR: I received 2x Fosi V3 mono with 2x 48V 5A.
Their phase is correct. they have 19-25dB gain switch on RCA. They have only 1 QC sticker.
They don't make any concerning amount of noise when turned on and off, even yanking or plugging in at mains with switch on amps set to "On".
They are not disturbingly warm after a few hours use at moderate volume, though the room is pretty cold at present.

They work. Music is being amplified and sounds as such. success! but it's only been mere hours.



I just received a pair of Fosi V3 Mono today, with 2x 48v 5A power supplies. (Previously...

• jEDGEc0m

• 

I think i might know my answer to your question, but someone better qualified may respond with better authority.

 

[staticV3]

1. Would swapping out the 48v 5 amp PS on the Fosi V3 mono to a 48v 10amp PS increase the heat of the components inside the amp, just due to the higher current figure?

No.

2. Would the 10amp PS be capable of more headroom, if the load (speaker) called for it?

Both the TPA3255 chip amp and the power supply have current limits.

If you were previously current limited by the power supply, but not by the chip amp, then upgrading the power supply would result in more headroom.

At 4Ω, the V3 Mono ist current limited by the chip amp, so 5A or 10A PSU makes no difference.

At 2Ω, the 10A supply might enable higher output power, but I haven't seen tests confirming this.

 

[antcollinet]

62.3 is into 4 ohms, which I rounded up to 70

Damnit so it was - brain fart at my end. Sorry.

Never the less - a 32V supply can generate a peak to peak sinewave of a little less than 64V in BTL - Lets call it 60V, or 21.2V rms. This is good for 56W at 8 ohm, 112 at 4 ohm.

Supported by this chart from the data sheet.

 

[jmdesignz2]

Power supplies: I remember that I read something like 2 x 48 V / 10 A = 196 Watt @ 1 % Distotion and
2 x 48 V / 5 A = 192 Watt @ 1 % Distortion
So, no difference and 2 x 48 V / 5 A are absolutely perfect!

And: Absolutely no difference in power between 1 x 48 V / 10 A with two outputs and 2 x 48 V / 5 A
I would prefer the last, because: why Monoblocks if you have just one power supply?

32 V / 5 A is a different thing: Roughly 50 % (or 3 dB) less power output (@amirm‘s measurement of the Fosi V3 stereo Version) than with 48 V / 5 A.
That’s maybe because it is possible that the 48 V / 5 A Fosi power supply is a bit more capable than rated in the specs.

Just to confirm, using a 32V PSU instead of 48V will result in lower thermals (and of course maximum power output), but using an equivalent voltage yet higher (maximum) current one will not increase thermals, right?

Check my recent post. A couple of $13 usb stand fans will resolve any thermal issues

 

[SoySalsaPicante]

Check my recent post. A couple of $13 usb stand fans will resolve any thermal issues

Yeah my questions were mostly academic. I use B200s which you get what you get and that's it as far as the power supplies.

 

[Blainethemono]

After running a Nakamichi PA7 Mkll power amplifier thru PBN Audio Montana EPS2 loud speakers for years, the big Nak finally failed. Being on a fixed income & low budget, I had to look for alternatives. A life long Friend & Audiophile, who runs stereo hardware that costs as much as a down payment for a house, suggested I check these out. After an initial failure of 1 of the amps, which FOSI promptly replaced post haste. I went with the 10amp power bricks for each amp, & the USB fan kit for cooling.

These lil guys perform just as strong as my Nak & sound just a clean, if not better. I listen to a lot of remastered LPs,180g quality from multiple genres of music, & the FOSI mono bricks have earned my respect.