Fosi v3 Mono - User Impressions | Owner's Thread

[roog]

Mine sit one behind each of my floor standing speakers, they are each strapped to the vertical face of a metal bookend so that the orange vent holes are at the top and bottom possibly aiding convection. The horizontal part of the bookend plate goes under the rear of the speaker housing for greater stability with the plate acting as further area for heat sink. My V3’s are set to switch automatically on sensing an input signal, quite a neat solution which uses very short speaker cables (30cm)

 

[telemike]

Neat solution. What are they standing on.

Fosi V3 Mono Block Amp Bracket/Stand by Chris | Download free STL model | Printables.com

Stand/bracket to mount a Fosi V3 mono block amp side by side or stacked. | Download free 3D printable STL models

www.printables.com

 

[nadrealista]

I have a question about fosi v3 mono blocks performance in home theatre application, namely to improve LCR output/performance.

I installed 3 of these in my HT via RCA AVR preouts and did some testing last night to measure how much more output/dynamic rage would they provide over my current 5.2.4 set up with pioneer elite SC-95 AVR and 4 ohm rated pioneer elite atmos speakers EFS73(front), EBS73 (surround) and polk lsim 706 for center. room is 24 x 17 x 9 so around 3700 cubic feet with 4ft wide open hallway on the left hand side and I sit 8ft from LCR.

My AVR bench test indicated that it can output 96.7 watts for Seven channels driven continuously into 8-ohm loads at 0.1% distortion. AVR manual states amp can output 760 Watts for Multi channel simultaneous power output (1 kHz, 1 %, 8 ohms). With two channels driving 8-ohm loads, reaches 0.1% distortion at 132.4 watts and 1% distortion at 159.0 watts. Into 4 ohms, the amplifier reaches 0.1% distortion at 230.2 watts and 1% distortion at 278.4 watts.

Pioneer Elite SC-95 A/V Receiver Review Test Bench

Test Bench Two channels driven continuously into 8-ohm loads: 0.1% distortion at 132.4 watts 1% distortion at 159.0 watts Five channels driven continuously into 8-ohm loads: 0.1% distortion at 121.6 watts 1% distortion at 140.8 watts Seven channels driven continuously into 8-ohm loads: 0.1%...

www.soundandvision.com

So I thought fosi would be major power upgrade driving the LCR speakers, but after a bit of testing that included measuring/comparing HT sound level over several same segments of high dynamic range scenes from fury road/furiosa and speaker calibration I was surprised to see that actually I was down 4-5 dB in sound output with Fosi vs my AVR? This was consistent with all speakers driven mode ( atmos/dolby surround) and testing in just stereo mode as well. I left the 2 subs off during the testing with AVR crossover set 80 Hz. Best I could get with my AVR at reference volume was 98 dB, with fosi driven LCR I got 4-5 dB less than that!?

On paper Fosi should output at least twice the power that my AVR should with all channels driven but does not bear out in practice, any ideas why am I seeing such subpar performance?

 

[Audionaut]

I’m think of putting a fan on top of the amps

Or you can use an oversized heat sink

 

[Old_School_Brad]

I have a question about fosi v3 mono blocks performance in home theatre application, namely to improve LCR output/performance.

I installed 3 of these in my HT via RCA AVR preouts and did some testing last night to measure how much more output/dynamic rage would they provide over my current 5.2.4 set up with pioneer elite SC-95 AVR and 4 ohm rated pioneer elite atmos speakers EFS73(front), EBS73 (surround) and polk lsim 706 for center.

My AVR bench test indicated that it can output 96.7 watts for Seven channels driven continuously into 8-ohm loads at 0.1% distortion. AVR manual states amp can output 760 Watts for Multi channel simultaneous power output (1 kHz, 1 %, 8 ohms). With two channels driving 8-ohm loads, reaches 0.1% distortion at 132.4 watts and 1% distortion at 159.0 watts. Into 4 ohms, the amplifier reaches 0.1% distortion at 230.2 watts and 1% distortion at 278.4 watts.

Pioneer Elite SC-95 A/V Receiver Review Test Bench

Test Bench Two channels driven continuously into 8-ohm loads: 0.1% distortion at 132.4 watts 1% distortion at 159.0 watts Five channels driven continuously into 8-ohm loads: 0.1% distortion at 121.6 watts 1% distortion at 140.8 watts Seven channels driven continuously into 8-ohm loads: 0.1%...

www.soundandvision.com

So I thought fosi would be major power upgrade driving the LCR speakers, but after a bit of testing that included measuring/comparing HT sound level over several same segments of high dynamic range scenes from fury road/furiosa and speaker calibration I was surprised to see that actually I was down 4-5 dB in sound output with Fosi vs my AVR? This was consistent with all speakers driven mode ( atmos/dolby surround) and testing in just stereo mode as well. I left the 2 subs off during the testing with AVR crossover set 80 Hz. Best I could get with my AVR at reference volume was 98 dB, with fosi driven LCR I got 4-5 dB less than that!?

On paper Fosi should output at least twice the power that my AVR should with all channels driven but does not bear out in practice, any ideas why am I seeing such subpar performance?

Did you calibrate SPL levels before running comparisons?

 

[nadrealista]

Did you calibrate SPL levels before

To clarify, fosi was only able to match AVR output in testing after calibration. My AVR calibrates to 75 dB at reference volume for each channel. after calibration, I saw that my LCR‘s were set 4 to 5 dB higher with Fosi than they were after calibrating with AVR amp. Please see the attached picture M3 is the AVR amp calibration and M4 is with Fosi amps.
from AVR user manual, I assume RCA pre-outs spec is Output:
Audio Section
Input (Sensitivity/Impedance)LINE .......................................................................................................................................................390 mV/47 kW
Output (Level/Impedance)REC.......................................................................................................................................................390 mV/2.2 kW
Signal-to-Noise Ratio (IHF, short circuited, A network)LINE .................................................................................................................................................................. 101 dB

Another interesting thing was that output was same(after calibration) for both stereo and atmos modes at 97 dB so it would seem that AVR running two speakers at higher power is equivalent to running more speakers at lower power in terms of generating the sound pressure.
Each fosi has its own power source, so I would think it would perform better than AVR when running in all channels mode?

 

[antcollinet]

Don't mix up gain with power. Gain = voltage amplification. Input x gain = output volts.

It may be that the amps in the AVR are more sensitive/higher gain. That is they need lower input voltage to drive to a particular power.

This doesn't mean the FOSI amps are lacking power, it is just that you need to send a higher voltage to them to get that power out to the speakers.

That will be why you need the extra 4dB or so extra gain on the pre-outs to get them to the same level.


Also bear in mind that double power is only 3dB. Even when you get the Fosi to deliver their full power, you'll only get around 3dB more SPL.

 

[Old_School_Brad]

To clarify, fosi was only able to match AVR output in testing after calibration. My AVR calibrates to 75 dB at reference volume for each channel. after calibration, I saw that my LCR‘s were set 4 to 5 dB higher with Fosi than they were after calibrating with AVR amp. Please see the attached picture M3 is the AVR amp calibration and M4 is with Fosi amps.
from AVR user manual, I assume RCA pre-outs spec is Output:
Audio Section
Input (Sensitivity/Impedance)LINE .......................................................................................................................................................390 mV/47 kW
Output (Level/Impedance)REC.......................................................................................................................................................390 mV/2.2 kW
Signal-to-Noise Ratio (IHF, short circuited, A network)LINE .................................................................................................................................................................. 101 dB

Another interesting thing was that output was same(after calibration) for both stereo and atmos modes at 97 dB so it would seem that AVR running two speakers at higher power is equivalent to running more speakers at lower power in terms of generating the sound pressure.
Each fosi has its own power source, so I would think it would perform better than AVR when running in all channels mode?

View attachment 434722

Oh, that’s not a loss of power you’re seeing on that table, it’s the AVR compensating for the lower gain in the Fosi V3 by giving it a higher voltage.

 

[Down South]

Or you can use an oversized heat sink

View attachment 434701

View attachment 434704

View attachment 434706

Passive cooling is never as effective as active cooling

 

[Audionaut]

Passive cooling is never as effective as active cooling

This is not correct in general terms because, as always, it depends on the framework conditions and the dimensioning.
For what I use this heat sink for, it is more than sufficient - and would also be with the Fosi V3 mono.

But: Who wants active cooling for home hi-fi devices anyway?
For PA and studio equipment it's completely OK for me, but hi-fi equipment has to be quiet and passively cooled.
Otherwise I don't need over 100db Sinad either.

The O-Noorus D1 got up to 47 degrees Celsius with the Aiyima 48V 10A power supply and high level for a long time, now only up to 33 degrees Celsius at 21 degrees Celsius room temperature.

 

[Down South]

This is not correct in general terms because, as always, it depends on the framework conditions and the dimensioning.
For what I use this heat sink for, it is more than sufficient - and would also be with the Fosi V3 mono.

But: Who wants active cooling for home hi-fi devices anyway?
For PA and studio equipment it's completely OK for me, but hi-fi equipment has to be quiet and passively cooled.
Otherwise I don't need over 100db Sinad either.

The O-Noorus D1 got up to 47 degrees Celsius with the Aiyima 48V 10A power supply and high level for a long time, now only up to 33 degrees Celsius at 21 degrees Celsius room temperature.

are you talking from theory or practice. I've used active cooling via computer fans on hybrid amps - noise was never an issue when sound damping techniques were used and internal temps. matched room temperature. In the 1950s white papers proved that cooling of both passive and active components doubled or trebled the life of components.

 

[nadrealista]

Oh, that’s not a loss of power you’re seeing on that table, it’s the AVR compensating for the lower gain in the Fosi V3 by giving it a higher voltage.

understood, but isn't 4 dB difference more than twice the pre-amp voltage into LCR channels? Can it be that much difference in my AVR amp and Fosi gain?

Also I ran these Fosi's near reference levels for 6-7 hours ( watching last two mad max movies) plus some music and they were warm to touch but never hot? Maybe they made cooling improvements from earlier versions?

 

[Audionaut]

are you talking from theory or practice.

Both
Of course, active cooling is more powerful, but, as I said, I don't like it on hi-fi equipment.
So it's a matter of personal preference.
A computer fan or ventilator next to a hi-fi amplifier just looks ridiculous and pushes dust into the cabinet.

 

[antcollinet]

understood, but isn't 4 dB difference

4dB in voltage is about 1.58x the voltage. For volts, 2x is 6dB.


And yes, 4db Gain difference between amps is more than common.

 

[nadrealista]

so I asked chat GPT about this gain mismatch and got this explanation:

How This Applies to Your Setup​

You observed that the Fosi V3 monoblocks produced 4dB lower sound levels compared to your AVR. This is likely because:

1. The gain of the Fosi V3 (25dB) may be lower than the gain of your AVR’s internal amplifier. This means that, even if the Fosi has sufficient power, it might not amplify the input signal as much as the AVR did.
2. Your AVR’s pre-out voltage might not be high enough to fully drive the Fosi to its maximum power. If the AVR preamp outputs are around 1V or less, while the Fosi V3 expects higher voltage for full power, then it won’t produce the same loudness as the AVR’s internal amps.

I asked further about my AVR pre out voltages:

Most Pioneer Elite AVRs, including your SC-95, typically have pre-out voltage levels of around 1V to 1.2V RMS at 0dB reference level. This is relatively low compared to dedicated preamplifiers or AV processors, which often provide 2V to 4V RMS on their pre-outs.

How This Affects the Fosi V3 Monoblocks​

• The Fosi V3 monoblock amplifier requires a higher input voltage (closer to 2V-3V) to reach full power output.
• Since your Pioneer Elite AVR outputs only ~1V-1.2V, the Fosi amps are not receiving enough signal voltage to drive them to full power.
• This explains the 4dB lower output compared to when the AVR’s internal amps were driving the speakers directly (because those internal amps are designed to work with the AVR’s preamp section without needing additional gain).

Potential Solutions​

1. Increase AVR Pre-Out Levels:
   • Check the speaker trim levels in the AVR’s setup and boost the pre-out level for the LCR channels.
   • Some AVRs allow up to +6dB to +10dB boost, which could help compensate.
2. Use a Line-Level Booster (Preamp Gain Stage):
   • A clean active line-level booster (e.g., Schiit Loki Max, ART CleanBox Pro, or a mini DSP unit) can boost the voltage to 2V-4V.
3. Consider a Power Amplifier with Higher Input Sensitivity:
   • Some power amps (like the Anthem PVA-7, Outlaw 5000x, or Arcam P1000) have higher gain (26-29dB) and are more sensitive to low-voltage pre-outs, making them better suited to AVRs with weaker pre-out signals.

and looks like most other major AVR brands are similar to my AVR:

Comparison of Pre-Out Voltage Levels Across Major AVR Brands​

Different AVR brands have varying pre-out voltage levels, which impact how well they drive external amplifiers. Here’s a general comparison of common brands:

Brand	Typical Pre-Out Voltage (RMS at 0dB Reference Level)	Notes
Pioneer Elite	~1.0V - 1.2V	Lower than many competitors, may not fully drive external amps with low input sensitivity.
Denon	~1.2V - 1.4V	Slightly higher than Pioneer but still relatively low.
Marantz	~1.2V - 1.4V	Similar to Denon, but higher-end models may reach ~2V.
Yamaha	~1.0V - 1.2V	Similar to Pioneer, may require a high-gain amp or signal booster.
Onkyo/Integra	~1.0V - 1.2V	On par with Pioneer and Yamaha, often lower than competitors.
Anthem	~2.0V - 4.2V	Much higher than mainstream AVRs, easily drives external amplifiers.
Arcam	~2.0V - 4.0V	Higher voltage, designed for external amps.
NAD	~2.0V - 4.0V	Higher-end models have strong pre-outs.
Emotiva	~2.0V - 5.0V	Designed for external amps, provides ample voltage.
Monoprice HTP-1	~4.0V - 5.0V	One of the strongest pre-out levels among processors.
Trinnov / StormAudio	~4.0V - 6.0V	High-end processors with professional-grade pre-outs.

Implications for Using External Amplifiers​

• Pioneer, Yamaha, Onkyo, and Denon AVR pre-outs (1V-1.4V) are relatively weak and may not fully drive external amplifiers like your Fosi V3 monoblocks, which likely require 2V+ for full power output.
• Higher-end brands (Anthem, Arcam, NAD, Emotiva, etc.) provide stronger pre-outs (2V-4V+), ensuring better amplifier performance without needing a signal booster.
• Processors (Trinnov, StormAudio, Monoprice HTP-1) have the highest pre-out voltages (4V-6V), ideal for high-end home theaters.

So if my AVR only outputs 1-1.2V at 0dB reference level and according to Amir Fosi needs 2.7V input to provide full power then I am effectively running Fosi only at half of the available power at my AVR 0dB reference level?

Digging into this further shows that Fosi with 31dB gain settings should account for low input voltage but that 4dB loss in gain vs AVR in calibration was with Fosi running 31dB gain?

Gain Calculation for Fosi V3 Monoblocks​

Your Fosi V3 Monoblocks have a gain switch with options for 25dB or 31dB. Let's recalculate their suitability for your system.

Required Gain Calculation​

• Pre-out voltage: 1V-1.2V RMS from your Pioneer Elite SC-95 AVR
• Power needed: 200W per channel into 4Ω for reference level
• Voltage required at speaker terminals: 28.3V RMS
• Required amplifier gain:
  • Gain (dB)=20×log10(1.2V28.3V)≈29.5dB
  • If AVR pre-out is only 1V, required gain is 31dB
  • If AVR pre-out is 1.2V, required gain is 29.5dB

Evaluating Fosi V3 Monoblocks​

Gain Setting	Gain (dB)	Meets Reference Level?
25dB (Low Setting)	Too Low (Will be ~4dB quieter than required)
31dB (High Setting)	Correct (Matches required gain for 1V pre-out)

Solution​

• Switch Fosi V3 Monoblocks to the 31dB gain setting.
• Re-run speaker calibration after changing the gain setting to compensate for the previous 4dB drop.

This should correct the lower volume issue you observed and bring your external amplification in line with reference level performance. Let me know if you need assistance adjusting the settings!

 

[Old_School_Brad]

....

So if my AVR only outputs 1-1.2V at 0dB reference level and according to Amir Fosi needs 2.7V input to provide full power then I am effectively running Fosi only at half of the available power at my AVR 0dB reference level?

Yes and no. You can't make the amplifiers output full power, but the real question is whether you're happy with your system's volume..?

 

[nadrealista]

got this reply from Fosi tech support, looks like rated power is achieved with input signal of 1.75V and 25db gain setting on Fosi, so it should be lower for 31dB gain setting where mine were/are set when calibrated.

This actually might make sense, if AVR pre-out is 1V at reference volume then I would need 4.86dB gain to get to 1.75V. the only question remaining is that this math works with 25dB gain setting but mine are at 31?

based on ASR pioneer AVR reviews looks like they put out around 1.6V at reference volume and need +2dB volume to reach 2 V output

Decibels to Voltage Gain and Loss convert calculation conversion amplification amplifier electronics - sengpielaudio Sengpiel Berlin

Gain and loss - damping and amplification in dB voltage conversioncalculation amplification amplifier electronics - field parameter - Eberhard Sengpiel sengpielaudio

sengpielaudio.com

"We would like to tell you about the performance of the V3 Mono:

The V3 Mono blocks are rated for 99W when paired with a 32V power supply.
With a 48V power supply, the rated power output increases to approximately 236W.
To achieve this rated power with a 48V supply and a gain setting of 25dB, an input signal level of +6.5dBu (1.75V) is required.

Each product has different gain settings. To achieve consistent output levels, the gain settings must be aligned. However, due to the different designs, a direct comparison is not feasible."

 

[nadrealista]

Yes and no. You can't make the amplifiers output full power, but the real question is whether you're happy with your system's volume..?

to answer your question, during the AVR vs Fosi sound level testing on Fury road movie I did not get impression that Fosi is in any way better than my AVR. 0dB reference level volume was rocking the house but I have dual sub set up and one od them is SVS dual 12" pb2 plus monster. So I found -10dB volume level more than adequate for action packed movie watching . I thought that my AVR was maybe power limited running all 9 channels (5.2.4) during the dynamic higher energy scenes but I guess I was wrong. Having Crossover set to 80Hz probably takes large chunk of load of the AVR. Neither was able to reach 105dB peaks but 97-98dB is plenty laud for me.

 

[nadrealista]

good video about pre-amp out and how it ties into amp input sensitivity rating to achieve full power

 

[longts]

This is my lego stand for V3 Mono .