Fosi v3 Mono - User Impressions | Owner's Thread

[Zek]

What's that peeking out from behind the TV?

 

[nadrealista]

Just wanted to share new info on changed gain values for latest version of Fosi, apparently they reduced gain from 31/25dB on RCA to 25 and 19dB which would make math work with what I saw on my AVR calibration - boosting pre-out by 4-5dB to reach reference calibration levels.

email form Fosi Tech support

We would like to confirm whether your V3 mono gain is the old version or the new version. Since V3mono does not have a volume adjustment knob. It has already obtained a large power output when it is turned on, so we have reduced the gain of 31db to 25db, and 25db to 19db in the new version, which can effectively ensure the stable output of V3mono, safe use and extend the service life.
You can check the voltage regulator 2576 on the board to see if it is LM2576HV or LM2576HVT,
or check the is it 1002 or 5101 written on the resistor next to the op-amp. Please refer to the attached.

If it is the old version of V3mono, 31DB meets the rated power input signal +1.2DB, and the rated power input signal is +6.5DB when the gain is 25DB. According to this condition, it only needs to increase 5.4DB to meet it.

Looking forward to hearing from you.

 

[nadrealista]

These are power levels I need for my HT set up to reach reference levels. Very interesting to see that for 85db Levels you do not need much power, however for peaks you need significant power.

Updated Power Calculation for 8 ft Listening Distance & 86 dB Speaker Sensitivity​

Step 1: Power for Reference Level (85 dB at 8 ft)​

• Sound drop-off: ~6 dB per doubling of distance
• 86 dB @ 1W/1m → 80 dB @ 2m (6.5 ft) → ~79 dB @ 8 ft
• Required gain: 85 dB - 79 dB = 6 dB
• Every +3 dB requires 2x power:
  • 2W → 82 dB
  • 4W → 85 dB

Each channel needs ~4W continuously to maintain reference level.

---

Step 2: Power for Dynamic Peaks (105 dB at 8 ft)​

• Required gain: 105 dB - 79 dB = 26 dB
• Every +10 dB requires 10x power:
  • 10W → 89 dB
  • 100W → 99 dB
  • ~200W → 105 dB

Each channel needs ~200W at 4Ω to hit 105 dB peaks.

---

Final Answer for 8 ft Distance & 86 dB Sensitivity​

• Reference level (85 dB): ~4W per channel
• Peak movie scenes (105 dB): ~200W per channel
• AVR should provide at least ~200W per channel @ 4Ω for full reference peaks.
• For real-world performance, an external amplifier for LCR is strongly recommended.

 

[Stoutblock]

Did they leave the XLR as 20db gain (fixed)?

 

[Settembrini]

Hi there,
has anyone tried the GAN power supply? Does it have any advantage over the regular 48/5 that comes with the single unit?
Cheers!

 

[wwjokerk]

I am having a serious problem. The power inlet just dropped off the main board... How can that happen?

I already contacted Fosi, that can get seriously dangerous.

 

[Deleted member 72219]

View attachment 440184
I am having a serious problem. The power inlet just dropped off the main board... How can that happen?

I already contacted Fosi, that can get seriously dangerous.

There can be some inconsistencies with the assembly. On one of my ZA3s, one of the RCA jacks just popped right out. They replace the whole board for free, though.

-Ed

 

[wwjokerk]

Aaaaarrrrrggghhh no music the next days. Just bought a new Burson Soloist 3XGT as preamp and now the amp is dead....

 

[wwjokerk]

Fosi will send two new boards, I will already receive the updated version with adjusted gain of 19/25 dB instead of 25/31 dB. That's why I insisted on getting two new boards and not only one. And I assume that my second amp will die shortly as well.... Maybe there is a mistake in one charge during production.

 

[Old_School_Brad]

Fosi will send two new boards, I will already receive the updated version with adjusted gain of 19/25 dB instead of 25/31 dB. That's why I insisted on getting two new boards and not only one. And I assume that my second amp will die shortly as well.... Maybe there is a mistake in one charge during production.

So you will now need to supply them twice the input voltage to reach the same output.

 

[antcollinet]

View attachment 440184
I am having a serious problem. The power inlet just dropped off the main board... How can that happen?

I already contacted Fosi, that can get seriously dangerous.

The solder hasn't "wetted" onto the connector pins at all. Suggests an incorrectly setup soldering process, and may mean there are dry joints on other components.

Definitely needs the whole unit (or PCB) replacing.

 

[antcollinet]

So you will now need to supply them twice the input voltage to reach the same output.

While true, any device that can supply 2V (unbalanced) or 4V (balanced) can still drive the amp to full power with some margin.

 

[wwjokerk]

I will use XLR wherever possible. Here the gain is anyway 'only' 20 dB.

And yes, the inlet sockets look pretty clean without any solder on it. That's why I wanted to have two new amps.

But seriously, we are talking about power supplies, 5 or 10A, about using different power cables, about DC noise in the power supply etc. and then all goes through those small cheaply soldered metal pins??? Isn't that ridiculous

 

[antcollinet]

and then all goes through those small cheaply soldered metal pins??? Isn't that ridiculous

Not really. Those pins are more than adequate for 10A if properly soldered. The poor soldering isn't an issue of cost, it's an issue of process control.

 

[Old_School_Brad]

I will use XLR wherever possible. Here the gain is anyway 'only' 20 dB.

And yes, the inlet sockets look pretty clean without any solder on it. That's why I wanted to have two new amps.

But seriously, we are talking about power supplies, 5 or 10A, about using different power cables, about DC noise in the power supply etc. and then all goes through those small cheaply soldered metal pins??? Isn't that ridiculous

Not really. It's the length that can be an issue if the current is high enough. And bad connections, of course.

 

[Stoutblock]

I know Fosi reduced the gain for the SE (RCA) input on the newer V3 mono amps from 25/31 to 19/25. Anyone know if the gain for balanced (XLR) input still 20db on the newer amps?

 

[Crash Alpha]

Anyone running a pair of these off the larger single PSU and with the Fosi DC power filter? I am just wondering if it is normal that when one amp is powered on, there is a noise coming out of the speaker for that amp. it remains until the other amp is turned on. It does it on both sides, so I know it isn't an amp defect.

 

[gwing]

Anyone running a pair of these off the larger single PSU and with the Fosi DC power filter? I am just wondering if it is normal that when one amp is powered on, there is a noise coming out of the speaker for that amp. it remains until the other amp is turned on. It does it on both sides, so I know it isn't an amp defect.

I also run a pair from single large supply with the filter and have no trouble with noise from the speakers, however I use optical connections to my DAC and then XLR cables to the amps. I also leave the amps in standby mode so both amps switch on almost simultaneously.

Running a quick test as you describe with either amp powered up on its own there is no perceptible noise whatsoever. This was at my normal gain settings with ears close to the speakers. Hope that helps ...

 

[Crash Alpha]

I also run a pair from single large supply with the filter and have no trouble with noise from the speakers, however I use optical connections to my DAC and then XLR cables to the amps. I also leave the amps in standby mode so both amps switch on almost simultaneously.

Running a quick test as you describe with either amp powered up on its own there is no perceptible noise whatsoever. This was at my normal gain settings with ears close to the speakers. Hope that helps ...

Interesting, I wonder where the fault is then. As both amps do it, I will have to do some testing of the DC filter and PSU.

 

[Crash Alpha]

I also run a pair from single large supply with the filter and have no trouble with noise from the speakers, however I use optical connections to my DAC and then XLR cables to the amps. I also leave the amps in standby mode so both amps switch on almost simultaneously.

Running a quick test as you describe with either amp powered up on its own there is no perceptible noise whatsoever. This was at my normal gain settings with ears close to the speakers. Hope that helps ...

So I just unplugged one of the amps from the PSU and turned on the other, it is now dead silent. I can only guess from this that the issue is with the PSU. I was hoping as such as I plan to replace it anyways.