Fosi Audio V3 Mono Amplifier Review

[jmdesignz2]

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.

Pics of setup ?

 

[jmdesignz2]

Mine are connected to 7awg cables which weigh more than the amps lol

 

[jEDGEc0m]

After an initial failure of 1 of the amps, which FOSI promptly replaced post haste

What do you suspect went wrong? What kind of failure symptom(s)? Dead on arrival?

I do hope we get good durability from them. Like 20+ years

 

[jEDGEc0m]

I've had a pair a few days now.
I'd previously reported that cutting or supplying power to them from the mains, with the Fosi's power switch left in "On" position didn't make any relevant audible sound through the speakers... I think that might have been when I was trying with the 32V 5A power supply. I purchased them with 2x separate 48V 5A power supplies.

Using the 48V 5A power supply, when cutting power to them by pulling the plug at the source/powerboard, I have noticed the speakers make a brief static sound - not much bass in it by the sounds. Maybe a quarter of a second in duration. Audible, but certainly not loud. Certainly doesn't seem hazardous in level.

Is this unique?

Will test again when I have the chance whether it occurs only with 48V, and not with 32V.

 

[jmdesignz2]

I've had a pair a few days now.
I'd previously reported that cutting or supplying power to them from the mains, with the Fosi's power switch left in "On" position didn't make any relevant audible sound through the speakers... I think that might have been when I was trying with the 32V 5A power supply. I purchased them with 2x separate 48V 5A power supplies.

Using the 48V 5A power supply, when cutting power to them by pulling the plug at the source/powerboard, I have noticed the speakers make a brief static sound - not much bass in it by the sounds. Maybe a quarter of a second in duration. Audible, but certainly not loud. Certainly doesn't seem hazardous in level.

Is this unique?

Will test again when I have the chance whether it occurs only with 48V, and not with 32V.

Never cut the power by any other method than the switch. Why’d you do this

 

[jEDGEc0m]

Never cut the power by any other method than the switch. Why’d you do this

There could be several reasons. Convenience - depending where the amps end up located, getting to the amp's power switches might be quite impractical, as we have little need to interact with them once installed.

But also, aren't some people using smart switches or whatever, which would be the same result?

Also, if the power happened to go out while they were in use, this would happen outside of your control, so it shouldn't be hazardous to equipment anyway, no?

 

[DavidR]

I have a question related to the V3 input impedance of the XLR. I've been experimenting with a hand-made balanced attenuator (two 10-turn pots) to test a idea for using it to make better distortion measurements with inexpensive audio interfaces. I'm using a Scarlett 2i2 Gen 4. All has gone as I hoped it would until I got to actually connecting to and testing the V3 Mono. Long story short, I have a DVM connected to the output of the attenuator connection to the V3 input, set for 422mV. The V3 should (has previously) output 4.47V. However, it was not. The input voltage (2i2/attenuator) output is 422mV...until I turn on the V3 Mono. Then the voltage as measured at the V3 input drops to 316mV. Every time. The V3 output voltage is 3.43V.

In that past with a direct connection of 2i2 to V3 I simply increased the REW generator until the 2i2 output was 422mV with V3 output at 4.47V. My question is: why would the V3 input load the incoming connection differently when powered on vs. powered off?

 

[antcollinet]

I have a question related to the V3 input impedance of the XLR. I've been experimenting with a hand-made balanced attenuator (two 10-turn pots) to test a idea for using it to make better distortion measurements with inexpensive audio interfaces. I'm using a Scarlett 2i2 Gen 4. All has gone as I hoped it would until I got to actually connecting to and testing the V3 Mono. Long story short, I have a DVM connected to the output of the attenuator connection to the V3 input, set for 422mV. The V3 should (has previously) output 4.47V. However, it was not. The input voltage (2i2/attenuator) output is 422mV...until I turn on the V3 Mono. Then the voltage as measured at the V3 input drops to 316mV. Every time. The V3 output voltage is 3.43V.

In that past with a direct connection of 2i2 to V3 I simply increased the REW generator until the 2i2 output was 422mV with V3 output at 4.47V. My question is: why would the V3 input load the incoming connection differently when powered on vs. powered off?

Difficult to say specifically for the V3 without knowing the circuit - but it is not a surprise.

For example. If the input is driving an op amp stage the voltage rails for the op amp will be zero V when powered off - this will result in current paths from the analogue input, via the input and output protection diodes to the 0V power rails (see picture). When there is power supplied to the op amp, these diodes are reverse biased - so no current path.

This will apply to any part of the input circuit, where protection diodes to power rails are provided - whether internally in ICs, or added to the circuit on the PCB.

 

[Nopiano]

There could be several reasons. Convenience - depending where the amps end up located, getting to the amp's power switches might be quite impractical, as we have little need to interact with them once installed.

But also, aren't some people using smart switches or whatever, which would be the same result?

Also, if the power happened to go out while they were in use, this would happen outside of your control, so it shouldn't be hazardous to equipment anyway, no?

I found that the auto sensing worked perfectly on my pair. Turn the preamp off by remote, then the Fosis shut down a few minutes later.

 

[jEDGEc0m]

I found that the auto sensing worked perfectly on my pair. Turn the preamps off by remote, then the Fosis shut down a few minutes later.

Glad to hear auto-standby working properly for you and your setup.

If I were going to use it, I would want to check how much power they draw in "standby" - seems some report they're still warm, and much more than 0.5W being drawn.

Do you know how much power yours are drawing from the power supply when in standby?

I just tested one. on 48V 5A power supply:
19W on, with no signal/sound.
it very quickly went to standby! just a few minutes it seems.
about 9W in standby. This is kinda stupid, and way too much. But hey, it's still half the power. That's something.

I'll be turning them off when not in use.

with 32V 5A power supply:
12W on, no signal.
7W standby. still too much.

Edit: unofficial values. Just measured with hardware store thing of this type:
and also values were lower a bit later... maybe after amps warmed up a bit?? as below...
edit2: next day, tried again, with them from cold, and got the lower values again, as below.

with 32V 5A powersupply, no signal, after amps been on a while - 5.8W
3.3W when it went to standby
With nothing connected, that 32V 5A fosi powersupply from a V3 stereo model is reading 0.1W

With nothing connected, 48V 5A powersupply is fluctuating between 0.3W and 0.6W
with no signal, amp on, 9.4W
went to standby quick - heard relay tick - 4.2W

 

[Nopiano]

Glad to hear auto-standby working properly for you and your setup.

If I were going to use it, I would want to check how much power they draw in "standby" - seems some report they're still warm, and much more than 0.5W being drawn.

Do you know how much power yours are drawing from the power supply when in standby?

I just tested one. on 48V 5A power supply:
19W on, with no signal/sound.
it very quickly went to standby! just a few minutes it seems.
about 9W in standby. This is kinda stupid, and way too much. But hey, it's still half the power. That's something.

I'll be turning them off when not in use.

with 32V 5A power supply:
12W on, no signal.
7W standby. still too much.

I see! Yes, they stayed warm, but so does my Linn preamp, as it’s from before the 0.5watt standby regime that rules today!

 

[jEDGEc0m]

Using the 48V 5A power supply, when cutting power to them by pulling the plug at the source/powerboard, I have noticed the speakers make a brief static sound - not much bass in it by the sounds. Maybe a quarter of a second in duration. Audible, but certainly not loud

With 32V 5A power supply, pulling the power while the Fosi amp's switch is "On" does not make the static noise. Just a tiny click. 48V does make the static noise.

 

[jEDGEc0m]

I see! Yes, they stayed warm, but so does my Linn preamp, as it’s from before the 0.5watt standby regime that rules today!

Yeah, fair to say this standby is not very well implemented.

 

[AnalogSteph]

For example. If the input is driving an op amp stage the voltage rails for the op amp will be zero V when powered off - this will result in current paths from the analogue input, via the input and output protection diodes to the 0V power rails (see picture). When there is power supplied to the op amp, these diodes are reverse biased - so no current path.

I could understand that, too, but according to @DavidR the amplitude goes down when the amp is on. That's the real puzzler. Based on the Scarlett's 100 ohm output impedance, this would signify an input impedance of a measly 300 ohms, which seems very low.

 

[DavidR]

Is it possible that this is due to the use of PFFB? REW Levels display shows a momentary delay of output after the "click" of the start in the V3, then a bit of a jump up about halfway to ultimate level, then a gradual output increase to the final output level. Curious action of the electronics.

 

[antcollinet]

That's the real puzzler

Perhaps it still makse sense. With power on, the input at the op amp will be 0V (virtual earth). With power off, the input to the op amp will be rail voltage (Probably 0V) + diode drop of 0.7V? assuming a positive input voltage. Or -0.7V for a negative input. That will result in a lower load when powered off depending on the input circuit. (Im thinking of something like the shunt feedback receiver shown here)

Practical Line Input Circuits

michaelfidler.com

Or am I getting this hopelessly wrong?

That's the real puzzler. Based on the Scarlett's 100 ohm output impedance, this would signify an input impedance of a measly 300 ohms, which seems very low.

But it is not just the Scarletts output impedance - it is that plus whatever is the impedance of the attenuator (Two pots) which could be MUCH higher.

 

[DavidR]

Perhaps it still makse sense. With power on, the input at the op amp will be 0V (virtual earth). With power off, the input to the op amp will be rail voltage (Probably 0V) + diode drop of 0.7V? assuming a positive input voltage. Or -0.7V for a negative input. That will result in a lower load when powered off depending on the input circuit. (Im thinking of something like the shunt feedback receiver shown here)

Practical Line Input Circuits

michaelfidler.com

Or am I getting this hopelessly wrong?



But it is not just the Scarletts output impedance - it is that plus whatever is the impedance of the attenuator (Two pots) which could be MUCH higher.

I just checked the 2i2 output voltage with and without the input connected to the V3 when the power is off. There is no change at all with power off. No load change or at least not enough to make a measurable change in voltage. Power in and the 2i2 output (V3 input) level drops. For reference there is a 4ohm load on the V3.

 

[antcollinet]

I just checked the 2i2 output voltage with and without the input connected to the V3 when the power is off. There is no change at all with power off. No load change or at least not enough to make a measurable change in voltage. Power in and the 2i2 output (V3 input) level drops. For reference there is a 4ohm load on the V3.

Do you know what the resistance is of your attenuator, at the setting you have it at?

 

[DavidR]

Do you know what the resistance is of your attenuator, at the setting you have it at?

No, I considered that, but I tested with both at the low level (maybe 10 of 100) and near maximum (90 of 100). The results were the same. My attenuator is 20k in total. I found online someone who says the V3 input impedance for XLS is 5k.

 

[antcollinet]

No, I considered that, but I tested with both at the low level (maybe 10 of 100) and near maximum (90 of 100). The results were the same. My attenuator is 20k in total. I found online someone who says the V3 input impedance for XLS is 5k.

Even at 90 out of 100 you are still putting 2K in series with your input, assuming the pot is linear - and the rated input impedance of the Fosi may only apply with power on - depending on what the input circuit is.

I think you just have to take the "it is what it is" approach - unless you are prepared to work out what the input circuit looks like - or ask Fosi to provide it for you.