Fosi Audio V3 Mono Amplifier Review

[burkm]

1 Ohm impedance ? That sounds quite problematic, because the V3 Monos are only specified down to 2 Ohm, if I remember correctly.
There are probably only very few amps, which can amplify at that impedance. if any. Most speakers nowadays are specified at 4-8 Ohm, going down to a min. 3 Ohm.
Those V3 Monos might get quite hot at that impedance, if they don't shut down before reaching that mark... I think, they will shut down before that.
I remember some old Infinitys, which went down to 2 Ohm, but they were already quite problematic because of the needed power amps for that impedance.
And those power amps where huge, because of the needed current because of amplification at low frequencies.
Note: The minimum impedance is usually close to those low frequencies, were the most power is needed.
There are none I know of today...

 

[gwing]

1 Ohm impedance ? That sounds quite problematic, because the V3 Monos are only specified down to 2 Ohm, if I remember correctly.
There are probably only very few amps, which can amplify at that impedance. if any. Most speakers nowadays are specified at 4-8 Ohm, going down to a min. 3 Ohm.
Those V3 Monos might get quite hot at that impedance, if they don't shut down before reaching that mark... I think, they will shut down before that.
I remember some old Infinitys, which went down to 2 Ohms, but they were already quite problematic because of needed power amps for that impedance.
And those power amps where huge, because of the needed current because of amplification at low frequencies.
The minimum impedance is usually at low frequencies, were the most power is needed.
There are none I know of today...

That's all true, but you can drive even ridiculously low impedance speakers without anything special in the way of amplification if you stay within its power limits, of course if that low impedance speaker is also of low sensitivity and you want high sound pressure levels this isn't going to happen. As a temporary stopgap I used to drive some big Magneplanar 3A's with a tiny 10W valve amp and it was OK but clearly lacking grunt - I'd expect the V3s here to do OK with the Apogees provided you are *very* careful with output level.

 

[antcollinet]

That's all true, but you can drive even ridiculously low impedance speakers without anything special in the way of amplification if you stay within its power limits, of course if that low impedance speaker is also of low sensitivity and you want high sound pressure levels this isn't going to happen. As a temporary stopgap I used to drive some big Magneplanar 3A's with a tiny 10W valve amp and it was OK but clearly lacking grunt - I'd expect the V3s here to do OK with the Apogees provided you are *very* careful with output level.

Exactly - at a typical low level (1 to 5W where most people listen) then most amps will happily drive very low impdeances - since the current required is much lower than when closer to full power of the amp.

An amp that can drive 4 ohms at full output, can drive 2 ohms at 1/2 of the voltage output - or 1 ohm with 1/4 of the voltage output.

(corresponding to 1/4 and 1/16th of the 4ohm power rating respectively)

Of course though - headroom for peaks will be lost.

 

[Sokel]

Exactly - at a typical low level (1 to 5W where most people listen) then most amps will happily drive very low impdeances - since the current required is much lower than when closer to full power of the amp.

An amp that can drive 4 ohms at full output, can drive 2 ohms at 1/2 of the voltage output - or 1 ohm with 1/4 of the voltage output.

(corresponding to 1/4 and 1/16th of the 4ohm power rating respectively)

Of course though - headroom for peaks will be lost.

Problem may be that TPA's thermals skyrocket as we go lower in impedance.
That would need special care.

(the chart does not even go down to 2 Ohm, more so at 1 Ohm)

 

[gwing]

Exactly - at a typical low level (1 to 5W where most people listen) then most amps will happily drive very low impdeances - since the current required is much lower than when closer to full power of the amp.

An amp that can drive 4 ohms at full output, can drive 2 ohms at 1/2 of the voltage output - or 1 ohm with 1/4 of the voltage output.

(corresponding to 1/4 and 1/16th of the 4ohm power rating respectively)

Of course though - headroom for peaks will be lost.

Here I start to wonder a bit about how everything is going to pan out. At 1 ohm said amp will indeed be only able to provide 1/4 of the voltage but it will still be providing the same current as it does driving 4 ohms at full output and that I think means the speaker coil is going to be driven just as hard so, in theory, if I am correct, we might get the same maximum output sound levels driving 1 ohm or 4 ohm speakers.

In practice that doesn't seem to happen, at least to my ears, so perhaps there is something else going on such as impedence drops in the speaker being caused by driver or crossover inefficiencies at that frequency so the extra amps sunk into the suckout points may be being wasted rather than actually driving the driver coils. Does that make sense or am I talking rubbish?

 

[burkm]

The question is, if the V3 Mono does accept the 1 Ohm load at all, even when driven at low volume. I don't know about the protection means of the amp and how they will be activated, thus...

 

[gwing]

The question is, if the V3 Mono does accept the 1 Ohm load at all, even when driven at low volume. I don't know about the protection means of the amp and how they will be activated, thus...

Your answer is given above by Antcollinet:

"An amp that can drive 4 ohms at full output, can drive 2 ohms at 1/2 of the voltage output - or 1 ohm with 1/4 of the voltage output."

So if you have your volume control turned down so the amp only produces 1/4 of the voltage you have it turned up to for 4 ohm speakers you will be OK.

 

[charlielaub]

Regarding "1 Ohm" and the amplifier: Keep in mind that if you are talking about a speaker, the impedance is both varying with frequency (it's magntiude) but also the phase angle is changing all over the place. Driving a 1 Ohm but completely resistive load (phase angle is zero) is different than when that load is very capacitive or reactive because of how the load will push and pull current differently from how the input signal dictates, and the output stage must be able to handle that. Also, if you are talking about a driver that is rated to be "1 Ohm" nominal, the actual impedance will often fall to less than that at some frequency above resonance before climbing again. The devil is in these details...

 

[antcollinet]

Problem may be that TPA's thermals skyrocket as we go lower in impedance.
That would need special care.

(the chart does not even go down to 2 Ohm, more so at 1 Ohm)

Right, but the first vertical line on that chart is 50 W. I'm talking about operating at an average power of 1/10th of that.

 

[Carlton80@0]

Regarding "1 Ohm" and the amplifier: Keep in mind that if you are talking about a speaker, the impedance is both varying with frequency (it's magntiude) but also the phase angle is changing all over the place. Driving a 1 Ohm but completely resistive load (phase angle is zero) is different than when that load is very capacitive or reactive because of how the load will push and pull current differently from how the input signal dictates, and the output stage must be able to handle that. Also, if you are talking about a driver that is rated to be "1 Ohm" nominal, the actual impedance will often fall to less than that at some frequency above resonance before climbing again. The devil is in these details...

I believe the Apogee presents a resistive load. Running in active mode, bypassing the huge MR capacitor bank and woofer coil.

 

[jEDGEc0m]

a couple V3 mono for the midrange and tweeter. The Fosi don't sound intimidated at all.

Hi JohnnyNG

If you feel like responding... How would you describe tweeter hiss with your 4-way active setup?
What tweeter are you using?
Any hardware attenuation between amp and tweeter?

(For a while I had an active project-speaker, with 2x Fosi V3 stereo, from MOTU Ultralite mk5, into an efficient JBL compression driver and waveguide (Studio 630 speaker) - Since the tweeter had so much volume anyway, and there was some audible hiss at ~2m listening position, I attenuated with some basic resistors, which toned down the hiss, and seemed to work otherwise)

 

[JohnnyNG]

Hi JohnnyNG

If you feel like responding... How would you describe tweeter hiss with your 4-way active setup?
What tweeter are you using?
Any hardware attenuation between amp and tweeter?

(For a while I had an active project-speaker, with 2x Fosi V3 stereo, from MOTU Ultralite mk5, into an efficient JBL compression driver and waveguide (Studio 630 speaker) - Since the tweeter had so much volume anyway, and there was some audible hiss at ~2m listening position, I attenuated with some basic resistors, which toned down the hiss, and seemed to work otherwise)

Just checked and with only the V3 mono powered my ear has to be about 3-4” from the tweeter to start hearing hiss. Only cable and binding posts between amp and speaker, no hardware attenuation.

I’m using this coax: https://sica.it/prodotto/5-5-c-15-cp/

 

[jEDGEc0m]

Just checked and with only the V3 mono powered my ear has to be about 3-4” from the tweeter to start hearing hiss. Only cable and binding posts between amp and speaker, no hardware attenuation.

I’m using this coax: https://sica.it/prodotto/5-5-c-15-cp/

Thanks!

Cool, that's very impressively quiet. Enjoy!

 

[norman bates]

Many loudspeakers hit a 45 degree phase angle (loudspeaker driver is kind of an inductor), that draws double the current than a resistor.

So your 100 watt amp probably starts clipping beyond 50 watts……

And hopefully the amplifiers power supply can provide it.

I’m old, so a 100w per Chanel amp should do 200 into 4, now double that for the phase angle.

So I’d love to see a 800va transformer.

But this is class d.

 

[antcollinet]

a 45 degree phase angle (loudspeaker driver is kind of an inductor), that draws double the current than a resistor.

You may need to rethink that statment.


An inductor which is 4ohm at a particular frequency - will take the same current as a 4ohm resistor if the voltage is constant - it will just be out of phase with the voltage.

 

[norman bates]

Google Search

"When an audio signal with a 45-degree phase angle is sent to a 4-ohm speaker, the amplifier must manage the electrical load with significant inefficiency.

• Greater stress on the amplifier: The combination of low impedance (4 ohms or less) and a large phase angle (like 45 degrees) at the same frequency creates a "difficult load." The amplifier has to deliver more current than the nominal impedance rating suggests, which can cause it to become unstable or clip (distort) at higher volumes.
• Increased heat dissipation: A 45-degree phase angle is considered a worst-case scenario for heat dissipation in a real-world loudspeaker. At this angle, an amplifier's output transistors may need to dissipate twice as much heat as they would with a purely resistive load. For example, if the amplifier delivers 100 watts of power to the speaker at this angle, it will heat up as if it were delivering 400 watts into a resistive load."

Worse than I thought............

 

[Palmer]

Bought a Fosi V3 Stereo amp based on the review(s) here, for an extra pair of monitors. So far I have to say I am impressed, hard to beat for that price.
Going to buy another one. Too many speakers in the house...

 

[antcollinet]

Google Search

"When an audio signal with a 45-degree phase angle is sent to a 4-ohm speaker, the amplifier must manage the electrical load with significant inefficiency.

• Greater stress on the amplifier: The combination of low impedance (4 ohms or less) and a large phase angle (like 45 degrees) at the same frequency creates a "difficult load." The amplifier has to deliver more current than the nominal impedance rating suggests, which can cause it to become unstable or clip (distort) at higher volumes.
• Increased heat dissipation: A 45-degree phase angle is considered a worst-case scenario for heat dissipation in a real-world loudspeaker. At this angle, an amplifier's output transistors may need to dissipate twice as much heat as they would with a purely resistive load. For example, if the amplifier delivers 100 watts of power to the speaker at this angle, it will heat up as if it were delivering 400 watts into a resistive load."

Worse than I thought............

But it is not the phase shift that results in higher current - as you stated it. It is an impedance dip that will do that - whether or not there is a corresponding phase shift.

It should also be pointed out that the difficult load caused by a phase shift (Peak of current corresponding to a peak of voltage across the ouput transistors resulting in high dissipation) is only a problem for traditional class A or AB designs, where the transisitors operate in the linear region. Class D amplifiers (On topic for this thread) don't have such a problem.

 

[norman bates]

guess I need to try one.

 

[burkm]

But it is not the phase shift that results in higher current - as you stated it. It is an impedance dip that will do that - whether or not there is a corresponding phase shift.

It should also be pointed out that the difficult load caused by a phase shift (Peak of current corresponding to a peak of voltage across the ouput transistors resulting in high dissipation) is only a problem for traditional class A or AB designs, where the transisitors operate in the linear region. Class D amplifiers (On topic for this thread) don't have such a problem.

Hight current has to be handled by the power transistors (for output) no matter, what class the amps have. It is this matter and its corresponding cooling, which i.e. limits the output of the amp.