Fosi Audio V3 Amplifier Review

[Lux_ury]

Thank you for your

Your asking a dicy question it's loaded one sorry for the pun. Some will say its not dependant upon loads Some will say its is and I can hear it. Some will say crossover, silver coated wires etc change the frequency sound dependant upon load and matching of amp and speaker etc. I keep it simple it has synergy or it doesn't I like it or I don't. Someone will proceed to produce a graph that will say contry to that difference or not. New speakers as new amplifiers are made to match as much as ohms law will permit. The curve it matches closer to 6ohms or 8ohms, but problems arise when you add load or music as the frequency is never the same load.
I could be talking rubbish and I'm sure someone will have data stating that fact (Smile emoji)

 

[ModDIY]

 

[strawberry644]

View attachment 296543

when I saw this I was wondering what a 6Ohm would look like? more like the 4, or a similar spike like the 8. maybe somewhere in-between?

 

[ddaudio]

when I saw this I was wondering what a 6Ohm would look like? more like the 4, or a similar spike like the 8. maybe somewhere in-between?

Yes, somewhere in between. But the problem is that speaker impedance will change over its frequency range, and the curve is very different for different speakers. So there may be ups and downs when using a "real" speaker, rather than the smooth slopes seen in the measurements against a constant 4 or 8 ohms.

In my understanding (someone please correct me if this is not a good explanation), the problem is that these cheap class D amplifiers have high (relative to the speaker's) and rising output impedance at higher frequencies (over 2-3 khz). This means that voltage delivered to the speaker is lost in proportion to the ratio between the amp's impedance and the speaker's impedance at any given frequency. At lower frequencies, the amp's output impedance is low enough for this not to matter. But if you have a speaker who's impedance swings appreciably at higher frequencies, you'll get corresponding peaks and dips in the frequency response of the amp/speaker combination.

@amirm's measurements show behaviour into 4 and 8 ohms, but some speakers can have swings to far higher impedances at some frequencies, which will result in corresponding swings in the frequency response of the amp/speaker combination. To get an idea of what might happen with a given speaker, try to find an impedance curve for it and look at its shape at higher frequencies.

But bear in mind that anomalies will be most pronounced at very high frequencies, perhaps beyond what many of us can hear anyway, depending on age. And also very narrow peaks will not necessarily be audible when listening to real music rather than test signals. You can also use equalisation. So having a "difficult" speaker doesn't necessarily mean these amps can't work with them.

 

[mccririck]

Why does the amp's output impedance increase with frequency?

 

[ddaudio]

Why does the amp's output impedance increase with frequency?

As far as I understand, it's because of the need for a low pass filter in the output of a Class D amp. There are solutions, but they would affect the power/cost ratio. Someone more knowledgeable than me may be able to go into more details.

 

[TurtlePaul]

Rising impedance with frequency is caused by inductance. This is no surprise because class D amps drive the load through an inductor, typically also in parallel with a capacitor. This makes a second order low pass filter to remove the ultrasonics.

Class D amps generate ultrasonics because of how they operate. Instead of having the output of the transistor chase the audio waveform, instead it turns the transistor fully on and fully off and varies the length of each pulse with how high the voltage of the wavefront should be. Transistors are very efficient at 0% and 100% power, so there is a huge power efficiency gain from this.

There are two problems the filter fixes:

1. The FCC problem. In class D, the audio frequency is modulated over a much higher frequency carrier wave. This frequency is typically 300-600 khz. The output of the amp is connected to several feet of speaker cable. This can accidentally make the amp into a 100 watt radio transmitter at the carrier frequency which can disrupt cell phone, short wave or AM radio signals.

2. The tweeter heater problem. Some speakers dont have an inductor in series with the tweeter and can have low inductance tweeters. You can end up with the tweeter’s thin voice coil burning several watts.

The answer to both is the same. The filter on most class D amps reduces the carrier frequency by about a factor of 1000x.

 

[duboisst7]

The fixed preout is annoying especially if you have it connected to a subwoofer. And not sure this was mentioned but the preout still works when the amp is off.

 

[wpo7]

The fixed preout is annoying especially if you have it connected to a subwoofer. And not sure this was mentioned but the preout still works when the amp is off.

the amps I'm currently running have same fixed out and work perfectly with a powered sub set to correct EQ roll off and Auto on off sensing. I don't see that being an issue as most modern day subs have these two features.

 

[Zek]

the preout still works when the amp is off.

This is because it is internally parallel connected to the RCA inputs.

work perfectly with a powered sub set to correct EQ roll off and Auto on off sensing. I don't see that being an issue

How do you adjust the volume of the subwoofer when changing the volume of the main speakers?
Only individually on the subwoofer itself, and on the amplifier for the main speakers.

 

[wpo7]

sorry once I set sub volume up to "disappear" in the blended mix I never adjust it's volume. it hasn't been a problem so far in the volume range of music levels I listen to.
my subs all have speaker level cable inputs as well so I would connect that way if I felt the need and completely ignore the line/sub out on any amp.
I should also add I always leave amp volume at 3/4 and use an app to control volume on streamer.

 

[GGroch]

sorry once I set sub volume up to "disappear" in the blended mix I never adjust it's volume. it hasn't been a problem so far in the volume range of music levels I listen to.

So, you are adjusting the "volume range" from your preamp or source, not the volume knob on the amp.


ADD off topic: I'm confused about the attention paid here to the Impedance dependence graph. It looks nasty because the steps are small, but can anyone really hear a +/- .5dB variance above 10K? Perhap we just feel better about our gear when we see straight lines?

 

[TurtlePaul]

So, you are adjusting the "volume range" from your preamp or source, not the volume knob on the amp.

I dont think that is what he is saying. I think the volume of his sub is fixed so he only adjusts the volume of the mains when he adjusts the volume knob.

 

[wpo7]

So, you are adjusting the "volume range" from your preamp or source, not the volume knob on the amp.


ADD off topic: I'm confused about the attention paid here to the Impedance dependence graph. It looks nasty because the steps are small, but can anyone really hear a +/- .5dB variance above 10K? Perhap we just feel better about our gear when we see straight lines?

So, you are adjusting the "volume range" from your preamp or source, not the volume knob on the amp.
Correct. but again if my needs change and will be adjusting volume regularly on the amp I simply connect the sub with the speaker level inputs.

 

[ddaudio]

So, you are adjusting the "volume range" from your preamp or source, not the volume knob on the amp.


ADD off topic: I'm confused about the attention paid here to the Impedance dependence graph. It looks nasty because the steps are small, but can anyone really hear a +/- .5dB variance above 10K? Perhap we just feel better about our gear when we see straight lines?

I agree that a 0.5dB difference (or 1dB as it is with 8 ohms) at 20khz is not significant. The issue we're discussing is slightly different - that real speakers do not have constant impedance and so when the amp has relatively high output impedance at higher frequencies it can lead to the frequency response tracking the speaker's impedance curve.

 

[GGroch]

Correct. but again if my needs change and will be adjusting volume regularly on the amp I simply connect the sub with the speaker level inputs.

Makes sense. I don't have a problem with it as I use it the same way, as a power amp. If they want to maintain the fixed out for V3mkII, they should go to a separate power switch so you never have to touch the volume knob, and relabel the rear output "Line Out".

 

[Zek]

I like the pre-out for daisy-chaining another V3 / Class D amp, and then use high-level inputs on my Monoprice subs.

And then you have to adjust the volume in two places - on the first amplifier for the main speakers and on the second amplifier for the subwoofer.

 

[mccririck]

I agree that a 0.5dB difference (or 1dB as it is with 8 ohms) at 20khz is not significant. The issue we're discussing is slightly different - that real speakers do not have constant impedance and so when the amp has relatively high output impedance at higher frequencies it can lead to the frequency response tracking the speaker's impedance curve.

Would the impedance curves of the amp and speaker not just be added together?

 

[Palomino]

I've got kind of a noob question here. If it's more appropriate in another area of the forum, I'll be glad to delete and re-post.

Anyways, I have an old Rotel RSP-1572 AV preamp/processor that was gifted to me by a friend. It's been sitting in my closet unused for about 5 years now. Its spec sheet says Preamp Output Level/Output Impedance = 1.0V/1k ohms.

I can't find any info on the V3's input sensitivity, but I think I've heard mentioned on ASR that 2V output is a "standard" of sorts for getting the most out of amps. Would using the Rotel to drive a Fosi V3 be problematic? If so, what would be the effects on the various measurements?

Thanks!

 

[Toku]

Rising impedance with frequency is caused by inductance. This is no surprise because class D amps drive the load through an inductor, typically also in parallel with a capacitor. This makes a second order low pass filter to remove the ultrasonics.

Class D amps generate ultrasonics because of how they operate. Instead of having the output of the transistor chase the audio waveform, instead it turns the transistor fully on and fully off and varies the length of each pulse with how high the voltage of the wavefront should be. Transistors are very efficient at 0% and 100% power, so there is a huge power efficiency gain from this.

There are two problems the filter fixes:

1. The FCC problem. In class D, the audio frequency is modulated over a much higher frequency carrier wave. This frequency is typically 300-600 khz. The output of the amp is connected to several feet of speaker cable. This can accidentally make the amp into a 100 watt radio transmitter at the carrier frequency which can disrupt cell phone, short wave or AM radio signals.

2. The tweeter heater problem. Some speakers dont have an inductor in series with the tweeter and can have low inductance tweeters. You can end up with the tweeter’s thin voice coil burning several watts.

The answer to both is the same. The filter on most class D amps reduces the carrier frequency by about a factor of 1000x.

It is true that class D amplifier products generate electromagnetic noise due to PWM. Noise radiates electromagnetic waves of noise components to the surroundings through speaker cables. A low-pass filter is attached to the output to prevent it. However, even with a filter, it is not possible to completely prevent leaks. Therefore, in each country, the permissible leakage of electromagnetic waves is stipulated by law, and each company's products are manufactured to comply with it. The EU has the EMC Directive and the US has the FCC standard.

In addition to audio equipment, there are many familiar products that emit electromagnetic noise, such as washing machines, refrigerators, vacuum cleaners, air conditioners, microwave ovens, televisions, solar power generators, and mobile chargers.
Noise from these devices appears mainly as noise in the LF/MF/HF bands. Noise can be detected by placing an AM radio close to the speaker wires of a Class D amplifier. But you can't hear it if you're 1m away. Fortunately, the noise from a Class D amplifier is very weak.

I have previously worked to investigate the actual state of disturbances caused by EMI and EMS in the EMC Directive. Modern society is built in a high-level electromagnetic noise space.

About the EMC Directive stipulated by the EU
EMI is Electromagnetic Interference. It means the electromagnetic noise emitted from the machine itself, and is evaluated by the emission test. EMS is Electromagnetic Susceptibility. This refers to the influence of electromagnetic noise emitted from other equipment, and is evaluated by immunity testing. EMC stands for Electromagnetic Compatibility. This means the influence of electromagnetic noise emitted from your own device and electromagnetic noise emitted from other devices. In other words, EMC is a concept that combines EMI and EMS.
Search online for details.