Fosi Audio SK-01 Headphone Amplifier Review

[IAtaman]

Strange. If I interpret your exerpt from the spec sheet correctly, most of the data seen here have been improved (impedance of the DAC) and some improvements from the spec sheet of the successor have not even been listed by you: i.e. the DAC has changed from HyperStream® II DAC to HyperStream® IV Dual DAC etc.

>>The ES9039Q2M SABRE® DAC improves on previous designs to include: • TDM & SPI support for more options in connectively • Lower power consumption than previous generations which includes the Hyperstream IV DAC modulator • New hardware mode (HW) programmability alleviates I2C/SPI programming for ease of use...
<<

To the contrary the size of the corresponding new chip has increased

Hyperstream IV and DAC chip resistance changed from 706 to 390ohms! Oh, I missed those. Tthey are game changers. I think you are right, we should never buy any DAC that has anything less than 9039 if we truly care about music.

 

[burkm]

Why so aggressive ? I just wondered, why they have chosen an "old" chip for a brand new headphone amp design, if the "old" chip has already a successor ?

 

[IAtaman]

You were wondering why they are using an "outdated" chip. I asked you if you know what has improved in the new chips. You did not know. So I checked the data sheets and pointed out to you that the differences were non impactful. You responded by saying, and I misquote,"strange, you haven't mentioned these two very important changes" I imagine with a vocal fry maybe, despite the fact that you have no idea what they actual mean. I find that irritating.

 

[burkm]

 

[Kimbrough Xu]

Thanks for the test Amir. Nice features and ok performance for $80.

Loundness functionality I think is a great feature to have. At least if you occasionally listen at low volume. It would have been even better if the Fosi Audio SK-01 had variable loudness, but that's perhaps too much to ask for the $80 it costs.

Loudness or not is discussed in this interesting thread:

Should you use Fletcher-Munson loudness compensation?

I know many are familiar with the Fletcher-Munson or equal loudness curves. In a nutshell: “The Fletcher Munson Curve contains a set of graphs that show that when you listen to music at a lower volume, mid-range frequencies will be sound more prominent, whereas high and low frequencies will be...

audiosciencereview.com

I think that Amir also created some thread about the Fletcher Munson Curve, but I couldn't find it now.

I havent heard of variable loudness, what is that?

 

[staticV3]

I havent heard of variable loudness, what is that?

Equal loudless compaensation is a feature that tries to make tonality more consistent when listening at very low or very high volumes.
You can look up fletcher munson curves, equal loudness contours, or ISO226 to learn more.

Ideally, this feature should be SPL referenced and be continuously variable to adapt to different SPLs.

The Fosi doesn't have that. It only has one fixed equal loudness compensation, for late night listening at low volumes.

 

[burkm]

"Loudness" describes indirectly the sensitivity of the human ear to the effects of different frequencies and sound pressures. The ears stays at medium frequencies with the highest sensitivity and looses this to the bass and (less) to the higher frquencies. depending on the sound pressure Your own age takes a toll here too for the highest frequencies, a 70 year old person hears (statistically) only just 8 kHz, a 40 year old about 13 kHz. There exists a formula for this...

The ISO norm 226:2003 describes this in detail (graphs). Fletcher / Munson detected this effect almost 100 years ago and it has been refined somewhat in the meantime by others. The "variable loudness compensation" creates a compensating profile so that the human ear will see those effected frequencies of same loudness even at lower sound pressures, because loudspeakers and earphones have their own efficiencies in presenting their output and human hearing varies individually a bit also, thus it should be "variable". "Old" implementations and less sophisticated ones simplify this and fix the loudness to an (statistically) expected mean, but it fails to be exact then.
The Fosi Audio SK01 has implemented a somewhat fixed loudness compensation curve and thus simplifies it often to the worse...

 

[Kimbrough Xu]

Equal loudless compaensation is a feature that tries to make tonality more consistent when listening at very low or very high volumes.
You can look up fletcher munson curves, equal loudness contours, or ISO226 to learn more.

Ideally, this feature should be SPL referenced and be continuously variable to adapt to different SPLs.

The Fosi doesn't have that. It only has one fixed equal loudness compensation, for late night listening at low volumes.

Thank you, so the fosi sk01 just for very low volumes, and the variable loudness is for both very low volumes and very high volumes.

 

[Kimbrough Xu]

"Loudness" describes indirectly the sensitivity of the human ear to the effects of different frequencies and sound pressures. The ears stays at medium frequencies with the highest sensitivity and looses this to the bass and (less) to the higher frquencies. depending on the sound pressure Your own age takes a toll here too for the highest frequencies, a 70 year old person hears (statistically) only just 8 kHz, a 40 year old about 13 kHz. There exists a formula for this...

The ISO norm 226:2003 describes this in detail (graphs). Fletcher / Munson detected this effect almost 100 years ago and it has been refined somewhat in the meantime by others. The "variable loudness compensation" creates a compensating profile so that the human ear will see those effected frequencies of same loudness even at lower sound pressures, because loudspeakers and earphones have their own efficiencies in presenting their output and human hearing varies individually a bit also, thus it should be "variable". "Old" implementations and less sophisticated ones simplify this and fix the loudness to an (statistically) expected mean, but it fails to be exact then.
The Fosi Audio SK01 has implemented a somewhat fixed loudness compensation curve and thus simplifies it often to the worse...

Is this the Fletcher-Munson curves? The lower sound pressure should gain more bass, so the variable loudness means different sound pressure should have different loudness curvers.

 

[burkm]

Because it is "fixed" it will work "spmehow" as intended just for low volumes. At higher volumes it is not useful because of the fixed adjustment. Thus it is not that useful and has to be disengaged manually then...