Fosi Audio SK-01 Headphone Amplifier Review

[activio]

Hello! My problem is the following. I bought fosi audio v3 to replace aiyima a07 and realized that aiyima sounds much louder. I know that fosi v3 has a different potentiometer, and even with the knob turned to maximum, it does not give the volume that aiyima gave. Then I started looking for information, and realized that fosi also lacks the input voltage from my DAC aiyima t10. Then I went and bought a DAC topping D50, and even now I understand that there is still no volume reserve and if you connect heavy speakers, it will not be enough. The question is next. Will it help me to buy sk01, will I be able to get fosi v3 to the maximum?
My power supply is 48/5 gophert cps-6005

 

[Bruce Morgen]

Hello! My problem is the following. I bought fosi audio v3 to replace aiyima a07 and realized that aiyima sounds much louder. I know that fosi v3 has a different potentiometer, and even with the knob turned to maximum, it does not give the volume that aiyima gave. Then I started looking for information, and realized that fosi also lacks the input voltage from my DAC aiyima t10. Then I went and bought a DAC topping D50, and even now I understand that there is still no volume reserve and if you connect heavy speakers, it will not be enough. The question is next. Will it help me to buy sk01, will I be able to get fosi v3 to the maximum?
My power supply is 48/5 gophert cps-6005

I just got mine, and (going by ear) it's got unity gain with the "VOL" knob dimed in "-10dB" mode and gets considerably louder in the "0dB" mode -- I can't say for sure if that's enough of a boost to solve your problem, but there's definitely substantial gain in that mode.

 

[Bruce Morgen]

Check again. I've had it for about 2 months and the headphone output is stronger than the line out I use for the amp! Overall, as a preamp with EQ controls it sounds incredibly good.

I can confirm that "the headphone output is stronger than the line out," probably by a few dB -- which might be useful in some preamp applications where the gain necessary to drive the power amp (or some such) is otherwise hard to come by. BTW, you can safely ignore the flashing blue LED in preamp applications -- according to the typically skimpy manual, that light has to flash blue and red to indicate (Fosi's idea of) the sort of "excessive volume" that apparently triggered protection during Amir's testing.

 

[Ingineer]

FYI, I did a teardown of this unit, and the main headphone amp is an SG Micro SGM8262-2 Ultra-low noise op-amp. (link below) The Line out is simply a divider off the headphone outputs. They also have a LM358 which is used to drive the flashing red/blue LEDs. Contrary to someone else's assessment, the LM358 is NOT in the audio path. There are also 4x JRC 4580 op-amps for the tone control/loudness processing. It is possible to increase/decrease the gain easily by changing 2 resistors if desired, which will then operate with the gain switch.

One cool aspect is the power supply is isolated from the audio circuit, so ground loops will not be a problem. They use a charging controller for the 2500mAh Lithium-Ion cell which regulates to 4.2v, then this and the battery is fed into a boost converter that feeds an isolated 5V to +/- 12v DC-DC converter, and the 12 dual-rail feeds all the op-amps.

Datasheet for the SG Micro SGM8262-2 Ultra-low noise op-amp: https://www.sg-micro.com/uploads/soft/20230303/1677822897.pdf
Datasheet for the JRC 4580 op-amps: https://www.ti.com/lit/ds/symlink/rc4580.pdf
Datasheet for the LM358 used as LED driver: https://www.onsemi.com/download/data-sheet/pdf/lm358-d.pdf

 

[Bruce Morgen]

FYI, I did a teardown of this unit

Thanks for sharing the knowledge!

 

[Chibrax]

Hey guys ! got mine a month ago really good for the price I use it with the SE out of my topping D90 in my living room for movies, music and tv and its really a blast.
Still I use quite a lot of software EQ when not using the SK01 controls (and while they are a really cool feature I must admit I prefer EQapo) and the power seem very impedance dependant it drives quite well some of my hp and not so good with others (not particularly power hungry ones like the HD280 pro (with EQ that HP is amazing).

What would be a powerful alternative I switch position a lot in my living room and the SK01 was very practical I want mostly a SE or Balanced inputs (both would be cool) from the D90 and dont care that much for a Dac+Amp but using the SKO1 made me want more.

Im looking into topping G5, Ifi xcan and gryphon and XDUO XD05 Pro they are not all measured and tested on ASR any advice ? maybe im missing another device adequate for my use... (so many releases nowadays).

NOTE : I apply my EQs with the coherent general gain decrease so that might be a huge draw of power when adjustements are heavy).

 

[staticV3]

What would be a powerful alternative I switch position a lot in my living room and the SK01 was very practical I want mostly a SE or Balanced inputs (both would be cool) from the D90 and dont care that much for a Dac+Amp but using the SKO1 made me want more.

Im looking into topping G5, Ifi xcan and gryphon and XDUO XD05 Pro they are not all measured and tested on ASR any advice ? maybe im missing another device adequate for my use... (so many releases nowadays).

I'd buy a Topping NX7, if you just want a pure headphone Amp without DAC or Bluetooth.

Here are measurements: https://www.l7audiolab.com/f/topping-nx7/

I'd avoid the iFi stuff, due to the borderline fraudulent way they rate output power: https://goldensound.audio/2023/11/24/ifi-idsd-diablo-2-measurements/ (scroll down to "Power:").

 

[Chibrax]

I'd buy a Topping NX7, if you just want a pure headphone Amp without DAC or Bluetooth.

Here are measurements: https://www.l7audiolab.com/f/topping-nx7/

I'd avoid the iFi stuff, due to the borderline fraudulent way they rate output power: https://goldensound.audio/2023/11/24/ifi-idsd-diablo-2-measurements/ (scroll down to "Power:").

Yeah forgot to put this one too so it has more power than the sk01... topping nx7 and g5 seem more down to what I need still the aux placement is quite bad imo.
And yep ifi gryphon or xcan are not measured (or I badly searched) and I know of ifi having issues... still I like a lot my zen dac v2 (desk setup).
Gryphon or xcan could be an endgame for me for on the go, at home and maybe even replacing my desk setups. But considering ifi is well known for bad info on power it is quite a gamble...
Also ifi allows me to use balanced !

 

[staticV3]

@Chibrax Here are measurements of the iFi Gryphon: https://www.l7audiolab.com/f/ifi-gryphon/

 

[Chibrax]

@Chibrax Here are measurements of the iFi Gryphon: https://www.l7audiolab.com/f/ifi-gryphon/

So tell me if im wrong it says for balanced in to balanced headphones 4 volts rms for 200 ohm load ? (impressive)
and unbalanced in to se headphones same load 2+ volts of power ?

On a side note I dont know if there is something capable of translating chinese on firefox ? this site seem to have some stuffs measured that we cant found elsewhere that would be nice. Im not a pro at reading those even on ASR (that is one reason I love it here every measurement has some explanations).

EDIT: Ok google translate does a usable job

 

[staticV3]

So tell me if im wrong it says for balanced in to balanced headphones 4 volts rms for 200 ohm load ?

200kΩ.

(impressive)

Meh. At the same load, the NX7 will output 7Vrms. The Gryphon just 5.

And the NX7 doesn't even need balanced headphone cables for that. Regular 3.5mm or 1/4" cables will do the job. That's impressive I think.

 

[Chibrax]

200kΩ.


Meh. At the same load, the NX7 will output 7Vrms. The Gryphon just 5.

And the NX7 doesn't even need balanced headphone cables for that. Regular 3.5mm or 1/4" cables will do the job. That's impressive I think.

So I bought the nx7 today I may return it if I dont see a substantial upgrade over the SK01 (unlikely).

 

[Chibrax]

Just to give you a little update (if you care)
First thanks a lot for your rec !
The NX7 is a huge stepup in terms of power and cleanliness the only downside imo is the design having every inputs/outputs on one side its not the most practical but its manageable after some use !

 

[TLEDDY]

Based on the (limited) ability of DSP and low cost, I purchased one!

I have several decent, but not great, headphones…all of them have, to my aged 82 y/o ears, benefited from the DSP!

I can sneeze hard and lose more than the cost of this little unit… I like it a lot! Try it - if you do not like it, give it to a kid/friend/acquaintance that has cheap headphones - they will worship at your feet for the improvement…

 

[Chibrax]

Based on the (limited) ability of DSP and low cost, I purchased one!

I have several decent, but not great, headphones…all of them have, to my aged 82 y/o ears, benefited from the DSP!

I can sneeze hard and lose more than the cost of this little unit… I like it a lot! Try it - if you do not like it, give it to a kid/friend/acquaintance that has cheap headphones - they will worship at your feet for the improvement…

yeah I kept the sk01 its a cool device when you dont want to bother with software EQ

 

[burkm]

Today Fosi Audio annouced a new headphone amp, the SK02, a desktop unit in almost the same (?) layout, available end of February 2024. it is supposed to be their "prime" headphone amp right now.
Just wondered, why they used an outdated chip in this new unit, the ESS ES9038Q2M, which has a successor by the same company already: ES9039Q2M.
But that is a different story...

 

[IAtaman]

Just wondered, why they used an outdated chip in this new unit, the ESS ES9038Q2M, which has a successor by the same company already: ES9039Q2M.

What is the difference between them that you are interested in?

 

[burkm]

The newer chip has better specs, which might make it sound "better"

 

[IAtaman]

The newer chip has better specs, which might make it sound "better"

No, it does not.

If I understood the datasheet correctly, it has time division multiplexing for multi-channel, which the 2 channel stereo headphone amp does not require, and a hardware mode which allows you to choose the mode the DAC operates in by setting its GPIO pins. All audible characteristics seems to be virtually the same.

SABRE Audiophile DACs – ESS Technology, Inc.

www.esstech.com

 

[burkm]

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