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FOSI SK01 Review: Headphone amp + preamp with ample tone controls

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Djano

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if you power the SK01 with a USB charger, Will SK01 be supplied with 5Vdc ? or does the USB charger would only charge the battery and then the unit runs with 3.7-4.2 volts ?
I think it is powered with 5V when plugged in. The red light, which means "battery charging", never lights on when it is constantly plugged. However, if I unplug and replug it instantly, it lights on. The fact that it never charges suggests that it runs directly via USB power.
 

conspibro

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I think it is powered with 5V when plugged in. The red light, which means "battery charging", never lights on when it is constantly plugged. However, if I unplug and replug it instantly, it lights on. The fact that it never charges suggests that it runs directly via USB power.
Problem with testing this claim is that I can see a constant 5V/0.5A draw upon turning it on with fully-charged battery while plugged into USB power. However, there's no way for me to tell if it's for charging the battery (that is fully-charged) or powering the amp itself. That 0.5A draw stays the same even when there's no input or load. I doubt the amp itself would draw 2.5W doing nothing, as that'd completely drain the battery in ~3 hours, which is definitely not the case because I could eke out about 5-6 hrs of use while listening with my headphones at a moderately loud level.

On a sidenote: the red light stays on for my unit if I use it with fully-charged battery and plugged into USB power at the same time, hence my scepticism. (i.e. amp is running off of battery, while it is being trickle-charged by USB charger)

Good way to check what is powering the device would be to disconnect the battery from the circuit, leaving just USB power, and see if the amp even turns on. However, unfortunately I don't have a soldering iron at the moment, nor the time to dismantle the device to have a look myself. I can't tell from the photo on prev page if the battery wires are soldered or mounted in a removable connector.
 
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Fred Pumpkinseed

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This review will primarily focus on the features of the device. I will touch very little on the technical performance because I lack the equipment to evaluate it properly. I wanted to do this review because the SK01 offers a bunch of very appreciable functions, yet nobody seems to talk about it (it was mentioned only once here in the topic about the Fosi V3).

View attachment 311052

We will discuss:
  • General overview
  • Tone control characteristics
  • Advantages of the tone defeat button
  • Implementation of the loudness function
  • Benefits of the unusual form factor
  • Potentiometer feel
  • Unnecessary and potentially regrettable presence of a battery
  • The LED that raises an important question
  • Comparison with the Sybasonic SD-DAC63106 to learn more
We will not discuss the headphone amplifier's performance since I only have easy-to-drive headphones and cannot provide useful information about its behavior with high-impedance or low-sensitivity headphones.

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General presentation

The SK01 is a headphone amp and a preamp. These two functions are often combined (e.g., Topping L30II), but there's generally no real interest in the preamplifier function unless your source isn't at an adequate level. Most of the time, it seems like manufacturers add a pre-out as an afterthought, taking advantage of the fact that it costs almost nothing and artificially adds an unnecessary function to the product.

The SK01, on the other hand, is genuinely interesting for both functions because it offers an almost unprecedented number of settings for this price range and size:
  • Three-band tone control,
  • Loudness function,
  • Adjustable gain at two levels (applies to line out as well)
Power is supplied via USB-C (power only, this is not a DAC) or the built-in battery (we'll come back to that). The input is analog, and there's only one (3.5mm jack), so it's not a preamplifier suitable for switching sources. There's a line-level output (also 3.5mm), and two headphone outputs (6.35 and 3.5mm). The plugs are good quality (nice click and jack is well held).

So, on paper, we have a very interesting preamplifier and a headphone amplifier with analog input tone control. To my knowledge, the only affordable and well-known headphone amp with tone control was the K5/K5Pro, but it has a digital input, making it challenging to integrate into an already complete system with a DAC.

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Tone control. The tone control is richer than what is usually offered since we can adjust the mids (200Hz-4kHz, -10dB/+10dB), in addition to the usual bass (20-250Hz, -14/+14dB), and treble (3kHz-20kHz, -12/+12dB). The Schiit Loki offers an extra adjustment, but Amir's measurements showed it behaved rather strangely. All three potentiometers have a center detent.

I know that the Fosi K5 Pro had an issue with the tone control adjustment, noted by nightlite at the end of 2022. At the neutral position, the bass decreased by -4dB, and the treble by -2dB. I don't know if this issue has been fixed, but it's quite possible that it's not even a relevant question, since the SK01 is built around a completely different PCB, most likely produced by a third party (see Sybasonic section). In the worst case, it could be compensated for by adjusting the tone control, but I admit that would be a bit of a workaround...
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Tone defeat. There's a tone defeat button. Since it's very quick to use, I think it's the best possible implementation, unlike the PRE-TC10 where you have to push a potentiometer all the way, for example. Besides the advantages of avoiding sound degradation when you don't want any tone control, it's very useful if you have multiple devices in your audio chain. For example, if you like to apply tone to your headphones but not to your speakers, a simple click is enough to adjust the sound when you switch outputs. Similarly, if you've tweaked the sound for a particular track, you can quickly return to a neutral sound with a click for the next track.

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Loudness. The loudness function, particularly rare in mini amplifiers, is present here. I love so much the effect it provides that I leave it on all the time. It works as expected: boosting bass and treble inversely proportional to the output level. Activating/disabling it has a noticeable effect at low volumes but not at all at high volumes. Adjust your system components accordingly to achieve the desired effect at the desired levels.

I would advise caution if adjusting the source. It needs a safety margin because it could "forget" the attenuation you programmed. You would then send a much louder signal to the speakers than intended. This has happened to an ASR member with a Topping DAC. The poor guy's speakers got blown out... Personally, I leave the source at maximum and only adjust with the amplifiers downstream in the chain.

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Build quality. The unit feels heavy and sturdy, in line with the quality Fosi typically provides. This is the usual anodized aluminium they use. I think the buttons could be improved, they do not feel luxurious at all (but still OK).

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Form factor. The unusual form factor has several advantages. It allows for creative placement (I could place it on my monitor support). More importantly, its vertical spread rather than width provides comfortable access to the potentiometers, especially since they are more spaced out than on my BT30D Pro, for example.

View attachment 311054

With conventional placement, two rubber pads prevent the device from slipping. Its weight (250-300g) also stabilizes it. It won't slide around.

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Potentiometers. Feeling of the volume potentiometer. The volume potentiometer has a similar resistance to those on the BT30D Pro: smooth and consistent, with a slight mechanical resistance that diminishes as the device warms up. The SK01 doesn't heat up much, but enough to become more supple, even without an input/output signal (just leave it on all the time). The BT30D Pro heats up a lot (7W in idle, power supply included). The SK01's potentiometer gives a slightly "hollower" impression, less massive, less luxurious. But it's still very decent.

Feeling of the tone control potentiometers. These seem less smooth or "lubricated" and are far from the velvety paradise of a warm BT30D Pro. They have a center detent. I find this redundant with the tone control bypass button, but the combination of these two things might appeal to some (especially regarding switching between devices, as I mentioned in the tone defeat section).

Type? I don't know if the potentiometers are of type A or B (ideal would be B-linear for the tone control and A-logarithmic for the volume). I saw on a thread here that Fosi only recently (December 2022) became aware of the relative benefits of type A for volume control, so I'm not sure they had time to implement it on this model. Fosi, please feel free to enlighten us here!

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Battery. I really don't understand the integration of a battery. The form factor is excellent for stationary use but certainly not for portability. Who would want to carry around a hefty brick weighing 250-300g? The battery unnecessarily adds to the price and complicates transport. It's also a significant point of failure. Personally, I leave the device plugged in all the time. I hope it can still function via USB-C even when the battery will die. Fosi, if you're reading this, I would be interested in your response on this matter.

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Cute little light. In addition to the LED at the top, indicating the power status of the device, the LED at the bottom lights up when the output signal exceeds a certain power level. If the volume is set very low, it's always off; conversely, it's always on at maximum power. But in between, it can beat to the rhythm of the music, lighting up with strong percussion or bass hits, for example. The result is not a simple ON/OFF but rather variable intensity depending on the signal's power. It's subtle and pleasing, though it obviously doesn't match the visual joy of a VU meter.

Voltage problem? One thing confuses me, though. The user manual claims that the LED triggers when the output voltage exceeds 2V. So, does that mean the line level is exceeded? Could this potentially damage the amplifier to which the signal is sent? Or lead to clipping? I haven't noticed it audibly, but I don't have the technical knowledge to answer these questions definitively. A response from the manufacturer or reasoning from technical experts at ASR would be welcome.
EDIT: It seems like this could be answered based on the input sensitivity of your particular amplifier. I am however puzzled, as my BT30D Pro datasheet claims 700mV imput sensitivity, but I do not hear any problem with >700mV signals. I lack knowledge here. Does going beyond the input sensitivity rating only matters if you push the amplifier to a high level, or could it be a problem at all levels ?

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Potential insights from similarity with the Sybasonic SD-DAC63106. As often happens in Chi-Fi, the same PCBs appear to be reused and sometimes slightly customized by multiple brands (Fosi, Aiyima, and Douk offer many amplifiers that obviously use the same circuit with the same layout, and you can buy the circuits alone on Aliexpress).

Similarity of PCBs. I believe the SK01 uses the same PCB as the Sybasonic SD-DAC63106, with some adjustments. I think this because 1) the form factor is almost identical, 2) the features are very similar (tone control, loudness, cute little light, power light), 3) both manufacturers use exactly the same graphics to present their tone control. The only differences seem to be that the SK01 looks much better (in line with Fosi's recent aesthetic direction), has a built-in battery, a gain adjustment button (0/-10dB, also works on the line out), but no left/right balance control.

Insights about the tone control. If the same PCB was indeed used, then what we already know about the Sybasonic can help us learn more about the SK01. Regarding the tone control, we can learn, from the identical graph of the Sybasonic, the central frequencies of the adjustments, while Fosi only mentioned their range (but the written range doesn't quite match the visual graph):

View attachment 311055

Insights about the LED indicator. About the cute light discussed earlier, Sybasonic simply describes it as a gadget that beats to the music, not as an indicator of exceeding 2V as described by Fosi. I'm not sure what to make of this for the SK01 because both claims are not necessarily incompatible.

View attachment 311057

Insights on the hardware. Here's what cfcubed tells us in his Amazon review of the Sybasonic (link here, click on review pictures to see cfcubed's comments), which he disassembled due to an early failure (gradual death of one channel, his case seems largely isolated):
  • ZM2V0512D DC booster (5V to 12V)
  • 4 JRC4580s in preamp/tone stages
  • TI LM358 Headphone amp
View attachment 311059

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Advantages with certain Chinese 2.1 amplifiers. I'm particularly satisfied with the BT30D Pro – SK01 combination. My BT30D Pro had two issues. 1) The passive subwoofer output didn't send enough power even though it's capable (maxing out "Sub. Vol." results in too little power compared to the speakers, but increasing the overall volume shows that there's a significant reserve of power for my two crappy 3-ohm passive subwoofers connected in series). 2) The tone control only applies to the speakers (a strange choice...).

Integrating the SK01 upstream solves both of these problems. Boosting the bass gives the subwoofer amplifier channel more to work with, and the subwoofer/speaker ratio is now much more satisfying. This could also have been achieved with Equalizer APO, but there's a lot of enjoyment in manually adjusting analog potentiometers to match different tracks. Plus, the fact that the SK01 globalizes the tone control (subwoofer/speakers) while the BT30D Pro focuses it on the speakers allows for interesting cross-adjustments.

These advantages should logically benefit those with passive subwoofers (quite rare, I know) and an amplifier with the mentioned issues (which then become assets).

Hello, excellent review. I just registered to this forum to ask a question about this FOSI SK01. I'm oblivious about audio equipment. I am interested in purchasing an item that would assist in my 85 year old mother in hearing her television better. She listens to her TV in her bedroom, with wired headphones. (from the TV headphone jack).

She's been complaining lately of muffled voices, (probably needs hearing aids). Her hearing really isn't that bad, but I've given her a few pair of headphones to try, and she still has some issues hearing some voices. So I'm thinking it's not a better pair of headphones she needs, just ability to control (and or boost) tone.
I was thinking perhaps, a device like this might be the thing, to allow her to make out the voices more clearly.

There are a few, relatively cheap headphones out there, which are designed for seniors, with a built in tone control ( I question how well they work), so I'm thinking a device like this would probably be much better.

Just wondering what you think? You seem satisfied with the SK01. Wondering just how much of a difference in tone is noticeable when cranking the Treble to 11 lol.

The price point is about right. My plan would be to keep the SK01 constantly plugged in. Hope it doesn't cook the battery after a few months.

Thanks.
 

Bruce Morgen

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This is an intriguing little device for very few $$ -- I love how it revives (something akin to) the old "Fletcher-Munson curve" idea with a "Loudness" button for low volume listening, something missing from almost all modern gear...on the cheap! I don't understand why they bothered with that battery, though. :cool:
 

amirm

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Just wondering what you think?
I just played with my sample and the mid-range control nicely modifies voices so I think it is a good idea for this purpose.
 

paudio

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Hello, excellent review. I just registered to this forum to ask a question about this FOSI SK01. I'm oblivious about audio equipment. I am interested in purchasing an item that would assist in my 85 year old mother in hearing her television better. She listens to her TV in her bedroom, with wired headphones. (from the TV headphone jack).

She's been complaining lately of muffled voices, (probably needs hearing aids). Her hearing really isn't that bad, but I've given her a few pair of headphones to try, and she still has some issues hearing some voices. So I'm thinking it's not a better pair of headphones she needs, just ability to control (and or boost) tone.
I was thinking perhaps, a device like this might be the thing, to allow her to make out the voices more clearly.

There are a few, relatively cheap headphones out there, which are designed for seniors, with a built in tone control ( I question how well they work), so I'm thinking a device like this would probably be much better.

Just wondering what you think? You seem satisfied with the SK01. Wondering just how much of a difference in tone is noticeable when cranking the Treble to 11 lol.

The price point is about right. My plan would be to keep the SK01 constantly plugged in. Hope it doesn't cook the battery after a few months.

Thanks.
Might help. Some seniors ears get plugged up bad with wax too. Lots of times assessment for hearing aids are free.
 

activio

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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?
 

hellysmith

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The MarkAudio CHR-70 or the Dayton Audio RS100 are both good options for full-range drivers in a 27cm inner diameter sphere. Whether to seal or port the enclosure depends on your desired sound signature. A sealed enclosure will result in a tighter, more punchy bass response, while a ported enclosure will extend the bass response lower but may also introduce some resonance.
 

Ingineer

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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
 

Splork

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Hello all and HNY 2024! I got the SK01 imported China to India and got these after almost a month. Really enjoying these with my new Hifiman HE400SE. My question is, since they run out of battery after 4-5 hours of moderately loud listening, can I plug for charging with my laptop and listen to them at the same time? Will it damage/heatup the amp?
 

Ingineer

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It's completely fine to have the USB-C charging hooked up while listening, but you may have ground loops (noise) if the power comes from the same device that's generating the audio signal. Basically it will not use the battery if there is a USB-C connection, it will draw power from there instead.
 

Bruce Morgen

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Hello all and HNY 2024! I got the SK01 imported China to India and got these after almost a month. Really enjoying these with my new Hifiman HE400SE. My question is, since they run out of battery after 4-5 hours of moderately loud listening, can I plug for charging with my laptop and listen to them at the same time? Will it damage/heatup the amp?

Mine is constantly charging from a cellphone wall wart whether it's turned on or not, so there should be no problem in your scenario.
 

burkm

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Fosi Audio just announced a new headphone / preamp (desktop) amp, SK02, supposed to be even "better" than the SK01, which will be available end of February 2024 at somewhat higher cost. Power supply seems to be through an USB-C port. Analog and digital inputs are available...
 

Bruce Morgen

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Fosi Audio just announced a new headphone / preamp (desktop) amp, SK02, supposed to be even "better" than the SK01, which will be available end of February 2024 at somewhat higher cost. Power supply seems to be through an USB-C port. Analog and digital inputs are available...
They're not even remotely similar other than the case. The main appeal of the SK01 is the tone controls -- it's a preamp with a headphone output. The SK02 is a DAC with a headphone amp but no tone controls -- a very different beast for an entirely different use case.
 

burkm

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They "look" almost the same, but it's a desktop design, asked by many with somewhat different (and improved) functionallity depending on the individual use case...
Its not defined for mobile application as the "old" SK01 is, which I already got.
 

Kimbrough Xu

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It interesting that it doesn't have a DAC - I guess the idea is it sits between a DAC and the Fosi Audio V3 - provides tone controls for the V3 and optionally headphone output.

It is easy to understand when fosi released fosi audio SK02.
 
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