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).
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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.
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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.
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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
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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).
