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

Djano

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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).

sk01fosi.jpg


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.

setup bureau.jpg


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):

Sybasonic.jpg


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.

led.jpg


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
SK01.jpg


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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).
 

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Djano

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Additional information regarding my concern about exceeding 2V on the line level

This concern, in fact, appears to be unfounded.

I conducted the following test:
  • I connected my headphones (X2HR) to the line-out of my integrated sound card (Realtek ALC 1150, if I remember correctly). I specified in the software that they were speakers (not headphones) to have a valid point of comparison. It outputted 2V, which I couldn't tolerate at maximum volume.
  • I connected the headphones to the SK01's line-out. The maximum level was comfortably manageable.
Therefore, on the line-out, the SK01 outputs a voltage lower than 2V.

It's possible that the LED indicates the 2V overage only on the headphone output. Or it could simply be that the manufacturer's provided indication is inaccurate.

If you are still interested in the relationship between the preamp's output voltage and the preamp's input voltage (there is no issue here, but it's an otherwise interesting question), two members provided us with information on this matter here

Regarding the similarity with the Sybasonic

Based purely on the specifications provided by the two manufacturers, it should be noted that:
  • Sybasonic promises 120mW / 32 ohms at 1 KHz
  • Fosi promises: High Gain: RMS 2400mW / 32Ω, Low Gain: RMS 300mW / 32Ω
This difference raises very serious doubts about whether the components are indeed the same... Please approach the parallel I suggested with the utmost caution
 

AnalogSteph

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  • 4 JRC4580s in preamp/tone stages
  • TI LM358 Headphone amp
I don't quite think that's correct. The LM358 has famously terrible crossover distortion (you can see it on a scope!), is quite noisy and requires substantial coaxing to even become a borderline passable headphone driver. It would definitely not be my first choice by a long shot. Now driving the clipping indicator LED right next to it, that I could definitely see happening. Then one of the 4580s would be the actual headphone driver. They're not half-bad at that. See Behringer HA400.

The question of clipping indicator vs. funny blinkenlights may just be a matter of component values, with the same PCB easily accommodating both.
Based purely on the specifications provided by the two manufacturers, it should be noted that:
  • Sybasonic promises 120mW / 32 ohms at 1 KHz
  • Fosi promises: High Gain: RMS 2400mW / 32Ω, Low Gain: RMS 300mW / 32Ω
Both range anywhere from optimistic to wildly optimistic. The Fosi specs have to be erroneous copypasta, you need a full-grown headphone amp with a +/-15 V supply and serious oomph (e.g. at least one TPA6120A2, probably two) for figures like that. A 4580 is good for about 40 mA rms if memory serves, that would be roughly 50 mW into 32 ohms per channel. Maybe 60 at astronomical distortion, add both channels, boom, 120 mW. Needless to say, lots of following wind involved here.
 
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Djano

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I don't quite think that's correct. The LM358 has famously terrible crossover distortion (you can see it on a scope!), is quite noisy and requires substantial coaxing to even become a borderline passable headphone driver. It would definitely not be my first choice by a long shot. Now driving the clipping indicator LED right next to it, that I could definitely see happening. Then one of the 4580s would be the actual headphone driver. They're not half-bad at that. See Behringer HA400.

Reminder for others : we are here talking about the Sybasonic, which the SK01 has not been proven to be built based on the same PCB/components

So, your take seems to be that the guy on Amazon got the components right, but assigned them to the wrong function in the design, is that correct ?
The JRC4580s would be used for tone controls AND headphone amp.
The LM328 would only be used to trigger the clipping LED, as it quickly goes into clipping
That seems strange, but you are obviously more knowledgeable than me. Please correct me if I got something wrong

You seem to assume that Fosi uses the LED as a clipping indicator. But they only say it is for signals >2V, to prevent ear damage with headphones (yeah, sensitivity and impedance would change the dB, so rough approximation they made). And it lights up with variable intensity. I can't make any sense of this.

Both range anywhere from optimistic to wildly optimistic. The Fosi specs have to be erroneous copypasta, you need a full-grown headphone amp with a +/-15 V supply and serious oomph (e.g. at least one TPA6120A2, probably two) for figures like that. A 4580 is good for about 40 mA rms if memory serves, that would be roughly 50 mW into 32 ohms per channel. Maybe 60 at astronomical distortion, add both channels, boom, 120 mW. Needless to say, lots of following wind involved here.

Obviously, the power estimations can not be trusted, as we often see with Chi-Fi amps.
I just thought that such a difference could indicate that the components used were, in fact, not the same.
But it is indeed possible to give different estimations for the same components, depending on the optimism of the manufacturer...
 
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Djano

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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.
And there was a serious lack of headphone amp with tone controls and analog input ! Some time ago, I was desperately looking for this, and (in reasonable prices), could only find the K5/K5 Pro, which are digital input.

In the V3 thread, you mentionned similar devices. Could you name some, so I can see if I missed something ?
 

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And there was a serious lack of headphone amp with tone controls and analog input ! Some time ago, I was desperately looking for this, and (in reasonable prices), could only find the K5/K5 Pro, which are digital input.

In the V3 thread, you mentionned similar devices. Could you name some, so I can see if I missed something ?
You are probably right - I do have a portable "headphone amp" which sort of increases the output of a mobile or laptop headphone jack for harder to drive headphones but it's not really that I think. At an initial look I was thinking I could use this instead of the Fiio K7 that I have between an Win Pro and my V3 as mostly I just that as a simple way to switch to nice headphones in the evening rather than speakers. Would depend on how good the headphone output is as I do really like the K7. Then I was also thinking it might be good while travelling obviously not as portable as a Qudelix but might be good for my planar headphones on a longer trip.
 
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Djano

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You are probably right - I do have a portable "headphone amp" which sort of increases the output of a mobile or laptop headphone jack for harder to drive headphones but it's not really that I think. At an initial look I was thinking I could use this instead of the Fiio K7 that I have between an Win Pro and my V3 as mostly I just that as a simple way to switch to nice headphones in the evening rather than speakers. Would depend on how good the headphone output is as I do really like the K7. Then I was also thinking it might be good while travelling obviously not as portable as a Qudelix but might be good for my planar headphones on a longer trip.
I doubt the SK01 can even get close to the K7 output, judging only by the power supply.
You would however benefit from the SK01 from a features point of view (tone control and loudness mode are great)
You could also do DAC (Wiim Pro) -> SK01 (for tone control) -> RCA Y splitter, one goes into the K7 and the other into the V3.
This would allow you to use the SK01 features on all of your outputs

About the travel thing, the SK01 is, to me, too big and heavy to carry everyday.
Now, you could take it on a longer trip, why not. Especially if you accept to travel with planar headphones aha
 

Jeromeof

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I doubt the SK01 can even get close to the K7 output, judging only by the power supply.
You would however benefit from the SK01 from a features point of view (tone control and loudness mode are great)
You could also do DAC (Wiim Pro) -> SK01 (for tone control) -> RCA Y splitter, one goes into the K7 and the other into the V3.
I don’t think an RCA Y splitter would be a good idea and the Wiim has EQ ( and very basic PEQ ) now so basic tones controls are less important for me ( though nice for have ) and why have it in the same system as a K7 ?

But I think it’s an interesting product
 
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Djano

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I don’t think an RCA Y splitter would be a good idea and the Wiim has EQ ( and very basic PEQ ) now so basic tones controls are less important for me ( though nice for have ) and why have it in the same system as a K7 ?

But I think it’s an interesting product
From what I read, RCA splitter is OK as long as the source can handle the lowering of impedance caused by outputs in parallel.
I currently have a splitter to feed my speaker amp and my headphone amp, and I could not spot any change.

You could also do Wiim Pro - SK01 - K7 - V3. Switch the K7 in preamp mode when you want to use the V3, HPA for the headphones.

Having it in the same system as K7 would allow you to keep your better HPA, but benefit from SK01 controls.

Wiim EQ app won't come close to the pleasure of physically turning potentiometers ! I wish they feel a little more luxurious on the SK01, though.
 

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Reminder for others : we are here talking about the Sybasonic, which the SK01 has not been proven to be built based on the same PCB/components

So, your take seems to be that the guy on Amazon got the components right, but assigned them to the wrong function in the design, is that correct ?
The JRC4580s would be used for tone controls AND headphone amp.
The LM328 would only be used to trigger the clipping LED, as it quickly goes into clipping
That seems strange, but you are obviously more knowledgeable than me. Please correct me if I got something wrong

Dismantle yours, take some high resolution pictures and post them here, instead of guessing what's inside, based on someone's random amazon review and speculation...
 
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Djano

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Dismantle yours, take some high resolution pictures and post them here, instead of guessing what's inside, based on someone's random amazon review and speculation...
You're right, that would clearly be the best way to get a clear picture.
But well, since it's brand new, I don't want to damage it or anything.
I know it's a bit selfish to prefer to keep it in perfect condition rather than informing everyone. I still tried to provide all the information and insights I could within the limits of what I was willing to do (not to damage my new device).

Directly asking Fosi is probably the best way to go. I will just do that.
 

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I took the bottom off mine (easy, peel the ends back on the 2 bottom bumpers enough to reveal 4 screws), but, the front knobs are very tightly pressure fit. I pulled the 3 tone knobs off but could not budge the volume. So, some photos from the bottom. It does not seem identical to the Sybasonic, but without a full extraction these may not be much help.
FosiSK01.jpg
 
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Djano

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I took the bottom off mine (easy, peel the ends back on the 2 bottom bumpers enough to reveal 4 screws), but, the front knobs are very tightly pressure fit. I pulled the 3 tone knobs off but could not budge the volume. So, some photos from the bottom. It does not seem identical to the Sybasonic, but without a full extraction these may not be much help. View attachment 313438
Hero! Many thanks for your sacrifice. I can not make any comment of this, as I lack every competence to do so...


By the way, I wanted to update my review with a problem that is annoying me with the SK01
The three buttons (gain, tone, loudness) activate or deactivate during their "travel length". With the tone button, this causes problems. Just touch it a little (which can happen when adjusting the middle potentiometer), and sound will crackle. Sometimes, this even leads to a left/right disbalance, and strange sound, because you "misplaced" the button by touching it. The solution is to wiggle or push and release the button. But it makes me wonder if this behavior does not cause a permanent problem, which my non-golden-ears do not perceive.
This problem does not exist with the gain button and loudness button. It seems like the tone control button does this because it has to manage three "parameters" (potentiometers), and it does so gradually during its travel length. So even the start of travel does some things, which could lead do the described problem. This is just a hypothesis.

Not a deal breaker, but definitely something that should be improved. I would now prefer this tone button gone, as there are center detents on each tone pot.
 

restorer-john

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I took the bottom off mine (easy, peel the ends back on the 2 bottom bumpers enough to reveal 4 screws), but, the front knobs are very tightly pressure fit. I pulled the 3 tone knobs off but could not budge the volume. So, some photos from the bottom. It does not seem identical to the Sybasonic, but without a full extraction these may not be much help. View attachment 313438

Excellent and well done!

Is the casework alloy or plastic and the knobs are made of...?

Is that battery an 18650?

Looks interesting for sure.
 

LegionOfHell

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It could have a voltage boosting circuit inside ... Both Sabaj PHA2 and Douk Audio U3 need 5Vdc power sources and have an internal voltage boosting circuit.

I guess the 2.4W @ 32ohms figure is accurate ? Has anybody tested how powerful the amp is through the headphone outs ?
 

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It could have a voltage boosting circuit inside ... Both Sabaj PHA2 and Douk Audio U3 need 5Vdc power sources and have an internal voltage boosting circuit.

I guess the 2.4W @ 32ohms figure is accurate ? Has anybody tested how powerful the amp is through the headphone outs ?

It definitely has a booster, seeing that it runs off a single 18650(?) battery (3.7-4.2V).

I'm not sure if it's my source (laptop HP out) being weak, but like-for-like and with same setup, SK01 seems to have less gain compared to my U3 Mini when used with a pair of Sundara. I have to turn up the volume knob further (SK01 - 2-3 o'clock; U3 - 11-12 o'clock), and it has little headroom available if I were to apply EQ and -ve preamp gain because I have to turn it up past 4 o'clock. It maxes out at 5o'c.

It might very well be capable of reaching 2W if the input is more powerful. I've seen someone comment on a Bilibili video of this amp that it has SGM8262 op amp. Not sure if it's true though. But datasheet for this part does state 310mA short-circuit current, so 2+W output is plausible.
 
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LegionOfHell

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It definitely has a booster, seeing that it runs off a single 18650(?) battery (3.7-4.2V).

I'm not sure if it's my source (laptop HP out) being weak, but like-for-like and with same setup, SK01 seems to have less gain compared to my U3 Mini when used with a pair of Sundara. I have to turn up the volume knob further (SK01 - 2-3 o'clock; U3 - 11-12 o'clock), and it has little headroom available if I were to apply EQ and -ve preamp gain because I have to turn it up past 4 o'clock. It maxes out at 5o'c.

It might very well be capable of reaching 2W if the input is more powerful. I've seen someone comment on a Bilibili video of this amp that it has SGM8262 op amp. Not sure if it's true though. But datasheet for this part does state 310mA short-circuit current, so 2+W output is plausible.

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 ?

How did you power the SK01 when you were comparing it with the U3 ?

By "if the input is more powerful" you mean a USB charger with more amperage ?
 

conspibro

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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 ?

How did you power the SK01 when you were comparing it with the U3 ?

By "if the input is more powerful" you mean a USB charger with more amperage ?

I don't think so. It just charges the battery. Device itself still runs off the battery. I ran it as it is, on its battery (fully charged).

By "powerful input", I meant whatever is plumbed into the line-in needs to be at an ample level in the first place, because the amp gain might not be enough on its own. Amp gain does not directly equate max power output afterall if source is weak.
 
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