Adventures In Audio

Can a tiny box do all this? Testing the Fosi Audio SK01 headphone amp, preamp, EQ

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@AudioMasterclass:  Not seeing your comment? Oops, I deleted a reply to a comment that I meant to approve. If you don't see your reply here, please repost it. DM

@madcrabber1113:  Loving mine, wish I found it years ago if it even existed?

@dennisgossard6755:  I’m looking for a preamp like this with the tone controls on the go, as I travel a lot. I have Fiio’s but I have been wanting to “ fiddle” with the sound more and more. I listen to all types of music. I’m wondering if this would work with my iPhone 14? I have all the necessary converters already.

@Brina0820:  The Fosi SK-01 & also P3 Preamp both have excellent sound and are most enjoyable when using various Class D amps. Build quality is excellent, they are a couple steps above most similar tone/audio enhancement devices, and I've used quite a few. Great product!

@rsvp9816:  so if I were record something and then re-run it through and record a second time the phase will be sorted?

@AudioMasterclass replies to @rsvp9816: Well yes, if you invert a signal then invert it again it will back the way it was. Alternatively, you could ignore it. It does matter in a production environment but for listening few people would hear the difference.

@voskresenie- replies to @rsvp9816: @@AudioMasterclass Would anyone hear the difference? Could anyone hear the difference? I thought that the only issue with inverting polarity of signal was if things didn't match -- eg if one speaker was normal, the other inverted, or mismatched stereo mics. In isolation, isn't positive/negative on a wave completely arbitrary?

@Omar_ZX:  It's line in but does 3.5mm aux cable still work for it? Never used anything line in and out before

@klasstrandberg3207:  That the output is inverted and a little distorted. At which level was this?

@jojomag9822:  Does the VOLUME CONTROL affect the line out?

@djsoulfilter replies to @jojomag9822: It does which is perfect to use for a pair of powered speakers.

@SerTol:  Loudness button should be called tone compensation . What did I win?

@AudioMasterclass replies to @SerTol: A like.

@MrNEWDY:  It seems safe to assume this will power many different impedance headphones? I got a really weak headphone amp in my new interface and both my headphones are between 32 and 50 ohms and i hear a distinct lack of low frequency response as my interface amp can't properly match.

@AudioMasterclass replies to @MrNEWDY: This is an interesting topic and it has for a long time been considered important that a power amplifier should have an output impedance near zero. It shouldn't be any different for a headphone amplifier.

@MrNEWDY replies to @MrNEWDY: @@AudioMasterclass that is what I thought. My new interface has a headphone amp that has an impedance of I believe 22ohms and I have felt like I am going insane because I notice they don't have nearly as much low end as I had on my older interface. I'm thinking this amp might be my ticket to not going nuts.

@nerwin:  this is interesting. Could I connect this between my audio source and amplifier (fosi V3) to get a little more control of the sound? Since the Fosi V3 has a volume control, would this SK01 volume also affect it or does simply using the line in and line out bypass the volume knob? In my mind I'm thinking the volume knob would be only for the headphone amp. Any help would be appreciated. Thank you! And great review!

@Aubury:  Can l ask , there seems to be scope for sound improvements by the use of AI ? Can you comment on

@AudioMasterclass replies to @Aubury: Considering the speed that AI has come upon us I think there will be amazing things to come in the future. I just don't know what.

@1Moggi:  I would like to use one of these with my V3 but the rca's are being used by my turntable preamp. 🤔

@fabriziodidomenico3149:  I think the correct term for "loudness" is "compensator". Accuphase uses this terminology. Moreover, the best implementation of the "compensator" comes from Yamaha, with a variable pot that decreases the midrange instead of boosting highs and lows; the Yamaha circuits apply in a perfect shape the Fletcher and Munson curves.

@rayc1557:  The Scope Trace... The input is a squarewave and the output is inverted. Output is a fair representation of the input (despite being inverted). Slew is minimal. Rising and falling edges w/respect to the steady-state level indicate the signal is critically damped (which is a good thing). If this is an audio circuit, your ears will never hear the difference between the two because speakers cannot really regenerate squarewave tones. Reversal of phase only makes a theoretical difference. If this is from the headphone amp, I give it an A+ grade.

@AudioMasterclass replies to @rayc1557: I agree that for hi-fi inversion is trivial but it isn’t right. For pro audio it is lying in wait to cause a problem.

@rayc1557 replies to @rayc1557: @@AudioMasterclass : Yes, Agree... For Pro Audio, it's important to keep a tight reign on all the phases of output signals. Honestly though, for an $80 (USD) headphone amp with a 3-pot tone control and a rechargeable battery... I think (based on your review) they did a nice job. Personally though, I try to avoid things with rechargeable batteries so this thing doesn't pass muster. BTW, I just bought a Schiit Lokius (6 Band EQ) and really like it. -Later. Be well!

@lmkan:  Class A should not be out of phase...

@apigge8723:  Thanks for the crucial reminder at 13:26! 😮

@JorgedeLumiarFerreira:  Best audio reviewer on Youtube. Informative, innovative (love the sound testing segments) and humorous. Loved every second of it. Thanks.

@eugeniy_nibumbum:  Смешная ундервафля для технозадротов.. ))
Если конечно вы поймёте смысл этих слов )))))))
И да, даже при прослушивании на мультимедии, оригинал играет лучше, сорри (

@igornau4411 replies to @eugeniy_nibumbum: Только в слепом тесте ты ничего не отличишь.

@techcafe0:  I'm not sure why the input vs output waveforms are inverted, but it looks like probe on the second channel might like to have its frequency compensation trimmer pot adjusted slightly, so that you see flat-top square waves on the display. Channel 1 looks fine though, perfectly flat square waves.

@maidsandmuses:  20microVolt noise floor sounds a little on the high side to me; background hiss may be audible to some listeners on low impedance high sensitivity headphones. That is the real design headache anyway with all these dedicated headphone amps: catering for the wide range of headphone impedances and sensitivities out there. A standard 120 Ohm output impedance used to be the answer to that (some older headphones actually need that 120 Ohm output impedance to sound well-balanced). Nowadays everyone expects a very low output impedance which makes noise attenuation for low impedance headphones tricky, as well as volume control. In absence of an adjustable amp sensitivity (the -10dB switch on this amp here is a bit limited), at the same volume control setting a 600 Ohm pair of headphones will sound very quiet, whereas an 8 Ohm pair of headphones will blow your eardrums. Same effect on the noise level.
With many cheap headphone amps you end up with not enough volume for 600 Ohm headphones, and for 8 Ohm headphones you hear background hiss and need to have the volume control barely open, where even expensive potentiometers have left and right channel imbalance issues (not to mention cheap potentiometers).

@AudioMasterclass replies to @maidsandmuses: You are absolutely right about stereo pots being imbalanced at low settings. I've never thought to make a video about it though. Until now...

@stevanbatinic6733:  I have had it for about 2 months. All I can say is that this is a fantastic preamp with EQ and in combination with the V3 the sound is amazing. Everything sounds much more transparent with fine details and more powerful bass. All congratulations to FOSI 👍

@dangerzone007:  It sounds like you stole that unboxing music from Crazy Frog

@JSSTUDIO-wr2jq:  🙂👍

@1974UTuber:  I used to have a 1970's Stereo Receiver unit that had a button labeled "HI/LO GAIN" and it did the same as a "LOUDNESS" Button.
Of course, the other way to do this is just to drop the level of the midrange by a couple of notches when the volume is low

@michaelturner4457:  I eat in michelin star restaurant and put ketchup on everything.

@johnfisher3929 replies to @michaelturner4457: Everything?!?

@SubTroppo:  Oscilloscope trace interpretation: you have a Helly Hansen jacket but no yacht to go with it. ps "Don't fiddle!": "Yes, miss but I was only..."

@billmilosz:  In my experience it's easier to design a good sounding class A gain stage than it is to design a class AB one. For preamps / headphone amps class A is a simple choice, as these devices don't need to output any real current. For power stages, class A generates a boatload of heat so you will need lots of heat sinking and / or fan cooling, as well as a beefy power transformer to handle the constant current draw. A class A power amp will generally sound quite good. A class AB power stage takes a lot more finesse to design, but good sounding class AB amps do exist, it seems to me that one needs to be a little more choosy when buying a class AB amp.

@rabit818:  It looks promising, same question, do I need Fosi? Not a fan of volume on/off combination.

@jimdavis5230:  Yes the scope shows a 180 degree phase shift but at least there is no ringing which is good.

@JeffWardMusic:  Nice video (as usual). Looks rather good and you've got me wanting one even though I have no real use for it! Output 180 degrees out of phase with input and a touch of overshoot on the rising edges (but not knowing the test frequency it may or may not be a big deal).

@MrBenherrmann:  Purchased this one more than a month ago, as it just was coming out. Really well constructed - like a small tank. I have it hooked up to my desktop system to a Schiit Modi 3 and love the sound. The only two things that have me perplexed is the overload light comes on with very little volume (remember, desktop system here), and I get noticeable hiss coming through the headphone outputs. Connected to speakers, everything sounds wonderful - a real winner at the low price.

@ScottGrammer:  The scope shows that the device inverts the absolute phase of the audio - a no-no if used in a studio environment, but generally no problem for listening at home.

@mr-iz8cx replies to @ScottGrammer: Good to know. Recommend any similarly priced amps that don't invert the phase? I'm on the lookout

@ScottGrammer replies to @ScottGrammer: @@mr-iz8cx I don't know of any offhand.

@mr-iz8cx replies to @ScottGrammer: ​@ScottGrammer apparently the smsl dl200 is worth a look. Relatively cheap

@DrBovdin replies to @ScottGrammer: As long as the inversion is identical on all channels, not frequency dependent, and you don’t have another amplifier involved that is non-inverting playing the sounds simultaneously, the perceived audio will be nigh on impossible to distinguish from the correctly phased audio.
I never tried it myself, but especially in headphones I can imagine that where you could potentially notice something would be certain transients where you should get an initial overpressure rather than a rarefaction. I can imagine that maybe some people may be able to notice that.

@ScottGrammer replies to @ScottGrammer: @@DrBovdin The only real problem with a phase-inverting headphone amp is when using it to listen while one speaks or sings into a microphone. The phase inversion will cancel out much of the low frequencies one hears from their own voice via bone conduction, and it makes one's voice sound very thin and strange.

@jimc9823:  Oh no it's an inverting amp! LOL

@paulphilippart7395:  Input is anti-phase to output ,interesting, that would be a whole bunch of silence if summed at same level.

@ScottGrammer:  Great review - I might get one of these. BTW, back in 1939, when Fletcher and Munson were testing people's hearing at the world's fair, they discovered the fact that our ears' frequency response changes with volume. They documented this and it was called the "Fletcher-Munson Curve." However, in the 80's it was discovered that they had made an error, and that the high-frequency deviations they noted were due to the equipment they were using. So loudness controls SHOULD only affect bass, but since they're never really used for loudness compensation, they still boost the highs to this day.

@hellomeatrobots:  I love the unboxing music.

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Thursday January 4, 2024

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David Mellor

David Mellor

David Mellor is CEO and Course Director of Audio Masterclass. David has designed courses in audio education and training since 1986 and is the publisher and principal writer of Adventures In Audio.

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