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

[@AudioMasterclass]

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[@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 👍

[@JorgedeLumiarFerreira]

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

[@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.

[@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]

Good to know. Recommend any similarly priced amps that don't invert the phase? I'm on the lookout

[@ScottGrammer]

@@mr-iz8cx I don't know of any offhand.

[@mr-iz8cx]

​@ScottGrammer apparently the smsl dl200 is worth a look. Relatively cheap

[@DrBovdin]

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]

@@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.

[@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.

[@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!

[@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.

[@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).

[@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

[@jimdavis5230]

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

[@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!

[@apigge8723]

Thanks for the crucial reminder at 13:26! 😮

[@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.

[@SerTol]

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

[@AudioMasterclass]

A like.

[@1Moggi]

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

[@hellomeatrobots]

I love the unboxing music.

[@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]

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]

@@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!

[@Omar_ZX]

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

[@rabit818]

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

[@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.

[@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]

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]

@@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.

[@paulphilippart7395]

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

[@jimc9823]

Oh no it's an inverting amp! LOL

[@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..."

[@jojomag9822]

Does the VOLUME CONTROL affect the line out?

[@djsoulfilter]

It does which is perfect to use for a pair of powered speakers.

[@rsvp9816]

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

[@AudioMasterclass]

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-]

@@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?

[@Aubury]

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

[@AudioMasterclass]

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.

[@klasstrandberg3207]

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

[@michaelturner4457]

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

[@johnfisher3929]

Everything?!?

[@JSSTUDIO-wr2jq]

🙂👍

[@lmkan]

Class A should not be out of phase...

[@dangerzone007]

It sounds like you stole that unboxing music from Crazy Frog

[@bobshifimods7302]

Wrong! How come an audiophile like you David never realised that tone controls spell death to sound quailty? This thing seems to be usb powered too. That's compromised power supply. I don't get why you are touching this at all.

[@AudioMasterclass]

I suggest, my dear friend, that you are not the high priest of moral rectitude that you think you are.

[@eugeniy_nibumbum]

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