Fosi Audio P1 Tube Preamplifier Mini Hi-Fi Stereo Preamp 6K4 Valve Vacuum Pre-amp with Treble Bass Tone Control for Home Theater HiFi System(not for Turntable)

This little preamp provides a great introduction into tube amplification, both the benefits and drawbacks of using tubes.First of all, as this is a preamp, it is very easy to set up direct A/B comparisons with commodity recording equipment (namely USB recording interfaces). Run a line out recording through the preamp, and again direct, match the output levels, and you now have a better insight into what these tubes are actually doing to the sound with A/B testing (preferably blind ABX testing).The good news - this preamp is pretty quiet and does a good job amplifying the signal coming in. I've had zero issues with noise including this in my playback chain. For a component containing tubes it is surprisingly quiet.The bad news (depending on your point of view) - the EQ knobs on this device affect the sound output far more than the tubes themselves. This is not a knock on the quality of the EQ, no, in fact, the EQ knobs are actually very useful given there is only a treble and a bass knob. In fact, the preamp is almost worth the price of the device given how well these work.The tubes themselves work as advertised. Sending a sine wave through the preamp, I definitely see way more second-order harmonic distortion through the preamp than through a direct output without the preamp. For instance, with a 440 kHz sine wave, I see the expected bump of 880 kHz at -30dB from the main signal. And there is noticeable compression running high-volume sounds through the preamp, particularly when running into the preamp at higher outputs. In short the expected effects of the tube (even order harmonics and compression) are measurable with sufficiently juiced inputs using relatively inexpensive measuring equipment.The real question is whether these effects are noticeable by your ears. For me at least the answer is yes, but the effect is pretty subtle, nowhere near what I was anticipating. For better or worse the stock tubes actually perform very well and have a good amount of headroom available before you notice any obvious changes in the sound. You kind of have to go out of your way to hear obvious changes, though, whether that be with bass-heavy music (Sublime's 40oz to Freedom works great for this) or by deliberately overdriving the inputs.A fun quantification of how this preamp affects sounds is to do a null test. First capture a recording of music played through the preamp tube, and then do the same exact source without the preamp tube. Line up both recordings *exactly* to the sample and to the same level, then invert the phase of one of them. Play them together and you'll get an exact audio representation of the difference between both input modes. This is more difficult than it sounds but if you do it you will hear the obvious distortions being introduced by the tubes. The interesting part is that they are both almost inaudible (unless you are carefully listening for them), and that even though these distortions sound not so great by themselves, when mixed into the output they can actually improve the sound of the music.OK, enough navel-gazing for now. Is this worth purchasing if you want to understand what "magical" tubes do? Absolutely. Is the effect going to be obvious? Probably not, unless you swap in tubes that are either busted or have very low headroom.Oh, and if you want to get an easy check for whether the tubes are actually in the signal path, grab a pencil and (gently) tap on each tube. As long as you don't have a fake you should hear the taps through your output speaker/headphones. There shouldn't be any ringing from the taps - that would indicate the tubes have gone bad and are microphonic - but you should be able to hear something with the volume up high enough. And the sound volume should increase gradually after a cold start like any self-respecting tube would.Speaking of tubes, I wish the silly LEDs under the tubes were not included. Even though this is just a preamp there is enough voltage running through the tubes to emit a slight glow after the tubes have warmed up. The red LEDs that turn on when powered up are *sort* of useful to make sure the device is working, but ultimately silly as it looks super fake compared to a real tube glow.

For pics of the unit go check elsewhere , Ok the tubes do function . Meaning they are in the circuit and glow . This unit will utilize the tubes not just a passthrough unit .Lot of reviews say upgrade the power supply . I see zero reason to do so . I have ZERO static or distortion .If your rca cables are out of a receiver or grounded as they should be there will not be a buzz or hum. With the volume at zero on my main unit put my ear next to the speaker zero sound . Keep in mind my sensitivity is higher then most up in the 110 db !! with my tweeters . Most home box speakers are about 90db. Meaning if there was a buzz it will be six time as loud with my speakers .Ok things to upgrade . The tubes . I strongly suggest a rca tubes if you can find them . They have some of the best 3d sound tubes for a fare price . Then golden lion brand tubes .I am comparing this pre amp to much higher models audio control . Dynaco . Macintosh .winner of the three top units is audio control my opinion but costs 10k and up.If i did a A and B test between a 5000 k unit you may or may not notice any differences depending on your amps and loud speakers . This cheap under 50 unit vs 5000 Keep in mind i upgrade the tubes .Factory tubes work just fine . But rca will give much more 3d sound .So this preamp will give more 3d tube sounding music . Guitars piano crystal clear . You will also notice it may organize the sound that is huge from such a cheap unit .I give a 100 percent buy this product . No picking on the mess in my picture lol you understand .

Sound quality to price ratio = 5 Stars. Hardware = 4 stars.Sound quality is actually good. Enough bass, mids are just right, not too forward nor backward. Treble is sweet. I like that you can roll op amps to get the sound signature you want.The tube sockets are kind of loose fitting. If you roll tubes a lot, chances are the sockets will loosen further. Suca Audio and FX audio beats Fosi in this regard.The supplied power adapter is only rated for 1 amp. Fosi should have supplied a higher amperage like 1.5 or higher amps.Changing some capacitors in the power chain and in the audio chain improved the sound a lot and I mean a lot. Wider soundstage and I think even the frequency response - deeper lows and higher frequency extension. Sound quality changed from good to great.By the way, Fosi Audio's customer service is excellent. The first unit I received is faulty and Fosi Audio replaced it with a new unit.