TLDR - I love it with all of its foibles.
-- The content comes from SeeDee
So, how did we get here? I love little amps. They don't need to be high powered for my listening enjoyment. I like to have things tuned for the space I am in. I have been rocking the Fosi Audio T20 since 2021 and I love it. I really enjoy the little tube amp in tight spaces. I use it in my kitchen to play audio while I cook or prep for meals (which is a lot). I first paired it with some Pioneer SP BS22 LR speakers. I picked them up used for less than a C note. I loved them too but found the bass lacking even with the T20 which has a bass pot to boost it. I picked up a pair of Klipsch RP500M speakers (in Walnut because Walnut sounds better). They were originally underwhelming compared to the Pioneers but had much more bass response. Eventually, they broke in and I was satisfied-ish. During this time, I was also tube rolling the T20. It originally came with 6J1s which were okay - nothing special. I splurged a little and bought some GE JAN 5654 and they weren't much better. They were smoother but they were missing the attack I was looking for. Nosing around the interwebs, I found some Voskhod Rocket 6J1P-EV which are similar and bingo - I got my attack. There was tight bass and crisp highs - jazz recordings sounded particularly perfect but other music, while good, was a little clinical. Eventually, the Bluetooth module on the T20 died and I started using one of my old Chromecast Audio devices (love these too). I decided that since I was using an RCA hookup for the Chromecast that I wanted to try the Fosi Box X4 preamp which also has tubes for the buffers. I started out by switching out the T20 and using the V3 instead. I popped in my beloved Voskhod Rockets and by song #2 one of the tubes blew. Fortunately, I had picked up some more and I had some spares but I decided to put in the 6J1s first. They did okay. I switched to my GE 5654s (not the ones that came with the X4) because they were burned in - still just okay. Eventually, I put in another set of Voskhod Rockets. I determined that the V3 just didn't sound good with the Klipsch speakers (jury is still out on whether I like it at all). I decided to go back to the T20 with the Rockets and I put in my GE 5654s into the X4 - viola! Sonic bliss!
The Chromecast Audio delivers FLAC with support for high-resolution streams from Plexamp (768K/24-bit/96KHz) which is pretty much what all my ripped CDs are in and streamed from. This is plugged into the X4 via RCA since it doesn't have an optical in. Then, I am going RCA from the X4 to the T20. Since the on/off/volume pot are all in one, I keep the X4 at about 75%. I wish that it had a plain on/off switch like the T20 so that the volume could be left static - I try not to be a knob fiddler. It seems that the bass and treble pots are both additive and cut - I have not found specs on them. From the 12 o'clock position being flat, I have the treble on about 1 o'clock and the bass between 12 and 1. The pots are smooth with no detents. On the T20, I have the bass around 11 and the treble between 1 and 2. The GE JAN 5654s in the preamp section give the music a nice smooth and buttery sound and then the Voskhod 6J1P-EVs in the amp stage kicks it up a notch.
Now, It has it's drawbacks as well. For a preamp, it doesn't have very much pre-amplification hence the 75%+ on the volume knob. The headphone out is unusable. To make it sound semi-normal, I had to turn the bass all the way up and the treble all the way down. Even then, it didn't sound good. The highs are just a little sibilant. I am going to guess that is from the Klipsch horns. Your speakers may be better than mine. I didn't get the same sonic satisfaction with an MM turntable and this is a phono preamp. It sounded really good but was missing something - perhaps the T20 and the Klipsch speakers since the phono is connected to an old Sony receiver and some old rebuilt JBLs. Another thing I wished would be that the bass and treble be in the same spots across the product line. On the X4, it's treble on the left, bass on the right. On the T20 (and BT10), it's bass on the left, treble on the right. Minor complaint, I know but... The body is well made but I wish that the tube openings were large enough to fit socket savers for tube rolling. The T20 had the same problem which I had to bore out the openings - I would probably do this to the X4 (I just like the look of the brass socket savers). Like the Schiit family of products, I kind of wished that the X4 and the T20 were the same size - perhaps the V3 and P3 will be. I have my P3 on pre-order - perhaps another "review" in the works. FWIW, I did find that the V3 pairs nicely with the large 3-way JBLs - not so muddy in the middle. I bought a bunch of cheap Op Amps from Mouser but my eyes have been fixed on some from Sparkos Labs. Perhaps a documentary on my trials and tribulations is in the works... Maybe...
Fosi Audio Box X4 - Not just for record players!
