Fosi Audio Box X5 Phono Preamp Review

[@Emperor36]

A 20% off deal is great for a small amp like this

[@aspataro79]

Very cool amp! 🎉

[@DonLee480]

Nice 👍

[@bertroost1675]

Is this pure analog?

[@Thedarkdj4450]

So will this work in-between a fluance rt85 as well as some active powered edifier speakers?

[@HiFiBrothers]

If the active powered speakers have a RCA line-in, yes!

[@Thedarkdj4450]

​@HiFiBrothers so got my fosi box x5 in what is this ground? Does it take a banana clip cause my rt85 ground doesn't fit on it?

[@HiFiBrothers]

@@Thedarkdj4450 the ground is where you would connect your turntables ground wire. You have to unscrew the cap on the X5 ground to fit in the little ground clamp. This is what you should have/need to make the connection: www.amazon.com/Phonograph-Turntable-Magnetic-Cartridge-Turntables/dp/B09V2633G5/ref=asc_df_B09V2633G5?mcid=9c66297dca3832de99a2ce78735fbb08&hvocijid=6921019666665584976-B09V2633G5-&hvexpln=73&tag=hyprod-20&linkCode=df0&hvadid=721245378154&hvpos=&hvnetw=g&hvrand=6921019666665584976&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9030091&hvtargid=pla-2281435179738&psc=1

[@swagonman]

I think more than likely the outside terminal of the power plug is ground. So touching it to the phono ground post is probably no concern. However, I have a Fosi Box 5 on order and will measure this myself with a multimeter. I will also check if the 16v is AC or DC. It’s hard for me to find my way back to where I’ve left a comment on KZbin. But if anyone is interested in my findings, please put a reply comment on my comment to indicate your interest, and that will make it super easy for me to report my findings in this thread. Thanks.

[@HiFiBrothers]

That would be awesome, thank you!

[@swagonman]

@ I measured 18.39V AC. And the outside of the barrel plug is ground, so if you touch it to the phono ground terminal on the back of the Box X5, no problem and no worries. The AC-to-DC rectification is inside the Box X5 probably so they can make sure the power is clean enough to meet their performance specs.

[@HiFiBrothers]

That’s good to know! Appreciate you checking that out.

[@BreakSoFresh]

Thinking about this or a mani 2 ? What do yall think?

[@HiFiBrothers]

We don’t have experience with the Mani 2 but Schiit as a brand has been out far longer with a great reputation. It would have to come down to what you’re getting for the price, pull the trigger and enjoy.

[@Jeep_on_audio]

Get an Aiyima T3 Pro. Cheaper, better and more room for improvement later. I upgraded all 4 opamps and it even sounds great on MC. Which is downright amazing at any price! In 40 years using MC cartridges I never found one that can really do that under $1k.

[@D1N02]

What about it's power supply? It says 16v AC on the back? It would be hard to source a replacement for that.

[@HiFiBrothers]

Don’t see that as being an issue as these devices are regularly available on various marketplaces. Also, they have a pretty good reputation on those marketplaces which should help with buyer confidence.

[@swagonman]

Being an electrical engineer, I would be absolutely stunned if it is 16v AC. I’m sure it is mislabeled and really 16V DC. But I could be wrong. Also, the outside sleeve of most power barrel connectors is usually ground, so I bet if you touch it to the phono grounding post, it would be fine. To double check this, if you have any digital multimeter, even a cheap $5 one, you can unplug the power and then measure the resistance or continuity between the grounding post and the outside terminal of the power port. You will be looking for a low resistance well below 1ohm… so, in the milliohm (mohm) range. Or if using continuity, you want to hear your meter make a beep. Going back to the 16v AC, I guess it might be possible. You could measure the output of the wall adapter with the multimeter in VAC mode. If it is 16v AC, then the wall wort is likely just a transformer, and the rectification and regulation to DC would be in the Fosi box 5. But that would be very unusual and surprising to me. But I’m not a phono preamp expert and don’t know if there is a weird reason for AC power for this application. I think it is more likely a misprint. Good news, I’ve ordered one of these preamps now and will measure it myself. If anyone wants, please respond to this comment, and I will post my findings soon.

[@D1N02]

@@swagonman It isn't a switch mode supply. Turntables also run on AC. It's a step down transformer but it doesn't convert to dc.

[@swagonman]

@ Thanks. Just saw another review where they opened the Box 5 up. Inside the box, the 16v AC is rectified to DC (and probably regulated). You cannot run the OpAmps on AC power - they need DC. I still don’t know if AC power is needed inside a phono PreAmp for anything. In a turntable, AC is maybe used for the motor. The PreAmp doesn’t have a motor. But it is likely Fosi made the design decision for an external transformer and internal AC-to-DC as a linear and low noise power supply that they control, avoiding customers using crappy external power supplies that degrade the performance. If turntables use DC motors, then likely the same reasoning applies. It makes sense if you are dealing with very small signal levels.