Fosi Audio Box X5 Phono Preamp Review

[MidFi Guy]

Anyone know how the Fosi X2 sounds compared to the X5? The specs show an SNR of 98 for the X2 vs 92 for the X5. It’s only MM, which works for me.

 

[SIY]

Anyone know how the Fosi X2 sounds compared to the X5? The specs show an SNR of 98 for the X2 vs 92 for the X5. It’s only MM, which works for me.

Remember that the medium will limit you to 60-70dB. So that difference means essentially nothing.

 

[Heinrich]

I am certainly not dismissing the Fosi. I would expect that to achieve the lowest noise, the OPA1612 would be used as the MC head amp (I have used it in that function) and the NE5534/2 in the MM stage. Without seeing the circuit, I am presuming that is in fact what they have done. The OPA1612 is noisy as an MM amp because the input noise current is high.

As you said, we do not have a schematic, hence can only speculate. But according to some all speculation is about, as they say, „pretty crucial“ things. That‘s not a nice gesture towards Fosi.

But reiterated, in case vinyl regains a bit of importance for me, the X5 would be it once and for all. Maybe after some adoptions using my soldering iron and a smidge of brains.

People might be unforgiving because Fosi again achieved 95% of one can hope for, for relatively no money. High end without the expenses. How would you feel if you invested 10 times the price for big name and (way) less performance? Betrayed?

 

[mike70]

Remember that the medium will limit you to 60-70dB. So that difference means essentially nothing.

... and recordings are normally between 40 - 60dB ... so the difference means less than nothing

 

[cfwotzka]

As you said, we do not have a schematic, hence can only speculate. But according to some all speculation is about, as they say, „pretty crucial“ things. That‘s not a nice gesture towards Fosi.

But reiterated, in case vinyl regains a bit of importance for me, the X5 would be it once and for all. Maybe after some adoptions using my soldering iron and a smidge of brains.

People might be unforgiving because Fosi again achieved 95% of one can hope for, for relatively no money. High end without the expenses. How would you feel if you invested 10 times the price for big name and (way) less performance? Betrayed?

excellent statement!
I'm able to compare the "little" Fosi to much more expensive Phono amps collected over the years and my impression is this is a "big" one. even for 20 times the price Phono amps to my ears still competitive. really very much enjoy to listen to it's neutrality and punch. in relation to it's price a masterpiece in my opinion.

 

[Jimi Floyd]

Remember that the medium will limit you to 60-70dB. So that difference means essentially nothing.

We know

 

[KSTR]

Without seeing the circuit, I am presuming that is in fact what they have done.

IMHO, the PCB photo shown suggests otherwise, the 5532 being late in the chain.

 

[KSTR]

In detail:

Signal path appears to be:
The first relay switches load resistance between 47k and 100R, as a part of the MM/MC switching.
The second relay switches between two gains of the first 1612 gain stage, likely the MM/MC gain switch.
DC blocker caps.
RIAA network is passive.
Second 1612 forms one part of the four gain settings, selecting between two relatively large base gains (for the 38 and 56 dB settings) with the third relay.
DC blocker caps.
5523 is switched between another two gains 10dB apart, for the 10dB additional gain (48 and 66 dB) with the fourth relay.
DC blocker caps.
Fifth relay could be output muting to prevent power-on/-off pops.

 

[rongon]

Newly made scan attached here.

Still no input capacitance specified. The preamp's Cinput is pretty much a non-issue for MC cartridges, but it can be important for MM cartridges. It's probably best to keep the Cinput as low as possible, since there will be significant capacitance introduced by the tonearm wiring and interconnect cable from the turntable to the preamp, especially if a long cable is used.

The input signal voltage is shown as 5mV, but is that RMS or peak? I mean, it could be peak-to-peak for all we know. Or each signal voltage spec could be chosen for best results. (Sorry for being so cynical-sounding. I'm playing devil's advocate here.)

If the output impedance (Zout) is really 10k ohms, that's really high for a solid state phono preamp. Even a vacuum tube phono preamp with a cathode follower output stage could easily beat that. I'm puzzled how Zout could be that high from a 5532 op-amp. Is there a 10k ohm resistor placed in series after the output from the 5532 output stage?

???

PS - on the choice of input opamp -- If you design a low cost preamp with both MM and MC capability, you're going to have to pick the input opamp that represents the best compromise that works well enough for both. A JFET input opamp will have higher current noise, but since we're talking about phonograph records here (with their high noise floor) is this really that important at typical MM signal levels (nominal 5mV RMS)? I think Fosi made a valid choice optimizing the noise performance for low output MC cartridges, typically 0.25mA to 0.5mV nominal.

But come to think of it, how many people buying a $110 dollar phono preamp are going to pay $400 and up for a low output MC cartridge?

 

[Balle Clorin]

Selling a RIAA without informing about MM input capacitance is not good, actually unacceptable.Makes me think if there is anything to hide..

 

[Skeeter]

In detail:
View attachment 421928
Signal path appears to be:
The first relay switches load resistance between 47k and 100R, as a part of the MM/MC switching.
The second relay switches between two gains of the first 1612 gain stage, likely the MM/MC gain switch.
DC blocker caps.
RIAA network is passive.
Second 1612 forms one part of the four gain settings, selecting between two relatively large base gains (for the 38 and 56 dB settings) with the third relay.
DC blocker caps.
5523 is switched between another two gains 10dB apart, for the 10dB additional gain (48 and 66 dB) with the fourth relay.
DC blocker caps.
Fifth relay could be output muting to prevent power-on/-off pops.

Pity. If that’s the case they have thrown away a few dB SnR. Are you sure the relay is not just bypassing the 1612 and routing it down to the 5532?

I suppose we will not know until someone traces the circuit out.

 

[Skeeter]

View attachment 421952

Still no input capacitance specified. The preamp's Cinput is pretty much a non-issue for MC cartridges, but it can be important for MM cartridges. It's probably best to keep the Cinput as low as possible, since there will be significant capacitance introduced by the tonearm wiring and interconnect cable from the turntable to the preamp, especially if a long cable is used.

The input signal voltage is shown as 5mV, but is that RMS or peak? I mean, it could be peak-to-peak for all we know. Or each signal voltage spec could be chosen for best results. (Sorry for being so cynical-sounding. I'm playing devil's advocate here.)

If the output impedance (Zout) is really 10k ohms, that's really high for a solid state phono preamp. Even a vacuum tube phono preamp with a cathode follower output stage could easily beat that. I'm puzzled how Zout could be that high from a 5532 op-amp. Is there a 10k ohm resistor placed in series after the output from the 5532 output stage?

???

PS - on the choice of input opamp -- If you design a low cost preamp with both MM and MC capability, you're going to have to pick the input opamp that represents the best compromise that works well enough for both. A JFET input opamp will have higher current noise, but since we're talking about phonograph records here (with their high noise floor) is this really that important at typical MM signal levels (nominal 5mV RMS)? I think Fosi made a valid choice optimizing the noise performance for low output MC cartridges, typically 0.25mA to 0.5mV nominal.

But come to think of it, how many people buying a $110 dollar phono preamp are going to pay $400 and up for a low output MC cartridge?

There’s lots of relays on that board. They are not cheap and NE5532 devices are very cheap on the other hand. You make a good point about a user buying a $110 preamp and a $400 MC cart. But, you see that all the time with speakers, amps and cables. I’ve seen people spend a grand on class D amp, a grand on speakers and then get talked into $400 speaker cables

 

[Skeeter]

Remember that the medium will limit you to 60-70dB. So that difference means essentially nothing.

I’d love to know where the 92 dB comes from. In the interest of low noise, I tried shorting the input to my phono preamp but unfortunately when I do that, no sound comes out the speakers

 

[KSTR]

Pity. If that’s the case they have thrown away a few dB SnR. Are you sure the relay is not just bypassing the 1612 and routing it down to the 5532?

I suppose we will not know until someone traces the circuit out.

The routing is clearly visible, even the few tracks on the bottom side are very easy to estimate and the basic schematic could be drawn quite quickly for those "skilled in the art" as they say ;-)
I don't know if it's worth the effort to disclose it publicly and I also respect IP a bit. The curious can make the investment and buy one to fully reverse-engineer it.

 

[Hamrin]

Ok the second unit goes back too...
Same issues... +48dB hisses like a snake.
And the AC adapter is a crap on this unit too.

I've noticed both had the QC5 label...

This box is an epic fail on my experience.

 

[amirm]

You have some other issues causing the hiss. Nothing is wrong with the AC adapter either. It would not at all create a "hiss."

 

[Hamrin]

Two different things.

The adapters are noisy themself, just plugged them to the mains. No need to connect to the box... The noise is caused by something inside the adapter (DC offset?).

The hissing is something that happens just switching the knob, soon back after the relay.

Denon DL110 and Hana SH here... +48dB is a needed gain.

Other phono stages are silent.

 

[Heinrich]

Selling a RIAA without informing about MM input capacitance is not good, actually unacceptable.Makes me think if there is anything to hide..

The input C that you are referring to could have at least two meanings. In the ancient days it was unavoidable with valves, hence to be minimized as to show technical skill. It could also, if done right, serve as an RFI filter with a little bit of noise penalty, needing an additional resistor in the signal path.

What‘s your choice?

I mean, if you have strong preferences you could measure (or evaluate by critical listening) the amplitude frequency response, that results from said capacitance — and many other factors. I don‘t see how the isolated value of „input cap“ would actually help you. At least the input resistance is as big as a factor also. Asking for switches, best realized with relays, would raise the cost a little bit. Not everyone is that picky, and many who are would know how to use a soldering iron after scanning the technology for remaining unknowns.

 

[iLoup]

But come to think of it, how many people buying a $110 dollar phono preamp are going to pay $400 and up for a low output MC cartridge?

My Ortofon Rondo Red MC wasn't cheap.
For me the X5 is a bargain (€89 in 2024) and sounds great. A clear improvement to my Clearaudio Smart Phono-1, which costed me 3x in 2008.
And no Hiss or whatever (potential) problems discussed here.
No need to label it as cheap china-ware.
Are there better ones? Likely, but for me the X5 is perfect fit.

 

[rongon]

The input C that you are referring to could have at least two meanings. In the ancient days it was unavoidable with valves, hence to be minimized as to show technical skill. It could also, if done right, serve as an RFI filter with a little bit of noise penalty, needing an additional resistor in the signal path.

What‘s your choice?

I mean, if you have strong preferences you could measure (or evaluate by critical listening) the amplitude frequency response, that results from said capacitance — and many other factors. I don‘t see how the isolated value of „input cap“ would actually help you. At least the input resistance is as big as a factor also. Asking for switches, best realized with relays, would raise the cost a little bit. Not everyone is that picky, and many who are would know how to use a soldering iron after scanning the technology for remaining unknowns.

From the TI OPA161x datasheet:

Input capacitance is negligible if nothing has been added to the input circuit. However, there is usually a parallel capacitor added to ensure stability and/or block RF. Is that present? If so, what's that cap's value? Is there a ferrite bead on the input?

Note the very low input impedance for differential input, 20k ohms.