Fosi Audio Box X5 Phono Preamp Review

[carsknivesbeer]

Nice job Fosi. Any bets on whether the new JDS Labs can beat it? I had been eyeing that one.

 

[Skeeter]

How is input noise current on MM being accounted for in these measurements?

 

[morillon]

10k output? i imagine that s just a error(?)
no capacitance value..
Post in thread 'Fosi Audio Box X5 Phono Preamp Review' https://audiosciencereview.com/foru...box-x5-phono-preamp-review.59982/post-2198033

 

[Wiredforsound]

Old vinyl give me the original content.

It's like reading with paper books.

Not easily manipulated.

 

[DSJR]

Not sure about the "cheaper turntables". There is an unavoidable resonance at a subsonic frequency, regardless of the TT/arm/cartridge quality. The exact frequency will vary, of course, depending on mass and compliance, and damping can reduce the peak, but it's always there and it does no good to amplify it, even if it's at the optimum 10-12 Hz.

I'm thinking potentially deeper than this, as so many vinyl systems, especially cheaper ones where the deck isn't as fastidiously sited or used as those of us schooled in the 70s and 80s vinyl era were... Inappropriate pickup choice and the lf resonance can be 10 - 15dB high on the midrange level at all manner of frequencies below 15Hz or so. Add a solid-plinth style deck with lid raised when playing and acting as a sound-board, transmitting all manner back into the structure and it's a recipe for disaster in my opinion!

 

[mike70]

That discussion was not what I was referring to. There are plenty of posts upthread just rehashing the old vinyl v digital stuff (eg loudness wars).

I agree 100% ... but, I would prefer that a warning be put up when someone starts writing concepts (also repetitive concepts) against those who use vinyl.
It always happens in that way and then the snow ball rolls.

Everything I said is with respect and in favor of tolerance, i hope you find it in that way also.

 

[SIY]

How is input noise current on MM being accounted for in these measurements?

It isn't. IMO, doing an MM review without using an MM impedance as a source misses a lot of potential problems.

 

[morillon]

Updated!
Amazon AU: https://www.amazon.com.au/dp/B0DLWM2YPD

Post in thread 'Fosi Audio Box X5 Phono Preamp Review' https://audiosciencereview.com/foru...box-x5-phono-preamp-review.59982/post-2198087


anwers mr fosi ?

 

[KSTR]

It isn't. IMO, doing an MM review without using an MM impedance as a source misses a lot of potential problems.

Yep. Shorted input doesn't reflect current noise contribution and also it does not reflect the benefit seen for voltage noise from a synthesized load impedance ("cold" resistor).

 

[Skeeter]

I’m at a loss as to how SnR can be quoted at 92 dB on a phono amp or 83 dB SINAD. Straight physics says that is not possible. The thermal noise of a 47k MM load resistor shunted by a typical MM cart (taken as 500mH in series with 1.3k Ohms) is 3.1uV. This sets the absolute best case SnR to 76 to 77 dB ref 5 mV. To achieve that level of performance, the input devices will have to be carefully selected for lowest noise voltage and importantly, very low noise current. Any old opamp or discrete device(s) don’t cut it.

Amir, I do wish you’d measure these things correctly. To assess the noise performance you could have a metal box into which you mount a typical cartridge, bring the connections out to some RCA plugs. To get the true SnR, connect the box to the DUT and get the integrated noise output over 20Hz to 20 kHz on the AP. Calculate unweighted SnR from the output voltage which you get in step 2 which is to remove the box and inject 5 mV from the AP to get an output signal level (this will automatically account for gain variance from product to product).

Why is this important? Because injecting a signal into a phono amp from a low Z source like a AP completely ignores the biggest noise contributor in an MM phono amp: input noise current. BTW John Atkinson from Stereophile has also been taken to task for not measuring MM phono amps correctly.

 

[KSTR]

I’m at a loss as to how SnR can be quoted at 92 dB on a phono amp or 83 dB SINAD. Straight physics says that is not possible. The thermal noise of a 47k MM load resistor shunted by a typical MM cart (taken as 500mH in series with 1.3k Ohms) is 3.1uV. This sets the absolute best case SnR to 76 to 77 dB ref 5 mV.

This all refers to 1kHz as test frequency (and I can confirm your values, 78dB would be the 20Hz...20kHz SNR for 5mV 1kHz @ 40dB gain with a totally noiseless amplifier, only accounting for the load resistor and cartridge thermal noise).
So, maybe they measured at low frequency, 20Hz? Or A-weighted the noise?

Again, this shows that specs are useless unless all relevant conditions are stated.

 

[SIY]

I’m at a loss as to how SnR can be quoted at 92 dB on a phono amp or 83 dB SINAD. Straight physics says that is not possible. The thermal noise of a 47k MM load resistor shunted by a typical MM cart (taken as 500mH in series with 1.3k Ohms) is 3.1uV. This sets the absolute best case SnR to 76 to 77 dB ref 5 mV. To achieve that level of performance, the input devices will have to be carefully selected for lowest noise voltage and importantly, very low noise current. Any old opamp or discrete device(s) don’t cut it.

Amir, I do wish you’d measure these things correctly. To assess the noise performance you could have a metal box into which you mount a typical cartridge, bring the connections out to some RCA plugs. To get the true SnR, connect the box to the DUT and get the integrated noise output over 20Hz to 20 kHz on the AP. Calculate unweighted SnR from the output voltage which you get in step 2 which is to remove the box and inject 5 mV from the AP to get an output signal level (this will automatically account for gain variance from product to product).

Why is this important? Because injecting a signal into a phono amp from a low Z source like a AP completely ignores the biggest noise contributor in an MM phono amp: input noise current. BTW John Atkinson from Stereophile has also been taken to task for not measuring MM phono amps correctly.

Since the output Z of the AP is about 20R (negligible compared to the cartridge impedance), a tester can do what I do for phono measurement, put the cartridge in series with the generator output. This will allow the test to show the current noise of the input, the thermal noise of the input impedance, and modulation of noise floor with signal.

 

[restorer-john]

Since the output Z of the AP is about 20R (negligible compared to the cartridge impedance), a tester can do what I do for phono measurement, put the cartridge in series with the generator output. This will allow the test to show the current noise of the input, the thermal noise of the input impedance, and and modulation of noise floor with signal.

Does the AP offer the ability to short the gen outputs when you need to? (for S/N testing) Or does it do signal in the presence of noise instead?

I've got an AT-91 in a box with as dedicated MM (~410R+400mH) and MC (10R+22uH- components) switched load.

That plot you showed back in the thread has a lot more going on in that Project box and the 60Hz skyrockets with the cartridge connected- is it in a shielded box?

 

[AudioSceptic]

I'm thinking potentially deeper than this, as so many vinyl systems, especially cheaper ones where the deck isn't as fastidiously sited or used as those of us schooled in the 70s and 80s vinyl era were... Inappropriate pickup choice and the lf resonance can be 10 - 15dB high on the midrange level at all manner of frequencies below 15Hz or so. Add a solid-plinth style deck with lid raised when playing and acting as a sound-board, transmitting all manner back into the structure and it's a recipe for disaster in my opinion!

Yes, agreed, any of those will make the problem much worse, but a high pass filter is always a good thing, even in an optimal system (as long as it's steep enough not to affect the musical content, of course). I expect Fosi will be onto this very soon.

BTW I remember when "rumble" used to mean noise from a poor main bearing, but now it seems to be used for any unwanted LF signal. Have you noticed that?

 

[SIY]

Does the AP offer the ability to short the gen outputs when you need to? (for S/N testing) Or does it do signal in the presence of noise instead?

I've got an AT-91 in a box with as dedicated MM (~410R+400mH) and MC (10R+22uH- components) switched load.

That plot you showed back in the thread has a lot more going on in that Project box and the 60Hz skyrockets with the cartridge connected- is it in a shielded box?

No shielding. If you look at the suite of measurements, you can see that while the cartridge pickup isn't a negligible contributor to the noise, the preamp's hum is a good chunk of it. Note that the Y axis in that graph is noise density, not noise voltage. And actually, the 60Hz pickup isn't that bad: input referred, that's 0.9uV/√Hz.

I'm of two minds here- a cartridge WILL be out and unshielded in actual use, but possibly hum pickup will contribute to measurement uncertainty. Either way, I didn't have a fabricated metal box for this, so I'll rationalize that the former is better.

When I did shorted input, it was literally a phono plug with the center pin and outer shell soldered together.

 

[restorer-john]

When I did shorted input, it was literally a phono plug with the center pin and outer shell soldered together.

Like my really expensive ones?

 

[SIY]

Like my really expensive ones?

View attachment 421143

They could be quadruplets.

 

[amirm]

Amir, I do wish you’d measure these things correctly.

I have yet to seen it done "correctly." By far the main contribution to SINAD is mains hum (aggravated by RIAA equalization) . What you propose increases the propensity of measuring just that instead of any input contributions with long wires to a cartridge hanging in the air. The input signal level is incorrect resulting SINAD just being SNR, dominated by said mains hum. I used to use higher input voltage to get 2 volts nominal but folks insisted on 5 mv so here we are, completely ignoring distortion products.

Use of any cartridge raises the question of why that instead of another. And how the results are comparable to people using other cartridges.

This is on top of the fact that groove noise, etc. dominates actual use. Not input current, etc.

At the same time as all of this, folks then ask for shorted input noise and such which is worse than what I use currently.

Bottom line, I am not proud of this suite of phono tests but is what I have settled down on. It seems to differentiate products from each other and no one has shown actual uses that disagree with these findings.

 

[SIY]

Use of any cartridge raises the question of why that instead of another.

That's why you pick something of more-or-less average DCR and inductance representing the majority of MM. There may be a few dB of difference from cartridge to cartridge, but the difference to the shorted input (or 20R source of the AP) is 20dB. Don't let perfection be the enemy of the good.

 

[Heinrich]

This is a review and detailed measurements of the Fosi Audio Box X5 stereo Phono stage. It was sent to me by the company and costs US $109.99.

As always, questions, comments, recommendations, etc. are welcome.

The cost has raised on the Fosi site from 110 to 131. Alternative vendors followed or mark the product out of stock. Good thing is, the 110 has once been true. I‘ve seen such with DS2 and the V3 Stereo before. Very soon after a (truthfully glowing) review the price raises significantly.