Fosi Audio Box X5 Phono Preamp Review

[Jimi Floyd]

I still don't understand how this poll is rating the Fosi Audio Box X5 Phono Preamp great when no input MM capacitance is supplied in the specs

 

[KSTR]

I still don't understand how this poll is rating the Fosi Audio Box X5 Phono Preamp great when no input MM capacitance is supplied in the specs

Ha, Amir rates a limited set of circuit performance under idealized lab conditions, nothing more and nothing less. And people follow.
BTW, I voted "not terrible" exactly for the issues with specs and documentation.

 

[guy48065]

The proper function would be expected. All that noise theme goes away with the first touch of the needle with the vinyl groove . Same applies to distortion of the phono preamp compared to vinyl technology intrinsic distortion.

That's what I keep saying. The measurements seek to explain what our ears perceive...not the other way around. What matters is "Does it sound good?".

 

[mike70]

That's what I keep saying. The measurements seek to explain what our ears perceive...not the other way around. What matters is "Does it sound good?".

sound good for ... who?
what's good for you is good for me? (or vice versa)
what speakers?
what room?
what acoustic treatment?

nah, i prefer the measurements and to know what kind of sound i like and what to get with a component

 

[cfwotzka]

Ha, Amir rates a limited set of circuit performance under idealized lab conditions, nothing more and nothing less. And people follow.
BTW, I voted "not terrible" exactly for the issues with specs and documentation.

measurements are helping to decide if it makes sense to buy and listen to and yes, people follow, then listen to and individually rate what they are hearing.
isn't his part of the idea of this forum? if not perhaps I completely misunderstood ASR aims

 

[Heinrich]

I still don't understand how this poll is rating the Fosi Audio Box X5 Phono Preamp great when no input MM capacitance is supplied in the specs

Good question, easily answered. I voted for a top rank, considering the use case. First it looks pretty. It is virtually transparent with either MM and MC. In regard to the input C my take is as follows, assuming it is on the low side well below 100pF.

A user wouldn‘t care because he knows that analog, especially with mass distributed media as vinyl, is a bit vague anyway. Let alone flee market, well (mis)used rare copies. Take it as it comes.

A user would understand that in order to estimate the relevant C the tonearm wiring and the interconnect’s C are needed likewise. He would just measure it having a simple multimeter at hand, because such thing is quite naturally part of his household.

Each of both could go for personal preference in the end, tuning the C via critical listening, after minimizing the interconnect‘s C anyway.

A user asking for the amp’s C to be specified would most probably ask directly after for a switchable C, by numbers. That‘s a use case the X5 isn’t made for, obviously.

 

[amirm]

Likewise, the noise of the 47k resistor (shunted by any load capacitance) will be present under real world use, so should be part of the consumer-facing measurement.

But that would factor out as it would be constant for all phono stages tested.

 

[amirm]

Ha, Amir rates a limited set of circuit performance under idealized lab conditions, nothing more and nothing less.

There is little that is "idealized" in my testing or I would not have to spend 15 minutes every time trying to reduce ground loops/mains leakage which is absent in simulations, etc. that you have been offering. It is these real life conditions which render some of the other noise sources moot.

 

[amirm]

No, but they clearly show the noise floor and the change with source impedance (admittedly, it's clearer in my graphic presentation, but Pavel's data are consistent with mine). If we use your standard of 1kHz signal, the increased noise floor will absolutely drop the SINAD (or S/N). I mean, the noise rise in the top 1-1/2 octaves is 20dB at 10kHz, well within the hearing range of my aged ears; even weighting it for the RIAA curve, it's very significant.

I hear you but the discussion is about SINAD which I show in the dashboard. That SINAD depends on the actual signal level (numerator) which in turn depends on the gain of the phono stage. Measuring just noise floor (denominator) and then doing back of the envelop math is not going to get us a proper SINAD result.

As I said at the outset, I like to see actual SINAD measurement s as impacted by using a source cartridge vs AP impedance. If we can't show this then we are talking about something that cannot be measured and conveyed to readers.

 

[amirm]

BTW, I voted "not terrible" exactly for the issues with specs and documentation.

Then you voted wrong. Is this what you call "not terrible?"

You have no excuse to go by "specs" and "documentation" when I have provided you far more than any company provides.

 

[SIY]

But that would factor out as it would be constant for all phono stages tested.

That part of the increased noise, yes. But you'll miss noise due to input current noise and, of course, the consumer will expect (based on a near-zero source impedance measurement) lower noise than he'll actually get. I'd demonstrate the effect on SINAD, but after a week of sub zero temps here, my lab is uncomfortably cold to work in. I'll demonstrate it after this cold spell passes. And, of course, given the physics limits of both cartridge and load Johnson noise, it's a fairly meaningless spec once it gets better than 70-75dB or so.

Using a realistic source impedance is not a difficult thing to add to the test protocol, and if you eliminate the near-zero impedance source test (which represents near-zero reality), the number of tests still remains the same.

 

[nagster]

A simple measurement of the Box X5 input capacitance using an LCR meter. Each was set to an average of 128 times.
Also, photos, THD vs. input level, and output impedance.

 

[Jimi Floyd]

Good question, easily answered. I voted for a top rank, considering the use case. First it looks pretty. It is virtually transparent with either MM and MC. In regard to the input C my take is as follows, assuming it is on the low side well below 100pF.

A user wouldn‘t care because he knows that analog, especially with mass distributed media as vinyl, is a bit vague anyway. Let alone flee market, well (mis)used rare copies. Take it as it comes.

A user would understand that in order to estimate the relevant C the tonearm wiring and the interconnect’s C are needed likewise. He would just measure it having a simple multimeter at hand, because such thing is quite naturally part of his household.

Each of both could go for personal preference in the end, tuning the C via critical listening, after minimizing the interconnect‘s C anyway.

A user asking for the amp’s C to be specified would most probably ask directly after for a switchable C, by numbers. That‘s a use case the X5 isn’t made for, obviously.

1) @nagster measured it and it's not "well below 100 pF"

2) A well informed user actually CARES because it is better to get the best possible results even if vinyl has limits

3) I would not trust much a simple multimeter to measure C input of a phono pre

4) You lost my interest at the "tuning C via critical listening" bit

 

[Heinrich]

1) @nagster measured it and it's not "well below 100 pF"

2) A well informed user actually CARES because it is better to get the best possible results even if vinyl has limits

3) I would not trust much a simple multimeter to measure C input of a phono pre

4) You lost my interest at the "tuning C via critical listening" bit

For (1), I said „under the assumption“.
For (4), I wonder if there is in general a personal (p)reference, which in my book defines the „best possible result“ mentioned in (2)

I only tried to help. Again, I personally feel that the X5 is at least a „good enough“ basis, which I think makes it great as being good enough.

Sorry, didn‘t give a link: https://en.wikipedia.org/wiki/Principle_of_good_enough and from there following references, I‘m from the engineering camp

 

[cfwotzka]

A simple measurement of the Box X5 input capacitance using an LCR meter. Each was set to an average of 128 times.
Also, photos, THD vs. input level, and output impedance.

Many thanks nagster! After weeks of theoretical discussions estimations and guesses I appreciate your results that add perfectly to amrim‘s data. In particular MM users now have a direction with sufficient accuracy.

 

[morillon]

Many thanks nagster! After weeks of theoretical discussions estimations and guesses I appreciate your results that add perfectly to amrim‘s data. In particular MM users now have a direction with sufficient accuracy.

thank you mr nagster...

kindly answers the two unanswered questions from Mr. Fosi...capacitance* and output impedance (the 10k is indeed a "mistake")....
(but the rest... just the idea of improving in futur the protocol or warning about certain results...nagster has the elegance of not "going there"... (maybe no cartdrige on hand?) ;- ) )

;-)

(*it would have been good to remain in single value under 100pf...fosi may be taking a little more precaution on a future model)

 

[KSTR]

You have no excuse to go by "specs" and "documentation" when I have provided you far more than any company provides.

The heading of the poll is "Rate this phono stage", not "Rate Amir's review of this phono stage". Your review is fine as is.

 

[Hamrin]

I've never seen an impact below ~1kHz from loading, and certainly nothing in the bass region. "Thin" means different things to different people. On a VM95ML at 50pF all-in you'd be about -2dB at 5.5kHz, -1dB at 10kHz, and -2dB at about 15kHz. If flat is the goal, the best compromise for that cart is about 250pF. All assume 47k Rl.

You're right... Thin is a qualitative description. I'l try to define the best I can.

First of all, I want to carify: I'm not speaking of distortion.
With my AT VM95 and low capacitance values the problem is not harshness nor sibilance.
If i set values below 200pF the reproduction is shomehow "subtracted" from midbass and bass frequencies, but global volume is not affected.
Music has "less body", there are mainly the mid-hig and high frequencies of the spectrum (clean and undistorted).

Making a sort of PEQ comparison, it is like taking the knobs below 400-500Hz and moving these down.
Moving up to 200pF it sounds is as it should, like moving those knobs back to their position.

With 400 pF I can hear softened high frequencies.
With the Shure M75 the impact of capacitance is less pronounced.
This happens with the MX Vynl...

Using the same AT with the Fosi the reproduction is closer to the 50/100pF behaviour described above.
At this point I assume this could be related with gain maybe?

 

[Skeeter]

I am in the process of moving house, but I will try to do a measurement in the next few days comparing shorted input with one with a real-world source (I have an AT95 I can use). I think the measurements of phono stages here on ASR are good, but the noise measurement unfortunately is really just looking at the voltage input noise floor and unfortunately misses the current input noise contribution which for many RIAA phono preamps is the bigger contributor (especially in bipolar input designs).

 

[Skeeter]

A simple measurement of the Box X5 input capacitance using an LCR meter. Each was set to an average of 128 times.
Also, photos, THD vs. input level, and output impedance.

Does your generator do an inverse RIAA sweep? Looking at your overload sweeps, reference the 1kHz sensitivity, the signal coming out the cart at 20 Hz is 20 dB lower. So if you take it as 5mV at 1kHz, at 20Hz it will be 500uV while at 20 kHz it will be 50mV. This seems to indicate the Fosi has 26 dB o/load at 20 Hz (very good) and 15 dB at 20 kHz. But, why are your 10kHz and 20kHz traces overlaid? I would expect there to be a 6 dB difference.