Fosi Audio Box X5 Phono Preamp Review

[Jimster480]

Looking at this Phono Preamp (no idea what they are actually used for) and its performance is the same as the Michael Fidler one it seems?

 

[abrcdbra]

The scheme is built on the classical principle with a passive correction circuit located between two frequency-independent amplification stages.
For example, such a topology is presented in the datasheet for LME49860, where the gain of the first stage is 23.
I believe that even such a gain of the first stage is excessive for a high-quality device, because it will lead to overload at high frequencies when the stage is powered by 15 V. Note - Chinese designers like to use 12 V. What else will limit the output voltage of the stage.
I have already gotten rid of the device. Therefore, I have a question. Can anyone measure what voltage is used to power the OPA in Fosi X5?

The first stage of Fosi X5 (only MM!) is shown in the figure.
The gain of the first stage is excessively 38. Unfortunately, as a result of this, a signal at a frequency of 11 kHz, overloaded by only 20 dB, turns out to be limited in output. Which leads to a wide range of high-frequency harmonics. And as a result to hardness and dirt in the sound.
But that's not all.
The OPA is loaded at about 600 ohms! But with such a small load, this OPA experiences a sharp increase in distortion. See the OPA1612 datasheet, where the minimum permissible load for this OPA is at least 2 kOhm.
This is the reason for the mediocre sound quality of the entire device.
But that's not all. But a little later.

 

[jazzvinyl2020]

I read throught the entire thread. Would you say that Fosi X5 would compare favorably to my DIY Pass XONO or even likely sound better / outperform it?

 

[abrcdbra]

The second stage of Fosi X5 (MM only!) is shown in the figure.
The correction circuit is 7.32k and 1.5k resistors, 0.3 μF and 0.102 μF capacitors. For a 7.32 kOhm resistor, this is an incorrectly calculated RIAA correction circuit. The error from the standard RIAA curve was 1.7 dB. This is a clearly audible value!
The calculated frequency response of Fosi X5 with the set values of the correction circuit is shown in the second figure.

The OPA is again loaded with a low resistance of about 700 Ohm. In the OPA1612 datasheet, figure 7 shows a sharp increase in distortion at a resistance of 600 Ohm. This is also clearly audible distortion.

To be continued.

 

[antcollinet]

Looking at this Phono Preamp (no idea what they are actually used for) and its performance is the same as the Michael Fidler one it seems?

The spartan 20 from MF has much better high frequency headroom.

 

[DSJR]

I'm aware of that. Nevertheless one should keep in mind, that the RIAA cutting curve to a certain degree counters the usual roll-off towards the higher frequencies in typical music. And I'd guess, that especially checking 45 rpm maxi singles would yield quite a few more examples of pretty hot cuts than found in Shure's old study. And one could also argue, that a good phono stage should also provide a sufficient overload margin for cartridges with pretty hot output (like for example the Pickering NP/AC or the Ortofon Q.Bert...).

Greetings from Munich!

Manfred / lini

Hi Manfred,

I think we've spoken a couple of times on VE...

I wanted to ask please, if you've tested the relative output of Ortofon 2M models, as Lowbeats measured them at twice the specified output (10mV at 5cm/s instead of 5mV - ATs mm types specify 3mV at same velocity).

I remember an acquaintance having terrible trouble with setting up a 2M Blue, the lively hf being too much, even in comparison with a good Shure V15III. Reading all of this, makes me wonder if his issue was more overload in his phono stage (I forget what it was, but nothing esoteric and arguably one in a typical mid-level integrated amp).

I still maintain that, as many vinyl turntable systems are less than optimal in set-up these days, a filter below 20Hz and really decent headroom at all frequencies is vital for best perceived quality in vinyl playback. Difficult to put into words what used to be easily demonstrated.

 

[antcollinet]

The error from the standard RIAA curve was 1.7 dB

And yet Amir measures a deviation of only around 0.5db. And that as a gentle rise on one channel towards higher frequencies. No 1khz peak.

The OPA is again loaded with a low resistance of about 700 Ohm. In the OPA1612 datasheet, figure 7 shows a sharp increase in distortion at a resistance of 600 Ohm. This is also clearly audible distortion.

You are looking at the difference between 0.000016% and 0.000018%. These are not even remotely audible levels of distortion.

 

[nagster]

Does the DUT have to be ON during measurment of capacitance? I tried to use a simple LCR meter but got jut crazy values...nowhere near any spec. I wonder hat I am doing wrong

These are browser translations. They may be strange sentences, but please try to predict/complete them.

Turn on the power to the DUT and measure. There may be phono preamps that do not change even when turned off, but I always measure with it turned on. Set other settings to the same as when using or close to them.

Measurements of 47kohm/5mV/xpF with an LCR meter are not stable at all. Even with a long measurement time setting + 128 average settings, there is still some fluctuation.
It may be difficult with a handheld meter.

In my experience, the measured capacitance changes depending on the applied voltage and frequency, not just when measuring input terminals but also when measuring capacitors. Also, the higher the applied voltage, the less the meter fluctuates, and the higher the frequency, the less the meter fluctuates.

It does not have much of an effect in the case of MM input, but if you do not use ALC, the set voltage is divided by the generator and DUT, and the actual voltage drops, so direct measurement with a DMM or similar is necessary.
After checking the voltage, disconnect the DMM just to be sure.
Of course, this is not necessary for LCR meters with voltage measurement functions.

I would like to increase the applied voltage and frequency. This is because the measurement value will fluctuate less.
When measuring capacitors in the pF range or less, it is sometimes possible to achieve stability by increasing the frequency up to several hundred kHz, but this is significantly different from the frequencies actually used in phono amplifiers.
I would like to apply 1kHz/1V, but it is rare to find a phono amplifier that is capable of doing so.
It may be easier to guess if you add high voltage 50kHz and 100kHz to the low voltage 1kHz and 20kHz.

We want to increase the applied voltage as much as possible, but it is not reliable if the input section is clipping. Therefore, before measuring the capacitance, we perform a level sweep measurement for each frequency to find the upper limit.

Fosi Audio Box X5 Phono Preamp Review

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

www.audiosciencereview.com

The problem of low input impedance of Topping amplifiers

To complicate things, for some unknown reason, Stereophile tests pre-amp outputs into a 600 Ohm load. (as one of it's tests) Its a standard load from the wired telephone era and also used in professional broadcast gear (load and source impedance with coupling transformers) It is probably a...

www.audiosciencereview.com

According to trusted members, this measurement method is one of the best.

Fosi Audio Box X5 Phono Preamp Review

(*it would have been good to remain in single value under 100pf...fosi may be taking a little more precaution on a future model) Maybe some expert could explain to me, why the headroom estimations of @nagster show opposite results. Increasing headroom with frequency, as I would expect if the...

www.audiosciencereview.com

 

[nagster]

The RIAA deviation seems small enough.

Fosi Audio Box X5 Phono Preamp Review

There we have it, again. 240uV (10Hz ...22kHz) noise with cartridge, but only 28uV with AP as source, about 18dB worse and SINAD would take a similar hit. ... Finally strict on topic, see below. But here are some blank numbers from my LTSpice simulation at 20°C, as we are dealing with thermal...

www.audiosciencereview.com

 

[nagster]

A quick search revealed one piece of data showing that a current product achieves 20kHz/1V input. From the Accuphase C-47 catalog.
The maximum input voltage for MM at 20kHz is 1.58V. However, the price may be more than 30 times that of FOSI.

Accuphase Laboratory, Inc.　C-47

www.accuphase.com

 

[lini]

Hi Manfred,

I think we've spoken a couple of times on VE...

I wanted to ask please, if you've tested the relative output of Ortofon 2M models, as Lowbeats measured them at twice the specified output (10mV at 5cm/s instead of 5mV - ATs mm types specify 3mV at same velocity). (...)

Hi DSJR!

And yup, we did. Nice to see you here, too.

Well, and no, I haven't - but I can nevertheless explain these output voltages: Lowbeats simply has used the DIN 0 dB mark as reference, so the corresponding modulation velocity is 8 cm/s (peak and in 45°/stereo modulation) or respectively 11.3 cm/s (also peak, but in lateral/mono modulation). Hence the measured 9.4 mV (Red), 10 mV (Blue and Black) and 10.8 mV (Bronze) would convert to not quite 5.9, 6.3 and 6.8 mV at the usual 5 cm/s (peak and in 45°/stereo modulation). So a bit hotter than specified by Ortofon, but just a tad over +2.6 dB hotter than specified in case of the 2M Bronze - rather than not quite +6.7 dB hotter, as it might appear at the first glance.

That's a general problem with cartridge output voltage specs, btw. For instance several Japanese manufacturers (including Denon, for example) tend to specify their cartridges at the JIS 0 dB mark instead, which also is 5 cm/s peak, but in lateral/mono modulation only - and hence would equal not quite 3.54 cm/s peak in 45°/stereo modulation. So one has to heed the "fine print", when comparing. Provided one can find it, that is - if not, one can never be quite sure, whether one might not be comparing oranges to grapefruits or mandarines rather than oranges to oranges.

Greetings from Munich!

Manfred / lini

 

[abrcdbra]

Any sharp increase in harmonics is audible to the human ear. Regardless of the values, of course, if they are not negligibly small. In triode amplifiers on tubes, nonlinear distortions reach 5%, and we do not hear them. Precisely because of the monotony of growth and because the harmonics are of low magnitude.
In OPA, harmonics are of high orders, and any person with a fine ear (whose ancestor 10,000 years ago was a great hunter) can definitely hear them.
Why do developers in triode SE amplifiers not fight with their 5% harmonics, while developers of transistor technology fight with thousandths of a percent.

 

[abrcdbra]

The third stage of the Fosi X5 (only for MM) is a regular follower. With a gain of +1.
But why the developer installed an “arch” of resistors at the output, none of the professionals will be able to explain. As a result, he excessively overloaded the 5532, taking into account the impedance of the input of the next device. Because two paralleled 10 kOhm resistors are a 5 kOhm resistor.
The last resistor can certainly be removed, it is simply unnecessary. And the first 10 kOhm resistor should be replaced with a 47 kOhm resistor.
And be sure to throw out the first two separating capacitors with 10k resistors after these capacitors from the Fosi X5. They are simply not needed. The bias of the OPA 1612 is small and the accumulated bias at the Fosi X5 output will not in any way reduce its overload capacity.
Even for a normalized device with the overload characteristic required for real gramophone records.

Finally, it is necessary to replace the very first OPA1612 with OPA2228. The latter will finally reduce the noise of the device on MM cartridges. And will bring special transparency and natural liveliness to the sound)
And of course, replace the resistor in the correction circuit if you are familiar with the RIAA standard. ))

This is the simplest thing to do to the device to breathe life into it. If you put up with the dirt at high volume levels from the reduced overload capacity of the device.

 

[antcollinet]

of course, if they are not negligibly small

0.000018% is beyond negligably small - it is around -135dB.

In OPA, harmonics are of high orders, and any person with a fine ear (whose ancestor 10,000 years ago was a great hunter) can definitely hear them.

Human hearing has neither infinite resolution, nor infinite bandwidth. There is no human on the planet, no matter how huntery their ancestors, who can hear these levels even compared to perfection. Let alone a difference between two similarly low levels (0.000018 and 0.000016)

The vast majority of people can't even hear distortion 1000 times larger than this. (Ref klippel online listening tests)

 

[abrcdbra]

A quick search revealed one piece of data showing that a current product achieves 20kHz/1V input. From the Accuphase C-47 catalog.
The maximum input voltage for MM at 20kHz is 1.58V. However, the price may be more than 30 times that of FOSI.

Accuphase Laboratory, Inc.　C-47

www.accuphase.com

Any normally calculated most banal tube corrector. That's why music lovers and audiophiles love them, including.

In principle, if you continue to change the Fosi X5 (a little more), saying goodbye to the 66 dB gain, then you can listen to music with pleasure with it).

In fact, the RIAA corrector is a simple device for a professional developer. You just need to choose the right compromise from approximately 10 conditions when developing it. )

 

[Old_School_Brad]

0.000018% is beyond negligably small - it is around -135dB.



Human hearing has neither infinite resolution, nor infinite bandwidth. There is no human on the planet, no matter how huntery their ancestors, who can hear these levels even compared to perfection. Let alone a difference between two similarly low levels (0.000018 and 0.000016)

The vast majority of people can't even hear distortion 1000 times larger than this. (Ref klippel online listening tests)

But wait -what about those of us descended from noble bat-hunters?

 

[antcollinet]

But wait -what about those of us descended from noble bat-hunters?

Bat screeches can be pretty loud (up to 135dB spl). So even if your bat huntery ancestors had the bandwidth, it says nothing about their ability to hear anything around -35dB

 

[Miser_Audioman]

I just received my X5, swapping out a iFI Zen Air. My turntable is a Rega p3, elys2. The gain adjustment on the x5 really helps with gain management between the cart, Freya S and Fosi V3s. The fosi resists hum interference far better than than the iFI in my system, making a bigger difference in practical bg noise in my particular setup than just the base measurements. The sound is very clean and free of dirt, I am very happy with it.

 

[Balle Clorin]

Nice, it can be worth to try another cartridge too, while you are in shopping mode

 

[Miser_Audioman]

Nice, it can be worth to try another cartridge too, while you are in shopping mode

Yeah no love for the elys2, but the whole this is an upgrade from a Fluance with AT95. Im pleased with the improvements overall