Fosi Audio Box X5 Phono Preamp Review

[antcollinet]

Does this thing really have 10k ohm output impedance?

I hope not. Where've you seen that?

 

[Kolner]

It’s in the manual.

Others posted about it 30 pages ago. I haven’t had time to read the entire thread.

 

[antcollinet]

It’s in the manual.

so it is. Seems unlikely. The poster you are referring to suggested it is a typo.

@Fosi Audio - can you check. 10K output impedance seems wrong.

 

[Kolner]

so it is. Seems unlikely. The poster you are referring to suggested it is a typo.

@Fosi Audio - can you check. 10K output impedance seems wrong.

I was indeed considering purchasing but I cannot by a phono stage with 10k ohm output impedance. If they say it’s 10k ohm, then 10k ohm it is.

They should have been aware of this for a while now as it has already been mentioned.

 

[StevenSGT98]

I was indeed considering purchasing but I cannot by a phono stage with 10k ohm output impedance. If they say it’s 10k ohm, then 10k ohm it is.

They should have been aware of this for a while now as it has already been mentioned.

It’s a mistake. Closer to under 140 ohm. From another thread.

 

[Kolner]

Thanks @StevenSGT98

 

[chrisb1983]

Just purchased this unit for £76.79 including shipping and taxes to the UK from AliExpress. Slightly longer delivery timeframe but absolute bargain, not seen it cheaper anywhere else.

 

[mackat]

This is what I'm getting in 66 dB MC mode with a cartridge connected but not playing anything - a clearly audible buzz (when the volume is turned up) that looks to be 60 Hz AC noise.

I'm using a different AC brick of the same spec (was curious if the issue would still present), but the stock one sounds exactly the same.

Is this likely inherent to the design, something that could somehow be remedied, or a potential issue with my unit?

Here's the recording: https://voca.ro/13QF8zDxfGis

Amplified so you can hear the noise easier: https://voca.ro/1lmOEAHEKocS

Thanks!

 

[Heinrich]

... - a clearly audible buzz ...

Here's the recording: https://voca.ro/13QF8zDxfGis

Amplified so you can hear the noise easier: https://voca.ro/1lmOEAHEKocS

You're in the phono section. We know how buzz sounds

To some degree it is normal. I have an elder Thorens 126 that shows it in my spectral analysis, originating in the motor, but I cannot make it audible with halfway reasonable volume settings.

In order to check the power supply you may test if there's a connection between the "earth", safety null of the (3 pole?) wall plug and any part of the pwr/ device's output connector. Turn the plug around and look if it betterns things.

 

[mackat]

You're in the phono section. We know how buzz sounds

To some degree it is normal. I have an elder Thorens 126 that shows it in my spectral analysis, originating in the motor, but I cannot make it audible with halfway reasonable volume settings.

In order to check the power supply you may test if there's a connection between the "earth", safety null of the (3 pole?) wall plug and any part of the pwr/ device's output connector. Turn the plug around and look if it betterns things.

You make a good point!

I just find it unfortunate that the Fosi X5 has this noise, while I have other MC phono stages which do not.

I'm not sure I fully understand your suggestion - the wall plug is two-prong on both AC supplies I have.

 

[Heinrich]

You make a good point!

I just find it unfortunate that the Fosi X5 has this noise, while I have other MC phono stages which do not.

I'm not sure I fully understand your suggestion - the wall plug is two-prong on both AC supplies I have.

Please excuse the poor English. For somewhat subtle reasons, it may depend on the orientation of the two-pin plug to the mains connection. I observed an effect of -15dB or so with my Thorens (while still not audible)?! Otherwise, I can hardly imagine that the X5 is badly designed with regard to ground loops, or that it is even possible to copy the mains frequency through to the OPs used. They typically have at least100dB operating voltage rejection.

 

[mike70]

The best option for MC cartridges is a SUT and a MM preamp (40dB gain).
That sky high gains (>60dB) are so much for a cheap preamp.
I use that "formula" with great results since years, a decent cheap MC preamp is like a white fly ... very very rare.

 

[morillon]

The best option for MC cartridges is a SUT and a MM preamp (40dB gain).
That sky high gains (>60dB) are so much for a cheap preamp.
I use that "formula" with great results since years, a decent cheap MC preamp is like a white fly ... very very rare.

some prefer active pre-pre to the sut, in thdg approaches, a little more versatile and with generally more relevant bass responses... etc.
and quickly the ambitious ones end up being very expensive even more than high-end prepre... and turn the situation around... from affordable to expensive solutions

(even if the sut solution is of course easy in terms of cabling etc.)
;-)

 

[mike70]

some prefer active pre-pre to the sut, in thdg approaches, a little more versatile and with generally more relevant bass responses... etc.
and quickly the ambitious ones end up being very expensive even more than high-end prepre... and turn the situation around... from affordable to expensive solutions

(even if the sut solution is of course easy in terms of cabling etc.)
;-)

i have a SUT with 4 or 5 configurations (variations of internal wiring in the transformers), so it's very flexible.
About the theoretical properties of SUT vs Active Headamp ... you also have to say that transformers are the less noisy (much better SNR) ... as always, manufacturers puts the marketing to work in favour of one or another option.

But, from what i can see, SUTs are the preferred way generally ... and in my personal case ... it works excellent, subjectively more lively and detailed than the mc preamp option with zero noise.

 

[morillon]

on certain points the suts are more limited like the frequency response etc.... I don't care, I have both in the cupboards.. ;-)

but my entourage in my country of very very "vinyl hdg enthusiasts" are on the contrary essentially on prepre... ( big system loudspeaker etc frequency reponse impact?)
with many, a classic of classics...hiraga 2, diy, assembled in often expensive components
;-)

 

[guy48065]

Is this likely inherent to the design, something that could somehow be remedied, or a potential issue with my unit?
...

If it hasn't already been done by somebody I would like to see the AC part of the internal power supply removed and modified to use an external quiet DC supply. Many other preamps over the years have been modified like this.
When I bought mine I intended to do that hack but life interfered and I'm tied up with retiring/selling/moving and now my X5 is boxed up.

 

[antcollinet]

Many other preamps over the years have been modified like this.

Probably mostly a complete waste of effort. No doubt some will even perform worse.

 

[guy48065]

Probably mostly a complete waste of effort. No doubt some will even perform worse.

Or not.
Why do you like like AC near your high-gain prepre?
Rather counter-intuitive, isn't it?

 

[antcollinet]

Or not.
Why do you like like AC near your high-gain prepre?
Rather counter-intuitive, isn't it?

Not particularly if you understand the engineering.

Bear in mind any DC input is also derived from AC - unless you are using a battery, which is another expensive inconvenient waste of time. If switch mode then it is derived from high frequency fast switching edges.

What matters is the amout of noise that appears on the output of the device. Not the amount of noise that goes into it. Noise on the ouput will depend on the PSRR of the circuits inside the device. Regardless of what you put into it.


In this case, you are getting PSU noise (60Hz) leaking through at -90dB compared to a 420mV signal (at the output of the amp) AFTER 38dB of gain. That is about 10micro volts - and is going to be inaudible unless you have the volume set for ear damaging levels - and even then only when the music has stopped.

It is also going to be around 20dB below the noise floor of vinyl.

 

[avantbored]

Anybody getting issues with channel imbalance? My left channel is much louder than the right with my unit. Tried different line level inputs on my amp (Sansui AU-8500) and, while operating the "Mode" toggle, the imbalance follows from "Stereo" to "Reverse."

Isolated it to the unit and not the cartridge, TT, or amp.

I re-read the manual. Am I psyching myself out? This thing's got higher gain than a regular preamp? So should this unit be used even with a line level input on my preamp?