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Fosi Audio P4 Preamplifier Review

Rate this preamplifier:

  • 1. Poor (headless panther)

    Votes: 1 0.5%
  • 2. Not terrible (postman panther)

    Votes: 4 1.9%
  • 3. Fine (happy panther)

    Votes: 50 23.7%
  • 4. Great (golfing panther)

    Votes: 156 73.9%

  • Total voters
    211

amirm

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This is a review and detailed measurements of the Fosi Audio P4 budget RCA preamp. It was sent to me by the company and as of this writing, is on sale for US $80.

Fosi P4 Preamplifier Stereo budget cheap review.jpg

The look is attractive enough for the class. A momentary switch turns on the unit and cycles through inputs, with a reassuring relay click. There are tone controls with detents in the center. Finally and very surprisingly so, the volume control is motorized and activated through remote control (not shown)!!!

Back panel shows exactly what one would need in this class:
Fosi P4 Preamplifier Stereo budget cheap RCA switcher preamp power supplyreview.jpg

Three inputs are provided. A switch nicely bypasses the tone controls if needed. Gain settings are available from 3 to 6 and 9 dB. The only thing missing is trigger support.

Fosi Audio P4 Preamplifier Measurements
The preamp ships with the gain switch set to 6 dB so I ran the dashboard there:
Fosi P4 Preamplifier Stereo medium 6 dB Gain Measurement.png

This is excellent performance for the class with distortion at -114 dB or threshold of hearing. So we are noise bound:
Fosi P4 Preamplifier Stereo medium SNR Measurement.png

We have a wide margin to clear 16 bit audio without impacting its noise floor. Moreover, performance is almost maintained even at highest gain.

We can see the effect of gain on distortion+noise:
Fosi P4 Preamplifier Stereo noise and distortion vs gain and level Measurement.png


IMD test shows exemplary performance when compared to much more expensive reference up to 1 volt output:
Fosi P4 Preamplifier Stereo medium IMD Measurement.png


Frequency with or without tone control bypass is excellent:
Fosi P4 Preamplifier Stereo Frequency Response Measurement.png


Crosstalk is very good:
Fosi P4 Preamplifier Stereo medium 6 dB Gain crosstalk Measurement.png


As is wideband noise+distortion vs frequency:
Fosi P4 Preamplifier Stereo medium Frequency vs Distortion Measurement.png


Conclusions
The Fosi P4 surprises with incredibly low cost while providing features such as motorized volume control and excellent performance. It simply has no weak point, other than absence of trigger. This is what happens when a company is laser focused on customer needs and excellence in engineering.

I am happy to recommend the Fosi P4 to many of you who need to switch between multiple unbalanced/RCA sources.
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As always, questions, comments, recommendations, etc. are welcome.

Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
 
Reserved for @AdamG to kindly post the specs.

Manufacturer Specifications:
IMG_1009.jpeg

Link to Product information and webpage:
 
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Recognizable by now in the audio world, the orange knob is a trademark, excellent performance, excellent prices, wide and interesting range of products, performances always up to the task, indeed very often excellent! This is Fosi!!

well done, you hit the mark with this preamplifier too.

Thanks Amirm for the test!!
 
As preamplifiers go, I prefer the features offered by select WiiM streamers. The Fosi Audio P4 offers a lot of measured performance for the price.
 
Great little device!
Tone controls AND bypass, how thoughtful.
I really wished I had a use for it, but I don't, not at all.
This is excellent performance for the class with distortion at -114 dB or threshold of hearing.
Somehow I doubt that -114dB is the threshold of audibility for distortion for us mortals, unless maybe we are talking about all the HD being in the 127th harmonic (and not the 3rd like here).
 
Thank Amir for the great measurement and review!:D
I find it spectacular that the volume control is motorized
Sorry if I'm asking in the wrong place, but I saw that the BT30D Pro now supports 48V. I bought one about 2 months ago, but it says 24-36V. Is it possible to use a 48V power supply on mine?
 
Hilarious. A virtually transparent straight wire with gain for $80! And to think in the bad old days audiophiles used to pay literally thousands of dollars for this and none of them were nearly as good.

Would be great to see a balanced version, and, while everyone knows you could just buy multiple units to add channels, still would be nice to have 4-8 channels in one case.,

As always great review, Amir.
 
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I find it spectacular that the volume control is motorized
Sorry if I'm asking in the wrong place, but I saw that the BT30D Pro now supports 48V. I bought one about 2 months ago, but it says 24-36V. Is it possible to use a 48V power supply on mine?
Pre-amp vs amp?

The pre-amp is more of an input switch with a volume knob and a little bit of gain, in case you have a weak input that needs it. It doesn't have the power to drive speakers. So you could hook up, say, a DAC, a Phono Pre, and a radio's output to BT30D, and switch the input and control the volume with one remote control.

The BT30D then drives speakers. There is a fair amount of functionality overlap - The P4 would make a bit more sense for folks with a power amp that has no controls at all. Example: a pair of Fosi V3 Mono Amplifiers. Though as stated, it doesn't have trigger support, which would have been nice.

So, in my example, a P4 + a Fosi V3 Mono Amplifier would give you the ability to control the volume coming out of the system. (Other ways would include if a DAC or DSP allows you to control it in software, for example)
 
Basically a fully transparent 3-input switch with basic EQ and remote motorised volume control and extra gain, for 100 moneys? That gotta be hard to beat. Chapeau Fosi! Add a balanced output, and I'd buy it yesterday even for 150 moneys or 200.
51dTOaNrm7L._AC_UF1000,1000_QL80_.jpg
 
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When I think about the amount of money spent trying to find a transparent preamp...

Fosi is really building a solid fan base with inexpensive equipment and nice measurements. Don't need this preamp but already have a BT20 Pro, the little portable DAC, the phono preamp... And I see Fosi offering more and more products in the future, so there is a chance I found one again for my use case.
 
Outstanding performance for a simple device. It allows to use and upgrade amps or integrated amps with only one or two RCA inputs. Thank you Amir for an excellent review that shows how much the well informed can stretch their Dollars!
 
There's so much choice and value out there these days . Backed with measurement based reviews by people like Amirm , One can know the performance they are getting as opposed to the old days when one suffered a degree of doubt especially further down the price ladder.

Thanks, Amirm! and thanks to all the other guys looking into actual definitive performance metrics .

I particularly like the gain settings , again the data given here really helps one see performance so one can feel confident in the options one choses .

With company's like Fosi on the forum one also has direct access to the designers and or company representatives. Members can talk to them and they in turn have highly valuable feedback on their products and potential direction for future projects, a focus group of sorts . Lucky them !
 
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