Fosi Audio P4 HiFi Preamplifier Review – Fine Overdrive In EQ and Tone Control
Fosi Audio P4 is a $109 USD compact control hub and preamplifier for hifi stereo systems. While this is a first for this price range, we will be exploring whether it is a good option for your listening rig, especially as it does have EQ options with bass and treble tuning and volume control you can add between the DAC and Stereo AMP for your speakers.
Introduction
Fosi Audio is going all-in with designing interesting, useful products, although at times a bit niche usage scenarios for their users. Fosi products are available to purchase from Amazon, Aliexpress and sometimes even electronics and Hifi local shops, depending on your country.
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Product Link
Amazon – https://amzn.to/41zUA4o
Aliexpress – https://s.click.aliexpress.com/e/_oFHrXBn
Build Quality – Design
Fosi audio designed the P4 Preamplifier to be an answer – to the needs and voices of their customers. While the company was working on the V3 Stereo amplifiers, many customers asked for a PRE that has 2-3 RCA inputs with a sub output, bass and treble tuning, and which can be affordable too.
Although it may seem complex at first, P4 has multiple functions, it has three RCA inputs, one RCA output, a PRE output that could be useful for a subwoofer, and a tone control bypass, if you want the purest sound possible. There’s also a three-level gain adjustment so you can get the best quality regardless of how loud your DAC source goes.
Bass and treble tone controls allow for some compensation for the shortcomings of your system and room, and Fosi Audio even bundles their P4 Preamplifier with a remote for an easy control. Certain buttons do not work on P4 as they are not supported by its function. Fosi made the chassis of the P4 of all-aluminum wit CNC machining. The design used is simple, perfect to be paired with Fosi V3 and Fosi Q6, to get a matching system for your desktop. I kind of wish the company made a white version of all those, to match with white desktop setups.
On a technical level, Fosi Audio P4 has a 2.0 channel input, with three RCA inputs, and with one RCA output, one PRE output. SNR is 110 dB, Dynamic range 110 dB, THD+N is 0.003%, and crosstalk is 105 dB. With a noise floor of 9 uV and a tonal adjustment of + – 12 dB, P4 looks good on paper for its price.
Subjective Usage
Using the Fosi Audio P4 is simple, it just works, but the first time you turn it on, it takes around 3-5 minutes to actually work, and it outputs complete silence for those 3-5 minutes which made me think that it is not working. This is not the first Fosi Audio DAC AMP that behaves like this, and I recommend having a bit of patience with it when you first turn it on.
Overall, it works well, you have easy access to most settings, the volume wheel is smooth, has a nice amount of damping and bass and treble wheels have a dead center which is very noticeable when the wheel clicks into place. Switching between RCA line inputs is also quick and easy, at the press of a button. The unit stays nice and cool during usage. The only drawback is that it needs wall power and the cable of the adapter is rather short. This limits how you can use and set up P4 a bit.
Sound Quality
Pairings – Fosi Audio P4 need to sit in between a DAC and an AMP, so I’ve used it in between Fosi’s own setup with Q6 and a set of v3 monobloc AMPs, in between FiiO K17 and Keces S300+ Stereo AMP, in between Burson PlayMate 2, and HIFIMAN GA-10 Tube AMP, in between HIFIMAN Serenade and HIFIMAN Prelude, and in between JDS Labs Element IV and Topping B100 Monoblock amplifiers. It is an effective way to use a PRE on the RCA channels, but with tone control, to add a subwoofer in the mix and to spice up your system with minimal added distortion and quality loss. For the most solid testing setup, I’ve used a FiiO K17 DAC, then connected HIFIMAN GA-10 to the XLR line out of the K17. I set K17 in Line Out mode as a full DAC, then connected the RCA output to Fosi P4, then using a similar cable connected P4 to HIFIMAN GA10, both driving a pair of HIFIMAN Susvara unveiled. This should provide the most direct comparison since I can quickly switch between RCA and XLR on GA-10 and also volume match precisely to see what P4 brings in or takes out.
Overall Signature – At default, with both sliders in neutral, the sound of the P4 Preamplifier is rather neutral without much coloration, or better said without much coloration. Upon more testing, I found that P4 has some volume gain, which means that it amplifies the sound slightly compared to the direct line connection, which is surprising. This makes it prone to overdrive and added distortion. It has a certain signature to itself, but it is not a FR tilt, more of a texture / resolution tilt, it actually sounds a bit smoother, removes certain elements from sound, and feels like it filters the sound slightly. This sits right in between feeling welcome and a bit bothersome for a system because it reduces the clarity ever so slightly, but that’s nothing you would notice with a system based on Fosi V3 as the amplifier as it simply does not have the resolution needed to show the changes. Miodrange is a bit shouty compared to the direct line, even with both EQs set at neutral. Using bypass is also useful to hear what the two adjustment wheels can do.
Bass Tuning – Using the bass tone wheel allows you to dial in some low-end but it is more of a bass bloom, it adds mostly bass in between the 40 Hz – 180 Hz range, adding both some kick but also warmth, thickness and warmth, it heavily colors the sound. This can increase the distortion significantly and result in a high THD in the bass, but it can also influence a system to gain more strength and presence in the low-end which is useful for stereo systems usually. Removing bass works as intended and it reduces the low-end impact and volume.
Treble Tuning – Quite similar to the bass uplift, adding more treble makes the sound far brighter, and it adds energy from 6 kHz all the way to 16 kHz, a wide treble tilt adjustment. This is not exactly ideal because it can significantly increase THD and distortion in the upper registers. Most systems can add more treble without distortion than they can add bass, especially as bass added from the preamplifier does not increase the headroom of the system, and there is no real attenuation unless you lower the volume to match the added bass or treble. So keeping volume at 50% but adding treble or bass to the max can result in a lower distortion, but the overall tone is still overdriven. You basically need to lower the volume to 25% to add bass at max which is +12 dB, or treble at max which is also +12dB.
Dynamic range And Textures – Dynamic range is exactly the same as when using the line transmission, but Fosi P4 has the tendency of decreasing texture levels, resulting in a smoother, simpler sound that has a bit of laid-back, relaxed effect on most systems. This is neither welcome nor disastrous, but a coloration it tends to apply to most chains, things might change if you replace the OP-AMPs inside. There is also an inherent difference between certain XLR and RCA inputs and outputs for most systems. Even on neutral, with volume at 70%, comparing the P4 to straight line, P4 has added distortion, in bass heavy tracks, it has a certain overdrive for sound, and a certain too much THD to each piece it is playing.
Loudness Saturation Gradient – As we’ve studio the loudness saturation gradient until now, I have to say, P4 has a somewhat distorted sound at all volumes, like it is receiving too much gain, and especially if you sed the gain to the lowest level, the effect is worse, attenuation is worse than no attenuation at all. Considering that the same DAC was used for each test, this is not a DAC or AMP issue, but rather how P4 behaves. Like all preamplifiers there is a certain distortion level it will add to a signal path, which is audible with high-end systems but will not be noticeable with entry-level ones, where it is meant to play.
Value and Conclusion
Fosi Audio P4 is a rather affordable Preamplifier, offers some light EQ and tuning for a system, and adds no audible noise, having a strong value for the kind of product it is. While not necessarily the most versatile or universal product in the world, it has plenty going on for it, and especially when you need to add a subwoofer to a system, it allows that with ease and without sonic degradation usually associated with using splitters or running the signal through the subwoofer itself.
At the end of the day, I like the performance of the Fosi Audio P4, it is simple, effective, offers bass and treble tuning, and has a clean sound too. While the company didn’t add too many features to it, it is effective for what it’s meant to be and to do, being a fully recommended option for a high-quality preamplifier for stereo systems.
PROs
- Compact and effective
- Multiple RCa inputs
- Both RCA output with tone control and a preamp output for an active subwoofer
- Low sonic degradation, but you can switch the tuning with the user replaceable op-amps
- Multiple gain levels to match multiple DAC levels
- Nice orange volume wheel at the center
- Bypass mode for tone control
- Perfect for an entry-level system
Cons
- Requires a large power brick to be used
- No XLR option, only RCA
- Power brick has a short cable, needs to be very close to a wall outlet
- Has an added distortion / overdrive effect audible with high-end systems
Product Link
Amazon – https://amzn.to/41zUA4o
Aliexpress – https://s.click.aliexpress.com/e/_oFHrXBn
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Full Playlist used for this review
We listened to more songs than those named in this playlist, but those are excellent for identifying a sonic signature. I recommend trying most of the songs from this playlist, especially if you’re searching for new music! The playlists are different for Spotify, Tidal and Youtube, and based on the songs I enjoy and are available on each!
https://www.youtube.com/playlist?list=PL_cjBXGmwSHSdGcwuc_bKbBDGHL4QvYBu
https://open.spotify.com/playlist/5J3oloz8Riy9LxEGenOjQ0?si=979ba4f082414be7
https://tidal.com/browse/playlist/330fd544-8e5b-4839-bd35-676b2edbb3d5
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