The Fosi Audio P4 is a compact stereo preamplifier designed to put practical analog system controls in one place. Its feature set includes three stereo RCA inputs, a stereo RCA output, an additional 3.5 mm pre-output, motorized volume adjustment, three gain settings, and independently adjustable bass and treble. A rear-panel switch can remove the tone controls from operation when they are not required.
That combination makes the P4 relevant to systems that need more than a simple volume knob. It can serve as the control point between several analog sources and a downstream power amplifier, active speaker system, or other compatible equipment. Its extra output also creates opportunities for integrating an active subwoofer or feeding another component, without turning the preamplifier into a large or complicated centerpiece.
A practical hub for three analog sources
The P4 provides three stereo RCA inputs, allowing several conventional analog components to remain connected at the same time. This is useful in a system assembled from separate devices, where relying on the limited input selection of an amplifier or active speaker can become restrictive. A prospective owner might use the inputs for compatible sources such as an external DAC, streamer, CD player, or other equipment with stereo analog outputs.
Keeping source selection and volume control together also gives the P4 a clear role in a modular hi-fi system. Rather than requiring each source to connect directly to a power amplifier or powered speaker, the preamplifier becomes the central control component. Its 2.0-channel configuration is straightforward and focused on stereo playback.
The connectivity is deliberately conventional. Fosi Audio specifies RCA inputs rather than digital, balanced, or wireless connections. That makes the P4 most appropriate for users whose sources already provide analog output, or who prefer to choose a separate DAC or streamer instead of having those functions built into the preamplifier.
Motorized volume with hands-on system control
Volume is managed by a motorized potentiometer. Beyond the visual appeal of a physical control that can be mechanically driven, the design retains a familiar analog-style interface. Fosi Audio also positions the potentiometer as a means of making smooth adjustments while limiting noise and distortion.
The motorized volume control is accompanied by functions that address common setup differences between sources and downstream components. In particular, the three-level gain switch allows the output signal to be adjusted according to the connected source. This can be valuable when equipment in the same system has differing output levels or when the downstream amplifier or active speaker has particular input-sensitivity requirements.
Gain selection is not the same as everyday volume adjustment. Instead, it helps establish a more appropriate operating range for the system as a whole. Choosing among three gain levels may make the main volume control easier to use with different source and amplification combinations, avoiding a one-setting-for-everything approach.
Tone controls that can be removed from operation
Dedicated bass and treble controls provide adjustment over a range of ±12 dB. That is a substantial amount of tonal flexibility for owners who want to compensate for a recording, speaker placement, room behavior, or personal preference. The controls can also be useful in desktop and compact-room systems, where ideal loudspeaker positioning is not always possible.
Importantly, tone adjustment is optional. A switch on the rear panel turns the tone-control function off entirely, allowing the P4 to be used without bass or treble modification. This gives the product two distinct modes of use: active tonal shaping when correction or customization is wanted, and an unadjusted mode when it is not.
The bypass option makes the tone controls less of an all-or-nothing design decision. Owners do not have to choose between buying a minimalist preamplifier and permanently placing equalization in the operating path. They can keep the controls available for particular recordings or circumstances, then switch them out when a neutral setting is preferred.
Two outputs expand system possibilities
The primary output is a stereo RCA connection suitable for feeding a compatible power amplifier, active speaker system, or another downstream audio component. A second pre-output is provided on a 3.5 mm jack. Fosi Audio identifies this additional connection as suitable for an active subwoofer or other parts of a system.
That extra output is one of the P4’s more useful features because it supports systems that extend beyond a basic source-preamp-amplifier chain. For example, an owner could connect the main RCA output to the stereo amplification stage while using the 3.5 mm connection for a powered subwoofer with suitable input facilities.
No crossover, low-pass filtering, or dedicated bass-management specification is documented for the 3.5 mm connection, so it should be understood as a pre-output rather than assumed to be a fully managed subwoofer channel. Any required crossover settings would therefore need to be handled by the connected subwoofer or other equipment. Even with that distinction, having a second output format can simplify integration in compact or mixed-component systems.
Compact construction with a machined-aluminum chassis
The P4 is housed in a black chassis made from machined aluminum alloy. The material gives the preamplifier a clean, purposeful appearance while also helping to reduce the influence of electromagnetic interference. This is particularly relevant for a small analog control component that may sit near source devices, power supplies, amplifiers, or desktop electronics.
Its external power requirement is 12V DC at 1.5A. The low-voltage supply arrangement is consistent with the P4’s compact system role, although prospective owners should still plan cable placement around the external power connection and the multiple analog interconnects.
Fosi Audio specifies a frequency range of 20 Hz to 20 kHz within ±0.1 dB. The published figures also include a signal-to-noise ratio of at least 110 dB, dynamic range of at least 110 dB, crosstalk of at least 103 dB, noise of no more than 9 µV, and a distortion-plus-noise figure no higher than 0.003%. These specifications describe the intended technical performance, though they should not be treated as independent Stereoindex measurements.
Who the Fosi Audio P4 is most suitable for
The P4 is best suited to someone building a compact, separates-based analog system. It is particularly relevant when three RCA sources need to share a single downstream amplifier or active speaker setup, or when a basic power amplifier needs source selection and volume control. The adjustable gain also broadens its usefulness across components with different output and input requirements.
It may also appeal to listeners who appreciate tone controls but do not want them permanently active. The ±12 dB bass and treble range provides meaningful adjustment, while the rear bypass switch preserves a simpler operating option. Users planning a 2.1 arrangement may find the additional 3.5 mm pre-output convenient, provided the active subwoofer or associated equipment can perform any necessary crossover duties.
The P4 is less obviously suited to anyone seeking an all-in-one digital control center. No USB, optical, coaxial, HDMI, Bluetooth, balanced input, phono-stage, or headphone-amplifier functionality is specified. Those facilities would need to come from separate components. Its appeal instead lies in concentrating core analog preamplifier functions into a small, focused device without adding unrelated features.
Conclusion
The Fosi Audio P4 stands out through a well-judged combination of analog connectivity and system-adjustment tools. Three stereo RCA inputs make it a useful source hub, while the motorized volume potentiometer and three-level gain switch provide control over both daily listening level and broader component matching. Bass and treble adjustment adds flexibility, and the rear-panel bypass switch ensures those controls can be removed from operation when they are unnecessary. The stereo RCA output, additional 3.5 mm pre-output, and machined-aluminum chassis further support its role in compact, modular installations. It is most appropriate for owners of analog-output sources who want a dedicated stereo control center for a power amplifier, active speakers, or a system incorporating a compatible powered subwoofer.
Join the discussion
Share your thoughts, listening impressions or product experience.