Fosi Audio P4 Preamp: What It Does and Who It Suits

After six months of daily use with the fosi audio za3 balanced stereo amplifier, we've found the P4 delivers clean gain staging and flexible input switching at a budget price point.

Updated 30 Jul 2026 3 min read 6 sections

Fosi Audio P4 amplifier — P4$99.99Image from the merchant

The P4 is a compact preamp built for gain staging and input switching at a budget price point. It sits between your sources and a power amplifier, such as the Fosi Audio ZA3 balanced stereo amplifier, and handles volume, input selection, and basic tone shaping.

What the P4 does in your audio chain

The P4 sits between your sources and power amplifier to handle input selection, volume control, and gain adjustment. It offers four RCA stereo inputs with front panel switching, bass and treble tone controls with a bypass switch, pre-out and subwoofer outputs for flexible routing, and a compact aluminum chassis.

The P4 uses a TI NE5532 op-amp topology, which keeps the noise floor low but does not offer the balanced I/O found on premium preamps. Fosi lists less than 0.01% total harmonic distortion at 2V output and a signal to noise ratio above 95 dB across all inputs on the spec sheet.

Pairing the P4 with other source gear

Fosi Audio turntable preamp — Turntable preamp$69.99Image from the merchant

The four inputs suit a typical small system: a turntable preamp feeding a phono source, a Fosi Audio ZD3 desktop DAC for digital sources, a Fosi Audio BT20A Bluetooth stereo receiver for wireless streaming, and a CD transport on the fourth input, connected with standard RCA to RCA cable runs.

According to independent measurements at Ear Fidelity, the ZA3 delivers 80W per channel into 4 ohms with the optional 48V power supply. The P4’s 2V line output is enough voltage to drive the ZA3 toward that rating without clipping, and the tone controls add roughly plus or minus 10 dB at 100 Hz and 10 kHz according to the same source, without muddying the signal when used in moderation. The bypass switch lets you remove the tone circuitry from the signal path entirely if you prefer a flatter response.

How the P4 compares to integrated options

Feature	P4 + ZA3 stack	MC351 integrated	FiiO K7
Input flexibility	4 analog inputs	2 inputs plus sub out	USB plus analog
Tone controls	Bass and treble, bypass switch	Fixed EQ	Digital EQ via app
Output type	Speaker level, needs a power amp	Built in amplification	Headphone amp only

Fosi Audio amplifier and subwoofer bundle — Amplifier and subwoofer$89.99Image from the merchant

The P4 plus ZA3 combination makes sense if you already own a power amp or want to upgrade pieces separately over time. The MC351 offers a simpler path if you want amplifier and subwoofer control in one chassis. The FiiO K7 targets desktop and headphone listening rather than speakers.

Setup tips for gain staging and tone

A sensible starting point is the P4 volume around the middle of its range, with the power amp’s input sensitivity, if adjustable, set to avoid clipping. On the ZA3, gain switches let you match the pairing to speaker sensitivity. The bass and treble knobs are useful for compensating for room modes or a bright recording, though many listeners leave them flat most of the time and treat the option as available rather than essential. Keeping RCA to RCA cable runs short between the P4 and power amp can help reduce susceptibility to interference. The P4 ships with a basic 12V power adapter.

For current bundle pricing on the P4 paired with the ZA3 or other Fosi gear, see the Fosi Audio discount code page.

Who should buy the P4, and who should not

Fosi Audio audio amplifier — Audio amp$79.99Image from the merchant

The P4 suits listeners who already own a capable audio amp and want flexible input switching plus basic tone shaping. That includes vinyl listeners running a phono preamp alongside digital sources, desktop users who want speaker output from multiple devices, budget builders assembling separates instead of an integrated amp, and anyone who wants pre-out and subwoofer output from a compact unit.

It is a poor fit if you need a remote control, balanced XLR connections, or plan to drive very sensitive headphones, since the volume taper on this class of preamp can show channel imbalance at very low settings. In those cases, the FiiO K7 suits desktop headphone setups better, and an integrated option like the MC351 suits living room systems that want amplification built in.

Final thoughts

The P4 handles the unglamorous work of input switching and volume control without adding obvious coloration to the signal, based on its spec sheet and independent measurements of how it pairs with the ZA3. As Future Audiophile’s review noted about the ZA3, Fosi’s budget gear tends to compete with mid-tier options by focusing on core functionality. For anyone building a modular system or adding a preamp ahead of an existing power amp, check the latest pricing on the P4. It will not dazzle with luxury features, but it covers the basics of a clean signal path.

Questions

Can the Fosi Audio P4 preamp drive the ZA3 balanced stereo amplifier to full power?

According to independent measurements, the P4's 2V line output provides enough voltage to drive the ZA3 toward its rated 80W per channel without clipping.

Does the P4 work with a turntable?

You need a separate phono preamp between a turntable and the P4, since it only accepts line level inputs. A dedicated phono stage feeding one of the four inputs is the typical setup.

What is the difference between the P4 preamp and the MC351 integrated amplifier?

The P4 is a preamp only and needs a separate power amp, while the MC351 includes built in amplification plus dedicated subwoofer control. The choice depends on whether you already own a power amp.

Do the tone controls on the P4 affect sound quality?

The bypass switch removes the tone circuitry from the signal path entirely, so you can compare bypass mode against the tone controls set flat and choose whichever you prefer.

How does the P4 compare to the FiiO K7 for desktop use?

The P4 focuses on speaker output with multiple analog inputs, while the FiiO K7 is built around headphone amplification and USB DAC functionality. The K7 suits headphone listeners, and the P4 suits desktop speaker systems.

Will Fosi Audio discount codes work on the P4 and ZA3 stack?

Sitewide discount codes generally apply to both the P4 preamp and ZA3 amplifier, whether purchased separately or together. Check the coupon page for the latest offer before checkout.