P4 Preamplifier Review

[RJHJR2]

From Fosi Audio: "The P4 features three RCA inputs, one RCA output, and a pre-out, with bass and treble adjustments as well as gain control functions, and comes with a remote control. (There is also a Tone Control bypass switch.)"

If you’re one of the many who have a small Class D amplifier and would appreciate remote volume control and source switching, the P4 fills a gap in the market. I now have the option of placing my amplifier, DAC, streamer, CD player, and turntable where they belong and not cluttering my workspace. The supplied remote also works with other Fosi equipment, chosen by button.

So why the P4? It allows me to use a focused, dedicated amp with my choices of DAC (into which goes the streamer) and other source hardware like my CD transport and turntable. I’m using it with a 48V Fosi V3 today, and have a ZA3 on order. The switching works as it should, the volume control is smooth and accurate and the tone controls do a fine job if needed (and can be bypassed with a switch on the rear of the P4). Mode switching can be done via the remote or the front of the unit. Power on/off is remote only, which seems odd. The pre-out works with my Yamaha active sub but does not offer a frequency cutoff - an oversight for a preamp? I didn't detect additional noise when using the highest gain setting, so left it there.

In my, and likely the most common, setup, the hardware is close enough not to need more than 1-foot cable runs. Balanced XLR cables are indicated for long runs and are surplus to requirement here. Anyone who feels the need for balanced input at these distances is over doing things – or has noisy power supplies. Eliminating additional XLR (or TRS) inputs keeps size and price requirements down.

Fosi claims SNR and Dynamic Range of >110dB and THD of 0.003%. I have no reason to doubt this. I’ve been waiting for a product like this to get the best out of my setups. The P4 fills a gap in the market.

 

[Joseph E Smith]

I was also a beta tester! Here's an abbreviated version of my review:

The P4!

The P4 is exactly what I was hoping for in a nice, clean, solid-state preamp that would pair nicely with the V3 stereo amp, or with other "black box" class D amps. Fosi has a nice design aesthetic going with clean case design, minimal lettering and indices, with the only "flash" provided by their distinctive (optional) copper volume knob.

The P4 has a metal case matching the recent Fosi amp releases. It's just a little larger than the V3 amp - it is 4.5" wide, just .25" wider than the V3. Thus, they would easily be stackable, with the P4 on the bottom. I've chosen to use them side by side as you can see in the photos.

The nice feature set from Fosi includes tone controls (bass and treble) on the front panel, with a plus or minus 12 db range, and a tone defeat switch located on the back of the unit. There is an adjustable slider-type gain switch on the back of the unit that lets one select 1v, 2v or 4v gain, corresponding to +3db, +6 db, or +9 db. There is also a pre-out jack which I assume can be used for a powered subwoofer connection - I don't use one, so I can't test this feature. (The manual does not provide guidance on this feature.)

And - a nice remote control! The remote is comfortable to hold and apparently can be used for multiple Fosi products, including the ZD3, ZP3 and BT20A MAX as well as the P4. The remote is nicely laid out with a power button, mute, volume, back/forward keys, mode selection, reset button and switch device buttons. It is powered by two AAA batteries (not included).

Power is provided by a wall-plug type power adapter, a small one, that is rated at DC 12v 1.5a. It terminates in a barrel plug that goes into the unit.

I've used the remote to power up the P4 and adjust volume. One can turn on the P4 by pressing and holding the volume knob, or pressing longer to turn it off, but it's simpler to use the remote. The remote seems to work from a reasonable distance away from the unit.

So, the P4 definitely checks most of the feature boxes I as a user was looking for - 3 RCA inputs, a good remote, adjustable bass/treble, tone defeat option, adjustable gain levels, and a compact size.

So, How Does It Sound?

In a word - great! The sound reflects the unit's claimed equal or better than 110 db SNR, 0.003% THD, low noise floor (+/- 9 uV) and equal or better than 103 db crosstalk.

Even at the lowest gain setting, there is a nice sound boost from the V3 amp alone, and very very loud indeed at the highest setting. Right now, for comfortable listening volume, I am on the lowest gain setting, the V3 power knob is at 75% and the P4 volume knob is at 50% with my older KEF Q 15.2 speakers (rated at 91 db efficient). Sample track I'm playing is between 75 and 85 decibels in nearfield, approximately a meter and a half from speakers. (I use a 36v 6a power supply for the V3 amplifier, as I really have no need for a 48v supply with the room sizes I have.)

I am not aware of any noise coming from the P4. With nothing playing, the above settings cause no speaker noise or hiss.

The adjustable tone controls are very handy. I use speaker EQ now via Equalizer APO to smooth out the KEFs a bit, but I like to leave that as a base setting and not fiddle with the computer/streaming settings. But when listening at low volumes, I do like to be able to adjust bass and treble, similar to the old "loudness" circuit on vintage equipment. It's helpful to just have a basic EQ control without having to buy an entirely separate unit for that, such as the Schiit Loki.

The P4 adds a surprising amount of power given the very small size of its power brick (18 watts). Playing as noted above, the P4 is cool to the touch and the V3 is barely warm after playing for many hours.

I regularly listen to a wide range of music - jazz, blues, rock, folk, electronic/ambient and classical. The P4 sounded just great with everything I played.

Pros and Cons

So let me note some of my noted plusses and minuses about this unit:

Pros:

• Very nice case and control aesthetics and matching with the V3 and rest of the Fosi amp families, including the fun copper knob!
• Great, easy to use remote including input selection and muting.
• Lots of power boost and switchable gain levels.
• Simple to use tone controls and inclusion of a tone defeat switch.
• Decent channel matching at lower volume levels.
• Great overall specs, minimal amount of noise inserted into the music chain.
• Price? Fosi has not told me the release price for this unit.

Cons:

• The use of the pre-out jack is not explained in any way in the product manual - Fosi should provide some additional guidance on this.
• The gain switch feels a bit flimsy - like it could break if used repeatedly - not a problem for me, as I'm probably going to use at the 1v or 2v settling and just leave it alone - but if your use case would have frequent switching going on, I'd wonder a bit about the longevity.
• The bass and treble knobs are very close to the volume knob - a bit close - even with my normal size hands, you brush them when adjusting the volume via the knob. I understand the desire to have the case not get much wider than the V3 amp, but the spacing is a bit tight. Not a huge problem, as I expect most folks will be adjusting the volume via the remote?
• When using the remote, the volume really jumps in the lower volume range - not a lot of range at the lowest volume level. However, this can be improved by hand-turning of the knob, and even easier is to use the V3's volume knob to "dial in" a precise low volume setting.
• No headphone jack.
• Another power brick - albeit a small one - I personally would have preferred inside power, but probably no way to do in the case size they wanted.

Conclusion

I'm glad that Fosi now has a good, simple and design-coherent preamp solution to match for its power amps for those of us that are only using RCA cabling. I understand they are also developing a product for the balanced/XLR users.

It's a very solid entry and I think it fills a big product gap for people using multiple inputs and not having access to a legacy, powered preamp as in days of old. Three is enough for most. If using phono, just connect using a dedicated phono preamp before one of the P4 inputs. Then an input for CD and an input for DAC/streaming. That's probably going to be enough for 95% of current audio enthusiasts.

The P4 follows the current releases of class D amplifiers with a "less is more" kind of philosophy. The big boxes of my youth have been replaced by small boxes that don't cost an arm and a leg and sound really, really good. It's an exciting time to be enjoying audio, for sure!