Fosi Audio P4 Preamplifier Review

[gino1961]

They are nice looking units but too small imho
I don't understand this trend towards mini units at all
Midsize would be the best
Something like a 12" width
Connections on the back are too close and big locking rcas don't fit easily
The zp3 is in the right direction and what i am looking for now
it's very good looking

 

[norman bates]

Well, you don't need space for the power supply.......

And you don't need a huge faceplate...............................

I agree that larger would be nice, but now we pay for a redesign, thusly raising the cost....................

 

[Joe Smith]

I do wish someone other than Emotiva would release a new preamp that includes an adequate phono preamp...or has an open slot for an add-in module...

 

[gino1961]

Why not a standalone phono preamp placed as close as possible to the turntable?
You could change the phono more easily and keep the line stage

 

[Joe Smith]

Why not a standalone phono preamp placed as close as possible to the turntable?
You could change the phono more easily and keep the line stage

This works for many, and may be preferable... But in my main upstairs setup, it's a small workroom and only has one electrical outlet. For that reason, I mainly use a restored receiver or integrated amplifier, as I also have to connect turntable, lamp, computer, DAC, and sources. I rotate in my Fosi and Schiit stuff to this system, but having to plug in three things (amp, pre, phono pre) versus one integrated is less preferable.

Since it's a small room, I don't need a lot of power. Currently I use a fully restored Marantz 2216 (c.1978) with a switching input going into the AUX so I can have CD, tape, DAC and phono all available.

I admit my use case there is very specific. But given the vinyl resurgence, I continue to hope that Schiit, Fosi or someone else will have a preamp at some point that has a slot for an internal phono pre.

I do think the Fosi P4 and the Schiit Saga 2 are going to fit a much wider use case for most folks. As I use both over time in other setups, I find the main drawback of the P4 is the big volume jump at the lower end of the pot range. It's nice to have the remote, but I find the only way to adjust volume accurately at lower levels is to use the volume control on the amp (usually a Fosi or Aiyima product). The Schiit is much, much better at level matching and lower volume level adjustments.

 

[norman bates]

The P4 has a surprising heft for its size.

It is great for what it is.

Maybe I can loan mine to some audiophiles.
But I doubt that they could admit if it sounds good....................

Most of them preach the preamp is often more of an effect on the sound than the amplifier....

 

[Ropeburn]

The P4 has a surprising heft for its size.

It is great for what it is.

Maybe I can loan mine to some audiophiles.
But I doubt that they could admit if it sounds good....................

Most of them preach the preamp is often more of an effect on the sound than the amplifier....

If you can find a way of fooling them into thinking it's exotic and expensive, it'll sound terrific.

 

[ninja1]

Maybe little off ,but not to open a new page..., and there are lot of posts regarding similar, but...

At the moment chain is: Wiim pro plus directly to boxem 4216/EV (arthur) (on the way) - 3 gain settings.

Usually I listen at 70 to 80 db SPL on three meters (90db 8ohm) speakers, but sometimes I would like to listen at lower volumes.

I know the best situation is to have dac on 100%, but I do not have other volume control then WIIM...Reducing volume on wiim will degrade "quality" (sinad).
At which volume (0 to100) system will still not have sound degradation in SINAD ( knowing that boxem sinad is cca 112db, and wiim pro plus 115db).
Listening on low volume ?

or is better to put something like FOSI P4 in chain and keep wiim at 100 %

I assume that chain with FOSI P4 is "better" (also 2 more inputs which I like), but how much, and do I really need FOSI " not have sound degradation" wise ?

 

[solderdude]

Usually I listen at 70 to 80 db SPL on three meters (90db 8ohm) speakers, but sometimes I would like to listen at lower volumes.

When you listen to 80dB average you are getting up to 95dB peaks (depends on the music)
You can't hear anything below 30dB when listening at these levels.
Now suppose you would be listening at 60-70dB average... you would be hearing peaks up to 85dB.
You won't be able to hear anything below 25dB and in quiet passages approx 20dB.
This means 60 SINAD is more than sufficient in this case.
You thus won't get better 'sound' quality using a pre-amp nor better noise floor either and have a 'technical' performance that is well beyond 60 SINAD... simply because softer signals simply can't be heard so they don't matter.
Just dial down the volume and it will be fine.

 

[Carl-Fosi Audio]

Hi I'm a happy customer and thank you very much for providing excellent sound at unbelievable prices
I just don't understand the variable input impedance
Why not a 10k constant vs Hz ?
Quite standard value for line preamps fwiu

The output impedance is variable because of volume control, the input impedance is constant.

 

[solderdude]

The output impedance is variable because of volume control, the input impedance is constant.

So... the volume control is a 100k potmeter with a 10k resistor from the wiper to the output RCA ?
All amplification is done before the volume control ?
Why would there not be amplification/or at least buffering after the volume control (like any other audio device) ?
10k to 60k output impedance would be ridiculously high and impractical at that.

It would make more sense if the input resistance would vary in this range when setting gain (a simple resistor in series with a 10k volpot directly at the input) and can even see a reason for the input resistance to vary depending on the volume control setting.

 

[ninja1]

When you listen to 80dB average you are getting up to 95dB peaks (depends on the music)
You can't hear anything below 30dB when listening at these levels.
Now suppose you would be listening at 60-70dB average... you would be hearing peaks up to 85dB.
You won't be able to hear anything below 25dB and in quiet passages approx 20dB.
This means 60 SINAD is more than sufficient in this case.
You thus won't get better 'sound' quality using a pre-amp nor better noise floor either and have a 'technical' performance that is well beyond 60 SINAD... simply because softer signals simply can't be heard so they don't matter.
Just dial down the volume and it will be fine.

Thank you solderdude !

1. I agree with your observations
2. If one will use FOSI P4 anyway (because of analog VC and more inputs) , and keep wiim at 100%, I asume that "technical performance" wll be same or maybe even better ?, but inaudibly, comparing to wiim to amp solution ?
3. If I connect two devices to P4 (Wiim and for example turntable or CD player) , with different input levels (2,1 V vs XY volts), then to amp, will the output volume SPL (or voltage) will be same for both inputs ?

 

[gino1961]

The output impedance is variable because of volume control, the input impedance is constant.

Hi ! thank you so much for your very kind and valuable reply I think i understand better now
The inputs are buffered and the output of the volume control drives the output rcas directly like a passive Very interesting indeed
As i said above i am amazed by the transparency of the unit Very pleased with that You have a new customer
I will be looking for the other preamp with XLR ins and outs I have a good dac with balanced outs that i would like to test
Thanks again and kind regards
gino

 

[solderdude]

Thank you solderdude !

1. I agree with your observations
2. If one will use FOSI P4 anyway (because of analog VC and more inputs) , and keep wiim at 100%, I asume that "technical performance" wll be same or maybe even better ?, but inaudibly, comparing to wiim to amp solution ?
3. If I connect two devices to P4 (Wiim and for example turntable or CD player) , with different input levels (2,1 V vs XY volts), then to amp, will the output volume SPL (or voltage) will be same for both inputs ?

Given the apparent operation of the amp (amplifier with +3, +6 or +9dB) and seeing the amplifier shows the distortion to rise again above 1V it would be best to use the Wiim with the output set at -6dB.

Having a 10k to 60k output resistance isn't the best idea either as frequency extension becomes load capacitance dependent.
This means it will probably be fine to connect the P4 with short interlinks to a power amp but less usable when having 6m interconnects to monoblocks near the speakers.

Some examples... RCA cables have a capacitance between 60pF/m and 250pF/m (not even taking input capacitance of power amps into consideration)
When one uses 6m of lowest capacitance RCA cable and the volume control of the amp is at -6dB the output resistance will be around 60k (I assume 100k potmeter with 10k extra resistor from the wiper to output) gives a 44kHz -3dB point at max. volume setting and min volume setting but a 7.5kHz -3dB point at -6dB volume setting (when the output is at 60.1kHz.
But when using such nice audiophile cables with a capacitance of 200pF/m the -3dB point can even drop to 2.2kHz.

Based on this it would probably be best to use the P4 with short interlinks to power amps with a input resistance > 10kohm (as you would loose a lot of the expected gain)

 

[Ropeburn]

Perhaps there's just some confusion here. In earlier posts (#150 ff.) @Carl-Fosi Audio stated those variable 10k-60k are input impedance, not output. That would make a lot more sense (pot before input buffer) and be unproblematic.

 

[redjr]

They are nice looking units but too small imho
I don't understand this trend towards mini units at all
Midsize would be the best
Something like a 12" width
Connections on the back are too close and big locking rcas don't fit easily
The zp3 is in the right direction and what i am looking for now
it's very good looking

I agree. I have both the P4 and the ZP3. I thought the P4 was entirely too small and that's why I bought the ZP3. I have thin fingers! Their design team needs to make the units wider and spread out the knobs and I/O on the back a bit. The ZP3 is going in the right direction. They could also adopt a standard case (form, width and depth) that make the units in a model line stackable.

 

[solderdude]

Perhaps there's just some confusion here. In earlier posts (#150 ff.) @Carl-Fosi Audio stated those variable 10k-60k are input impedance, not output. That would make a lot more sense (pot before input buffer) and be unproblematic.

I agree that would make a lot more sense but then this was posted...
I can easily see the input R behaving this way when a 50k potmeter were used with a circuit behind it with a 15k input resistance or so.
It would appear that there is an amplifier circuit followed by a passive volume control.
That would be fine when the volume control was just 5k or so (max 10k).

The output impedance is variable because of volume control, the input impedance is constant.

I guess either someone has to measure in and output impedance (at 1kHz as there might be output caps) or the topology is explained in more detail.

 

[gino1961]

However ... this performance is absolutely irreproachable

one info is missing The Zin of the audio analyzer used
Knowing that could help the discussion
I see 100kohm ?
If i understand well the output impedance should decrease with the increase of the volume level
I am using it with an amp with 20kohm input and the sound is really fine

 

[ninja1]

Based on this it would probably be best to use the P4 with short interlinks to power amps with a input resistance > 10kohm (as you would loose a lot of the expected gain)

Amp input impedance is 94 kohm

 

[solderdude]

However ... this performance is absolutely irreproachable

View attachment 472173

one info is missing The Zin of the audio analyzer used
Knowing that could help the discussion
I see 100kohm ?
If i understand well the output impedance should decrease with the increase of the volume level
I am using it with an amp with 20kohm input and the sound is really fine

Yes the AP in 100k input impedance.
If it would have been 10M then the volume knob setting during the testing would have been 0.8dB lower in setting, hardly noticeable in practice.
When one would be using a 20k input R amplifier the volume control would just be set a little higher, how much is hard to say as the output resistance varies per volume setting.

You can use it with an amplifier that is 20kohm.
You can even use it with an amplifier that is 1kohm input R but you would need to use a higher gain setting in that case and the volume adjustment taper and max. level would be different.
The sound will be fine and not suffer in anyway.

Would be great if some beans were spilled by Fosi about the circuit.