Fosi Audio P4 Preamplifier Review

[gino1961]

Hi thank you very much for your advice
On the basis of what i am hearing i think that buffering the inputs is really beneficial
In the same way i would have buffered also the output of the pot
The pot impedance seems a little high for a preamp
But I am not an audio designer

 

[solderdude]

On the basis of what i am hearing i think that buffering the inputs is really beneficial

Amplification before the volume control has the benefit that the self noise of the amplifier part also becomes lower when the volume is lowered.
Think the famous O2 design from NwAvGuy...
It also has downsides though namely distortion is higher and clipping can occur when a high gain + higher input voltage is present.
With the NE5532 self-noise with low to medium gain is not an issue though.
The downside is the high and variable output resistance which is in no way beneficial.
It isn't beneficial when the input impedance of the amp behind it is low due to volume setting dependent attenuation (voltage division) and the fact that a load capacitance can cause (volume setting dependent) high frequency roll-off when a higher cable/amp input capacitance is present.
This can easily happen with 'exotic' cables of more than a few meters and with amplifiers having a HF filter capacitor on the input (which is common).

When using a 100k pot at the input of the amp buffering is not needed and the input resistance will either be constant or just vary a little depending on the volume setting. The input resistance will vary less with 50k, 20k inputs and even with 10k input potmeters.
The downside of a high impedance is noise which is higher.
The upside is lower distortion but the self noise is constant. Using op-amps like the NE5532 the selfnoise is low enough not to become audible.
Another upside is low output resistance so no issues with any loads, can drive anything effortlessly. No volume control dependent issues.
For this reason all the high SINAD devices have low input and output resistances.

As far as pre-amps go the best design is a buffered input (maybe a few times gain), a low resistance volume control followed by a buffer (with or without some gain) and a low output resistance.

When it comes to benefits from a topology the amp first, high output resistance volpot after it then it would rank about lowest on the list.
Mind you ... it is really hard to make a (technically) poor performing pre-amp.
The voltages are low, the currents are low ... easy peasy.
So in most cases any of the 3 configurations will work fine. The P4 config, if it really is amplification followed by a high resistance volpot directly at the output, is the least desirable of the 3 because of the limited max gain and volume control setting dependent potentially audible HF roll-off>
It would still need to be confirmed by @Carl-Fosi Audio that this is the case (but measurements suggests it is).

 

[norman bates]

makes sense

 

[gino1961]

Amplification before the volume control has the benefit that the self noise of the amplifier part also becomes lower when the volume is lowered.
Think the famous O2 design from NwAvGuy...
It also has downsides though namely distortion is higher and clipping can occur when a high gain + higher input voltage is present.
With the NE5532 self-noise with low to medium gain is not an issue though.
The downside is the high and variable output resistance which is in no way beneficial.
It isn't beneficial when the input impedance of the amp behind it is low due to volume setting dependent attenuation (voltage division) and the fact that a load capacitance can cause (volume setting dependent) high frequency roll-off when a higher cable/amp input capacitance is present.
This can easily happen with 'exotic' cables of more than a few meters and with amplifiers having a HF filter capacitor on the input (which is common).

When using a 100k pot at the input of the amp buffering is not needed and the input resistance will either be constant or just vary a little depending on the volume setting. The input resistance will vary less with 50k, 20k inputs and even with 10k input potmeters.
The downside of a high impedance is noise which is higher.
The upside is lower distortion but the self noise is constant. Using op-amps like the NE5532 the selfnoise is low enough not to become audible.
Another upside is low output resistance so no issues with any loads, can drive anything effortlessly. No volume control dependent issues.
For this reason all the high SINAD devices have low input and output resistances.

As far as pre-amps go the best design is a buffered input (maybe a few times gain), a low resistance volume control followed by a buffer (with or without some gain) and a low output resistance.

When it comes to benefits from a topology the amp first, high output resistance volpot after it then it would rank about lowest on the list.
Mind you ... it is really hard to make a (technically) poor performing pre-amp.
The voltages are low, the currents are low ... easy peasy.
So in most cases any of the 3 configurations will work fine. The P4 config, if it really is amplification followed by a high resistance volpot directly at the output, is the least desirable of the 3 because of the limited max gain and volume control setting dependent potentially audible HF roll-off>
It would still need to be confirmed by @Carl-Fosi Audio that this is the case (but measurements suggests it is).

Good morning thank you very much for the very kind and valuable advice
Fwiu the analyzer has a 100k input and i guess almost purely resistive
The amp i am using has an input impedance of 20k don't know how many pF
To my ears the P4 sounds very clean
I would much prefer a midsize case with ins and outs more spaced
I am also using a better PSU i had at hand a 10 euro thing but good
The stock one looks poor
IMHO the key is in the pot
When the pot is right in value and quality it's a very good start
Thanks again and kind regards Gino

 

[solderdude]

The power supply is 'redone' in the amp as there are internal DC-DC converters so the 'quality' of the power supply is of little (no) consequence as long as it is regulated (which all of those light-weight wide voltage range adapters are).

As long as you are using short interlinks the P4 will work fine.

The P4 is just a NE5532 + potmeter (instead of the usual potmeter + NE5532 configuration) so is audibly transparent.

 

[gino1961]

Thank you very much again
No more doubts about ne5532 and opamps in general
They can sound really good
If i were an audio expert i would be tempted to rewire it connecting the pot directly to the inputs and the opamp after the pot
I guess it should be a matter of cutting traces and add some wires
I have also another curiosity
Clearly a dual opamp simplifies things
Butter what are the differences between using one double or two singles simplification aside?

 

[solderdude]

You can get a lot of op-amps in different 'housings'.
The round metal can, the 8-pin DIP (single and dual version where with some single op-amps one gets the option for offset correction) and in 14-pin DIP version (4 op-amps)
Some op-amps are also available in ceramic DIP housing.
Then there are the small surface mount SMD SOIC-8 and SOIC-14 housings.

In theory one could get a higher channel separation (if a dual op-amp is used for stereo) but this is more dependent on the PCB layout than the op-amp.
Besides .. one does not need a super high channel separation anyway as you always hear both speakers with each ear anyway.
30-40dB channel separation is way more than sufficient already.

Modifying it to be potmeter - op-amp config won't be very easy as the tone control also is in there.
Besides it is kind of moot when short interlinks are used.
I'm not sure but expect the tone control to be 'active' (using its own SMD opamp).
When the tone control would be passive as well, which could explain the choice for a 100k potmeter (if that really is the case, it is an assumption based on the 60k output Z).

 

[Davey]

Me thinks there is some confusion here.
My P4 measures an output resistance of 1.1k ohms at all volume control positions. (Regardless of gain switch position.)
Minimal output noise change with volume control position.

This unit has all the characteristics of a buffered output stage with potentiometer upstream.

Edit: I removed the board from the case and took a good look. The input jacks route directly to the volume control, and pins 1 and 7 (outputs) on the NE5532 route over to the L/R output jacks. So, case closed on that.

 

[solderdude]

In that case @Carl-Fosi Audio is giving us the wrong info.

The problem here is conflicting information.

P4:

• RCA input*3
• RCA output, PRE OUT
• S/N ratio: ≥110dB
• Distortion: ≤0.003%
• Dynamic range: ≥110dB
• Less noise: ≤9uV
• Frequency response: 20HZ-20KHZ (±0.1dB)
• High and low gain Bass gain: ±12dB, Treble: ±12dB
• Output impedance: ≥1kΩ
• Input voltage and current: DC 12V/1.5A

If this is the case then it is potmeter first and then the opamp.
It could also be that the spec above means that the load resistance to the P4 must be higher than 1kohm.

Judging from the layout of the PCB it appears as if the input signals go through the replaceable op-amp first and the outputs of the op-amp go to the volume control from there.

On the PCB there is also a tone control and an SMD opamp after the volume control (which may be a buffer or belong to the tone control or the SMD chip isn't a dual-opamp and has another function related to the motor drive.

I would need detailed pics of front and bottom of the PCB to determine what is done internally (and of the front PCB) or a set of reliable input/output measurements (AC) of the impedance at various settings.

 

[gino1961]

You can get a lot of op-amps in different 'housings'.
The round metal can, the 8-pin DIP (single and dual version where with some single op-amps one gets the option for offset correction) and in 14-pin DIP version (4 op-amps)
Some op-amps are also available in ceramic DIP housing.
Then there are the small surface mount SMD SOIC-8 and SOIC-14 housings.

In theory one could get a higher channel separation (if a dual op-amp is used for stereo) but this is more dependent on the PCB layout than the op-amp.
Besides .. one does not need a super high channel separation anyway as you always hear both speakers with each ear anyway.
30-40dB channel separation is way more than sufficient already.

Modifying it to be potmeter - op-amp config won't be very easy as the tone control also is in there.
Besides it is kind of moot when short interlinks are used.
I'm not sure but expect the tone control to be 'active' (using its own SMD opamp).
When the tone control would be passive as well, which could explain the choice for a 100k potmeter (if that really is the case, it is an assumption based on the 60k output Z).

Thank you very much for the very helpful explanation
Just a word on tone controls
I hate them so much that i am always use the bypass
To tones control addicteds i would recommend a nice equalizer analog or digital between pre and power amp
Maybe one of those with tens of sliders.
They should make them happy

 

[Count Dacula]

I just ordered my second one, like $75

These things are magical

 

[olds1959special]

My speaker rigs use digital volume control direct from DAC now (better,) so I no longer need this. It was okay when I did need it, but as I stated before, the remote control was barely usable.

 

[Carl-Fosi Audio]

I just ordered my second one, like $75

These things are magical

Why do you need a second one?

 

[olds1959special]

I started using my P4 again, mostly for the treble control. Still wish the remote control worked better.

 

[Carl-Fosi Audio]

I started using my P4 again, mostly for the treble control. Still wish the remote control worked better.

What do you mean by "worked better"?

 

[olds1959special]

What do you mean by "worked better"?

It can’t finely control volume, only in big jerky movements.

 

[Davey]

It can’t finely control volume, only in big jerky movements.

This is a remotely operated, standard motorized potentiometer. You understand this, correct?
The operation really can't be improved upon, significantly.

Also, I don't think Carl-Fosi is really a member of their team. I think maybe this is an AI bot.

 

[olds1959special]

This is a remotely operated, standard motorized potentiometer. You understand this, correct?
The operation really can't be improved upon, significantly.

Also, I don't think Carl-Fosi is really a member of their team. I think maybe this is an AI bot.

I understand that’s it’s a design limitation but it’s a good reason not to buy this unless you control the knob manually.

 

[Davey]

To me, being old school, this type of control action is, tolerable.
This being an audio-taper control, the best area of operation is above the 12 o'clock position.
If that is not the area you are normally in, then your system gain structure should be adjusted/optimized.

I prefer a digital volume control. But I could live with this no problem......if I had to.

 

[Carl-Fosi Audio]

It can’t finely control volume, only in big jerky movements.

Thank you for your feedback, we will try to improve it.