My view of this amp is that it's clearly a great product: PFFB to eliminate load-dependence; sufficient power output for a wide range of typical hi-fi audio applications; sufficient gain with both balanced and unbalanced connections; ability to handle 2 ohm loads; and noise and distortion performance that ranges from Good to Excellent, with most performance measures being excellent.
Personally I would like to see lower distortion in the top octave and better IMD distortion performance. However, given the content of music, as
@amirm says, this appears to be inaudible in practice. And crucially, these two measurement results would not stop me from buying this amp
if I were looking for a very small form factor amp in this price range. I do not happen to need an amp this small or this cheap in price, so I would probably go for a more expensive amp, perhaps with a different chip/module. But, to
@AdamG 's caution about keeping the thread relevant to this review, my point here is that the couple of less-impressive measurements of this amp are not in my view a deficiency of this amp
within its class; they are, rather, a more than reasonable performance tradeoff given the value - and a performance tradeoff which will be inaudible in 99-100% of applications.
As to why PFFB has not been implemented more often or for a longer period of time with these kinds of amps, I appreciate
@Toku 's very helpful information on that in post 157. I would also say, look at the frequency response graphs Amir posted in his review and the graph Fosi posted of the final production version. Even with Fosi's final, improved response, the PFFB has gained you about 1.75dB in improved linearity (4 ohm up 0.25dB; 8 ohm down 1.75dB) - and that's at a frequency 99% of adults can't even hear.
Don't get me wrong: I love the PFFB linearity and the lack of load dependence definitely is a make-or-break difference for me and I'm sure many others in terms whether we would consider purchasing such an amp. But when these TI-based amps were delivering lower wattage with 15-20dB lower SINAD, less ability to drive sub-4 ohm loads, and competing with similar amps going for $50-$70 online, I can see how there was no use case or business case for the added expense and design work to implement PFFB. Now with these amps going upmarket relative to where they started, it makes sense.
You need a pre-amp to control volume.
The only way you could connect OPPO analog out to Fosi directly, is by utilising OPPO's USB DAC feature... it will work no problem, but the volume would have to be controlled at the source... I used HQPLayerPro and JRiver when experimenting with my OPPO205. If you decide to try this, limit the maximum volume (in JRiver for example) to something like 50% and do NOT forget to disable all Windows OS sounds (!!).
Just to clarify the use-case with this amp, I'm not quite sure what you're saying here. When using the Oppo's analogue outputs - as one would with this Fosi amp - the Oppo's built-in volume control will work fine, no matter what input you use into the Oppo, with no need to use a second volume control in any device or software upstream of the Oppo. The Oppo is a digital preamp, and its volume control is disabled only at its own digital
outputs. Same goes for any DAC or other source device with a built-in volume control.
Why are people asking for a Stereo version?
I thought the original FOSI Audio V3 (STEREO) is what prompted people to ask for a Mono Amp version. Here it is.
If you want Stereo go for the original V3 which also gives you a sub-out.
Am i missing something?
You are not missing anything. It's the internet: Folks say "It should be B, not A" and then when B is provided other folks jump in and say "It should be A, not B," as if the previous comments and discussion never happened.
