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Fosi Audio V3 Amplifier Review

Rate this amplifier:

  • 1. Poor (headless panther)

    Votes: 10 2.0%
  • 2. Not terrible (postman panther)

    Votes: 44 8.8%
  • 3. Fine (happy panther)

    Votes: 236 47.0%
  • 4. Great (golfing panther)

    Votes: 212 42.2%

  • Total voters
    502
Hello rongon :) .

If you look at the input circuit of the FOSI V3 STEREO that I posted, then you will deduce that its input impedance is 25k ;)

Merry Christmas :D
 
Hello rongon :) .

If you look at the input circuit of the FOSI V3 STEREO that I posted, then you will deduce that its input impedance is 25k ;)

Merry Christmas :D
I missed that post.
25k ohms is nice. Relatively easy to drive, even by vintage tube preamps like the Dyna PAS. No worries at all with any modern sources.
 
People hi :D

It's good that FOSI makes a 'PFFB' version of the V3 stereo ;)

They can be inspired by the 2 'MODs' that I made on the O-NOORUS D1 or 'copy' the input diagram of the TOPPING PA5 II a bit like they did on their A70 model... :rolleyes:
 
If FOSI does like AIYIMA with their A80 model then no (LOL) :eek:

For my part, the ONLY undeniable flaw on the FOSI V3 stereo is the volume knob which does not allow you to listen at low volume and which even seems 'unbalanced' in level for the 2 channels at the beginning of its travel :(

After their diagram could be modified and simplified by using good components on the audio circuit such as 'MELF' resistors, better capacitors and especially better OP-AMP ;)
 
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That should be a nice improvement.
Will it make an audible difference?
:D
That would depend on your speakers' impedance curve (assuming you aren't already correcting for it), and your degree of HF hearing loss
 
People hi !

The 'degree of HF hearing loss' is not too important since some seem to 'watch' the music rather than 'listen' to it (LOL :p )
 
Hello rongon :) .

If you look at the input circuit of the FOSI V3 STEREO that I posted, then you will deduce that its input impedance is 25k ;)

Merry Christmas :D
Hi ICIETDIYEUR , I have followed your discussion about the V3 input circuitry with great interest. As you at some point correctly noted, a good input transformer is the best to use from an esoteric point of view.
Noticed the input impedance is not 25k, following your schematic. Zin varies from 24.2k when pot is at 0 %, 24,1k at 25%, 23,8k at 50%, 17,3k at 75% and 7,1k with the pot 100%. This due to the first opamp´s Zin=10k. As most of you probably never is over 50%, this is of little consequence;).
 
Just stumbled upon this thread - is the Fosi v3 the best stereo, desktop amp for under $100? Anything that gets recommended more? I have some extra speakers I wanted to connect to my PC.
 
rsl ia255 better in some respects. hasn't been measured here so no sinad info, but it has a hpf and is only about $15 more than the fosi. and no load dependency
 
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Hello reVintage.

I don't do esotericism or 'fiddling' with input impedance: everyone knows that a mount with a potentiometer varies the resistance (here the impedance) according to his position.

This was just an indication for a member who needed it, he hadn't asked for an electronics lesson... ;)

@ SlowJim

Who told you or wrote to you that the 'rsl ia255' doesn't have a dependency on the load ?

If this is true then they have to give lessons to TOPPING because even the PA5 II has a load dependence although very small, at least very 'contained'....
 
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Hello ICIETDIYER,
Seems you took it the wrong way, I was just trying to be friendly by pointing out a not so well choosen solution by Fosi, as an addition to your work here. Really appreciate the schematic.

By the way the Zin of a 50kohm pot does not vary if used the normal way, ie with a the pot resistance into the high Z input stage of a non-inverting opamp. But I am sure you are aware of this ;) .

A question that I am sure you can answer: TPA3255 is said to have Rin 20k in the data sheet . Does does that go for each input, ie 40k between the balanced input_A and input_B?

I have one V3 per stereo channel for my triamp system. One channel for 20-60Hz and the other for 60-350Hz. The 20-60Hz will be modded according to your suggestions, but the 60-350 will be with transformer input, call it esoteric if you want:).

Fosimod.png


Cheers
Lars
 
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Hello reVintage,

You didn't understand my attempt at humor either: I'm a bad 'joker' so :(

For the 'Rin' of the TPA3255, rather than tell you something stupid, I prefer not to answer but you can always ask the question to AIYIMA who seem to be great experts in TPA3255 :facepalm:

Regarding 'esotericism', I hope you will use LUNDAHL LL1544A transformers (it's expensive but it's the best) :cool:

I'll help you a little with this diagram ->

LUNDAHL LL1544A.jpg

It's up to you to 'play' now ;)

Regards.
 
I have done some sims of the compensation for C46 left channel, if you want to compensate when having added C41 220p. I have used TI´s own spice models for OPA164x. With C46=4.7-6p everything is OK up to 1Mhz, though phaseshift is alittel high at 20kHz.

Draw your own conclusions;).
FosiV3comp.png
 
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Hello again ICIETDIYEUR,

Thanks for the recommendations. I have LL1544A and LL1676 amongst others in my workshop , but I prefer LL1544A due to its smaller footprint and adequate overload margin.
When connecting LL1544A, R4U and L1 should be used. As TPA3255 gain is to high for my high sensitivity midbass driver 99dB/1m, will be perfect, giving a total gain of 3x.

1736176951949.png 1736177257184.jpeg
 
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Hi.

My final scheme for the 'MOD' was as follows ->

DIAGRAM FOSI AUDIO V3 STEREO + MOD V3 - FINAL (17 -12 -2024).jpg


Regarding the LUNDAHL transformers, so much the better if you have found what you are looking for ;)
 
Sorry didn´t see your latest mod, phase shift ca 20 degrees at 20kHz, in addition to 20-40 degrees due to output filtering. Have not seen many measurements and don´t know how it affects sound, though. With C41 22p phase shift is close to zero.
 

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