Fosi v3 Mono - User Impressions | Owner's Thread

[Miser_Audioman]

The listening test should only check if both speakers are in the same phase. The reversing only matters if you bring in an amp that has a different polarity, such as a sub. That is how i detected the issue with rew, change in bass mode

 

[SMen]

The listening test should only check if both speakers are in the same phase. The reversing only matters if you bring in an amp that has a different polarity, such as a sub. That is how i detected the issue with rew, change in bass mode

I think @Fosi Audio will need to establish the units affected.

If I was not on this thread I would not have a clue ... and then (they are not currently set up) find unexplained issues in a couple of months when I will be using with subs connected.

Maybe they will respond to this.

 

[SMen]

Is your system out-of-phase? Using the cited test tracks, my system is in phase without a doubt.

When I have time, I can try the RCA connections and check phase again.

I'm not sure if the out of phase test only works if one speaker is out of phase vs another - but if both have reversed polarity ... is that not slightly different?

The listening test should only check if both speakers are in the same phase. The reversing only matters if you bring in an amp that has a different polarity, such as a sub. That is how i detected the issue with rew, change in bass mode

Which I think is what you are saying here - so if both speakers are reversed identically we won't hear - only when another amp used - or sub as you say.

I have not heard anything - they are sounding excellent with my stereo speakers ... I'd like to know if it is just a speaker terminal issue though - I think I can probably swap the color coded plastic elements of the sockets when I have a DIY few minutes ... but if it is at RCA level and not Balanced that is a bit of a production error by Fosi ... gone unnoticed by lots of reviewers

 

[rhing]

Fosi Audio will need to determine what might be at the root of out-of-phase behavior.

The XLR balanced inputs do not go through the single NE5532 op amp next to the RCA single ended inputs, whereas the RCA inputs do. Not having seen a schematic diagram of the amp, it is difficult to tell how these op amps are wired into the circuit as it is different from the Texas Instruments TPA3255EVM evaluation module circuit, which does have a publicly released circuit schematic diagram.

With the TI TPA3255EVM, the NE5532 op amps are input buffers going from RCA single ended signal inputs into the differential (balanced) inputs of the TPA3255 amplifier IC. Interestingly, the Fosi Audio ZA3 has an NE5532 op amp at the XLR inputs to convert the balanced signal into a single ended circuit leading to the pair of NE5532 op amps on the left and right channel inputs to the TPA3255 amp IC.

I bought the pair of V3 Monos for my specific use case, which is to use all-balanced (differential) amplification and to take advantage of the load-independent frequency response of the PFFB circuit. This setup drives a pair of low efficiency KEF LS50 speakers that can dip down to 4 ohms impedance with Class D amps capable of delivering current without breaking a sweat (why I bought a pair of the 48V/10A power supplies). The mission is accomplished and I am very pleased with the highly musical sound quality.

 

[LukW]

Witamy w ASR!

Przedstawione wartości pochodzą z karty katalogowej TI TPA3255 . Pierwsza wartość to maksymalna moc wyjściowa (przy 1% THD+N) przy użyciu w trybie stereo (2 kanały wyjściowe). Niebieska linia przerywana wskazuje napięcie zasilania 48 VDC. Przy obciążeniu 4 Ω (czarna krzywa) moc wyjściowa wynosi ~230 W. Zakładając, że wzmacniacz osiągnął maksymalny prąd wyjściowy przy obciążeniu 4 Ω, maksymalna moc wyjściowa przy obciążeniu 3 Ω wynosi 230 * (3/4) = 173 W. (Ponieważ moc = prąd^2 × impedancja obciążenia, a po osiągnięciu limitu prądu maksymalna moc wyjściowa spada wraz z impedancją obciążenia). Oszacowanie 173 W jest ostrożne, ponieważ limit prądu prawdopodobnie nie jest nagłym, twardym limitem, a wzmacniacz stereo prawdopodobnie może wyprowadzić nieco więcej niż 173 W (1% THD+N).
View attachment 381599

Druga wartość dotyczy trybu mono (1 kanał wyjściowy). Przy obciążeniu 4 Ω (szara krzywa) maksymalna moc wyjściowa jest nieznacznie zwiększona do ~255 W (wzrost o ~0,5 dB, różnica prawdopodobnie niezauważalna). Jednak ponieważ aktualna pojemność wyjściowa jest podwojona dla trybu mono, maksymalna moc wyjściowa 3 Ω (czerwona krzywa) jest zwiększona do 325 W, co jest o około 2,7 dB (maksymalnie) więcej niż w trybie stereo.

Podsumowując, korzyści są niewielkie, jeśli impedancja obciążenia przekracza 4 Ω, przy ustawieniu mono rozpraszanie ciepła będzie rozprowadzane do 2 amperów zamiast do 1, a zatem mniej stresu dla mono, gdy zapotrzebowanie na moc wyjściową jest wysokie. Musisz sam zdecydować, czy optymistyczna ocena wzrostu mocy o 2,7 dB przy 3 Ω jest warta wyboru 2 mono czy 1 stereo.
View attachment 381594

 

[LukW]

Witamy w ASR!

Przedstawione wartości pochodzą z karty katalogowej TI TPA3255 . Pierwsza wartość to maksymalna moc wyjściowa (przy 1% THD+N) przy użyciu w trybie stereo (2 kanały wyjściowe). Niebieska linia przerywana wskazuje napięcie zasilania 48 VDC. Przy obciążeniu 4 Ω (czarna krzywa) moc wyjściowa wynosi ~230 W. Zakładając, że wzmacniacz osiągnął maksymalny prąd wyjściowy przy obciążeniu 4 Ω, maksymalna moc wyjściowa przy obciążeniu 3 Ω wynosi 230 * (3/4) = 173 W. (Ponieważ moc = prąd^2 × impedancja obciążenia, a po osiągnięciu limitu prądu maksymalna moc wyjściowa spada wraz z impedancją obciążenia). Oszacowanie 173 W jest ostrożne, ponieważ limit prądu prawdopodobnie nie jest nagłym, twardym limitem, a wzmacniacz stereo prawdopodobnie może wyprowadzić nieco więcej niż 173 W (1% THD+N).
View attachment 381599

Druga wartość dotyczy trybu mono (1 kanał wyjściowy). Przy obciążeniu 4 Ω (szara krzywa) maksymalna moc wyjściowa jest nieznacznie zwiększona do ~255 W (wzrost o ~0,5 dB, różnica prawdopodobnie niezauważalna). Jednak ponieważ aktualna pojemność wyjściowa jest podwojona dla trybu mono, maksymalna moc wyjściowa 3 Ω (czerwona krzywa) jest zwiększona do 325 W, co jest o około 2,7 dB (maksymalnie) więcej niż w trybie stereo.

Podsumowując, korzyści są niewielkie, jeśli impedancja obciążenia przekracza 4 Ω, przy ustawieniu mono rozpraszanie ciepła będzie rozprowadzane do 2 amperów zamiast do 1, a zatem mniej stresu dla mono, gdy zapotrzebowanie na moc wyjściową jest wysokie. Musisz sam zdecydować, czy optymistyczna ocena wzrostu mocy o 2,7 dB przy 3 Ω jest warta wyboru 2 mono czy 1 stereo.
View attachment 381594

Ok, znam tabele, nie o to mi chodziło. Czy mono fosi poradzi sobie z moimi głośnikami i przedwzmacniaczem o poziomie wyjściowym:
Niezbalansowane (RCA): 1V
Zbalansowane (XLR): 2V
Mam nadzieję, że nie rozmawiam z botem.

Translated by mods. Please ensure any translator apps are posting in English. Thanks

ok, I know the tables, that's not what I meant. Will the mono fosi handle my speakers and preamp with an output level of:

Unbalanced (RCA): 1V

Balanced (XLR): 2V

I hope I'm not talking to a bot.

 

[Miser_Audioman]

this should help clear things up- sub is out and no crossover except what is in paradigms. Red is the polarity reversed from marked (and what is the same as the receiver and sub amp, what I think is ‘standard’ absolute polarity). Regardless of absolute polarity, we get the same response curve (i dont live in an anechoic chamber, so slight deviations). Then i reversed the polarity on ONE (right side) amp so the channels are out of phase (blue). There, you see cancellation etc, especially at bass frequencies. I hope this helps.

 

[Miser_Audioman]

So without another amp involved, as long as left and right are in phase absolute polarity does not matter

 

[EddNog]

So without another amp involved, as long as left and right are in phase absolute polarity does not matter

Correct.

Phase is a time-related issue. If both channels are on the same time signature, there will be no issue. It’s when two amps are on different phases, that means they are out of sync, and so you have issues.

-Ed

 

[Jon L]

With the TI TPA3255EVM, the NE5532 op amps are input buffers going from RCA single ended signal inputs into the differential (balanced) inputs of the TPA3255 amplifier IC. Interestingly, the Fosi Audio ZA3 has an NE5532 op amp at the XLR inputs to convert the balanced signal into a single ended circuit

Very odd difference from products from same company. One could argue if one is using RCA signal, ZA3 would offer purer signal path by bypassing one extra opamp, and vice versa for XLR signal into V3 Monos.

 

[rhing]

Maybe so. The TI TPA3255 is designed as a differential amplifier. The first time I heard it running in differential mode came when a friend loaned me a PonoPlayer loaded with high resolution files and special cables to connect the Pono’s balanced outputs to my TI TPA3255EVM evaluation module powered by a 48V/10A Meanwell switching power supply that I bought from Mouser Electronics at a deep discount, because TI was trying to promote the use of the TPA32xx series of Class D amplifier ICs. The PonoPlayer employed a DAC and analog circuitry designed by the late Charles Hansen of high end audio manufacturer Ayre Acoustics. Neil Young was adamant that the PonoPlayer would not sound digital, and Charles Hansen delivered on his design. When I heard this combination in my own system six years ago, I was floored at how great this sounded. I almost bought a PonoPlayer, but by then, Neil Young’s product had been discontinued and it wasn’t really supported with any updates for the desktop app used to upload files to it.

That memory of that impressive sound stuck with me, so when I read Amir’s review on this forum, I closely followed Fosi Audio’s release of this product. With the almost all-balanced system I’ve assembled, I am very pleased with the results. These amps running in differential mode really make my KEF LS50’s sing.

 

[Basstölpel]

Fosi in orange. Can someone else verify this result? If the HK230 is correct, the Fosi is reversed.

Can confirm for another pair.
Connected to Denon 4400h via RCA, so it is definitely the fosi that are inverting the phase.
After plugging in the external amps i simply reapplied my old MultEQ-x profile instead of remeasuring and noticed something sounding wrong at the crossover frequency to the subs and perhaps up to 200Hz.
Rerunning Audyssey verified both front amps were out of phase (did this after seeing Miser Audiomans measurements)
So at the momet its established that either the Fosis speaker outs are inverted or the RCA Inputs. Can anyone test with XLR in?
Edit: Reread some comments. EddNog reported this behavior first while using RCA, retested with XLR and found only one input inverted.
So RCA is inverted, XLR is not.

Many thanks to @EddNog and @Miser_Audioman, without your testing i would have taken much longer to find the cause!

 

[DavidM1]

After plugging in the external amps i simply reapplied my old MultEQ-x profile instead of remeasuring and noticed something sounding wrong at the crossover frequency to the subs and perhaps up to 200Hz.

Pardon the newbie question but is a phase issue only a concern if hooking up a subwoofer along with the V3 monos in stereo?

If so, is it a concern if using a preamp to provide a signal to a sub or is it a concern if taking the signal to the sub from the speaker outputs or is it a problem in both scenarios?

 

[Basstölpel]

Pardon the newbie question but is a phase issue only a concern if hooking up a subwoofer along with the V3 monos in stereo?

If so, is it a concern if using a preamp to provide a signal to a sub or is it a concern if taking the signal to the sub from the speaker outputs or is it a problem in both scenarios?

It is the summation of multiple speakers/amps were the polarity is audible. If just running stereo with the Fosis (or driving the sub with a Fosi, too) everything sounds fine.
As soon as one speaker is not in phase with the others you will hear issues, regardless of signal routing.

Edit:if you are taking the signal for the sub from the output of a Fosi amp, via highlevel connection, that signal is inverted before it reaches the sub. The sub would play in phase with the main Speakers.

 

[SMen]

Maybe so. The TI TPA3255 is designed as a differential amplifier. The first time I heard it running in differential mode came when a friend loaned me a PonoPlayer loaded with high resolution files and special cables to connect the Pono’s balanced outputs to my TI TPA3255EVM evaluation module powered by a 48V/10A Meanwell switching power supply that I bought from Mouser Electronics at a deep discount, because TI was trying to promote the use of the TPA32xx series of Class D amplifier ICs. The PonoPlayer employed a DAC and analog circuitry designed by the late Charles Hansen of high end audio manufacturer Ayre Acoustics. Neil Young was adamant that the PonoPlayer would not sound digital, and Charles Hansen delivered on his design. When I heard this combination in my own system six years ago, I was floored at how great this sounded. I almost bought a PonoPlayer, but by then, Neil Young’s product had been discontinued and it wasn’t really supported with any updates for the desktop app used to upload files to it.

That memory of that impressive sound stuck with me, so when I read Amir’s review on this forum, I closely followed Fosi Audio’s release of this product. With the almost all-balanced system I’ve assembled, I am very pleased with the results. These amps running in differential mode really make my KEF LS50’s sing.

View attachment 382147
View attachment 382148
View attachment 382149

I am just wondering if the v3 monos are following the circuit design of the Evaluation boards you have photographed here, and when you refer to running as a differential amplifier, is this an added benefit to the mono vs the stereo boards? At least theoretically, or are they both differential. Newbie question I know.

Whatever, the monos are sounding excellent. I am enjoying the extra headroom - I can run my low output MM cartridges on a lower gain setting through the balanced input. Much fuller and natural sounding than my previous 3255 board if I may be forgiven a small subjective remark, just saying "sounding excellent" does not say very much!

Or perhaps I should say, much more revealing of source, be it 1964 or 2024...

 

[rhing]

In simple terms, differential = balanced. The TPA3255 amp IC runs in differential (balanced) mode in stereo—typically in BTL or Bridge Tied Load, and in mono—typically in PBTL or Parallel Bridge Tied Load. I believe the V3 Mono is run in PBTL mode enabling the amp to drive speaker loads that drop down to 4 ohms. The single ended RCA signal input is buffered and converted to a differential or balanced signal via the first op amp closest to the RCA jacks, and then fed through the rest of the input circuitry including two op amps that appear to be in series.

The TI NE5532 op amps in the TI TPA3255EVM evaluation module served as buffers to convert single ended signals from the RCA inputs to differential mode to be fed into the TPA3255 amp IC’s differential inputs. The modules had a set of four RCA jacks so that the first pair and second pair could be designated to carry the “+” and “-“ halves of the incoming balanced signal along with GND.

The TI TPA3255EVM module had very good quality components, but the Fosi Audio V3 Mono appears to have higher quality “audio grade” components like the really nice Sumida inductors and Wima film capacitors in the amp’s low pass output filter network.

It’s a shame that some people have out-of-phase response using the RCA inputs, but they should try reversing the + and - speaker connections on one of the amps (channels) to determine if this will resolve the phase problem.

Hopefully, Fosi Audio can duplicate the problem and then troubleshoot the problem and come up with a solution for all these new V3 Mono owners. I’ll have to review many of the early YouTube reviews to determine if most of the reviewers used the balanced XLR inputs, or if they used the single ended RCA inputs in their evaluations. It seemed like everyone was wrapped around the noise coming from use of the split power cable with the 48V/10A power supply. I just chose to buy two of them, even if it sounds like overkill. I like headroom.

 

[SMen]

In simple terms, differential = balanced. The TPA3255 amp IC runs in differential (balanced) mode in stereo—typically in BTL or Bridge Tied Load, and in mono—typically in PBTL or Parallel Bridge Tied Load. I believe the V3 Mono is run in PBTL mode enabling the amp to drive speaker loads that drop down to 4 ohms. The single ended RCA signal input is buffered and converted to a differential or balanced signal via the first op amp closest to the RCA jacks, and then fed through the rest of the input circuitry including two op amps that appear to be in series.

The TI NE5532 op amps in the TI TPA3255EVM evaluation module served as buffers to convert single ended signals from the RCA inputs to differential mode to be fed into the TPA3255 amp IC’s differential inputs. The modules had a set of four RCA jacks so that the first pair and second pair could be designated to carry the “+” and “-“ halves of the incoming balanced signal along with GND.

The TI TPA3255EVM module had very good quality components, but the Fosi Audio V3 Mono appears to have higher quality “audio grade” components like the really nice Sumida inductors and Wima film capacitors in the amp’s low pass output filter network.

It’s a shame that some people have out-of-phase response using the RCA inputs, but they should try reversing the + and - speaker connections on one of the amps (channels) to determine if this will resolve the phase problem.

Hopefully, Fosi Audio can duplicate the problem and then troubleshoot the problem and come up with a solution for all these new V3 Mono owners. I’ll have to review many of the early YouTube reviews to determine if most of the reviewers used the balanced XLR inputs, or if they used the single ended RCA inputs in their evaluations. It seemed like everyone was wrapped around the noise coming from use of the split power cable with the 48V/10A power supply. I just chose to buy two of them, even if it sounds like overkill. I like headroom.

Thank you for that. Interesting and clarifying. Those two op amps that appear to be in series are the ones that have my head scratching I have to say. @antcollinet pointed out to me on the V3 Mono review thread that they could well be in parallel - even when laid out that way apparently in series. This (in series) seems different (with my limited knowledge) to what I would expect with PBTL.
I don't mind switching speaker leads if I had to using RCA because it only really becomes an issue as using balanced are different (or correct) but Fosi sure ought to put a note up on this and inform purchasers. Great stuff

 

[MCH]

The TI TPA3255EVM module had very good quality components, but the Fosi Audio V3 Mono appears to have higher quality “audio grade” components like the really nice Sumida inductors and Wima film capacitors in the amp’s low pass output filter network.

I am going to disagree.
The selection of components for the LC filter is very well documented by TI, and the choices made by Fosi, specifically for the capacitors, does not even meet TI requirements. If they are "audio grade" I think doesn't mean much in this case.

https://www.ti.com/lit/pdf/slaa701

 

[rhing]

Fair point. The Texas Instruments white paper you reference doesn’t include implementation of Post Filter Feedback (PFFB), which is a significant feature of the Fosi V3 Mono, and can have an effect on selection of output filter component values. Not having a circuit schematic from Fosi to determine if output filter values are correct or not, getting into a disagreement over this is pointless.

 

[MCH]

Fair point. The Texas Instruments white paper you reference doesn’t include implementation of Post Filter Feedback (PFFB), which is a significant feature of the Fosi V3 Mono, and can have an effect on selection of output filter component values. Not having a circuit schematic from Fosi to determine if output filter values are correct or not, getting into a disagreement over this is pointless.

I was not referring to the values but to the ratings (section 5.2), but I agree with you on this