Fosi ZA3 Amplifier Deep Dive Review (with Data)

[@ErinsAudioCorner]

Re: Audible differences in amps, opamps, etc My personal take is this: Can differences be heard? Absolutely. Do measurements-only matter? Nope. Am I skeptical of claims of audibility? Yep. Now, let me explain... As someone who does have a bit of a following (albeit, quite small) I will always default to not talking about hearing things that I can't at least point to evidence with. It's not "just about measurements". It isn't. It's about being able to back up a claim. Whether that be providing measurements or results of a proper blind test. Either is acceptable to me as proof. The last thing I want to do here is to fool my audience into believing things that may not be true. None of this means that differences can't be a) heard and b) measured. In fact, I think there very well can be differences heard. But are those differences real or are they some form of sighted listening bias? In the case of Fosi ZA3 amp I have shown that its load dependency can effect the response of the speaker and that may very well be audible. The key is this: If someone can so easily hear differences in amplifiers, opamps, etc then it really should be easy to prove via a simple blind listening comparison. You don't have to have hardware or measurement experience. You just have to do a proper blind listening test with the help of a friend or friends. No one is doing that, though. At least not in the review space that I'm aware of. None of the above makes me a "non-believer". It just means that I take a reserved stance and am cynical of claims of audibility without some sort of evidence. I think that's a pretty reasonable approach to take. Especially when it comes to spending money based on claims. And, in my specific case, especially when I'm helping people decide if they want to part with their money on a product. I wouldn't feel good telling a friend to drop $300 on something I wouldn't buy. I am not going to tell my audience to do that, either. 👍 I prefer to report on things that I can prove or at the very least have some sort of pointer to. Whether that be from a blind listening test or measurements, doesn't matter to me. I just need to feel confident in telling you all that what I am reporting is accurate and not just words I used to fill space. I'm not saying any of this to get anyone to agree with me. I'm saying it so you understand where I'm coming from. Hopefully you can respect that. - Erin

[@joentell]

I reviewed a Burson amp a long time ago where they sent opamps that I could change. I could "sense" some kind of difference, but didn't have the tools to measure the change. See if you can get some of those or if they even still sell them. I'm curious as to what they did.

[@Douglas_Blake_579]

On the op-amp thing... (edit: In relation to these mini-amps) Get a copy of the NE5532 spec sheets and look at the various curves and ratings. Two things become readily apparent ... the chip has uneven gain below about 1 volt of input (which is where we use them!) and they don't even specify differential input specs because the two inputs are diode clamped together giving a maximum of 1.2 volts across the two inputs. (On the TI data sheet take a look at graphs 5, 6, and 7) Now you have your difference... a cheap as chips chip with uneven gain and very uneven input characteristics vs. any other chip that does not suffer these limitations. The real difference in op-amps with these small amps is NOT putting in an expensive chip ... it is getting rid of the stock chips. I commonly pull out all the NE5532 chips supplied and put in either RC4558 or TLO72 ... under $1.00 each in lots of 10. Yes that first step does make a noticeable difference ... beyond that, not so much.

[@editorjuno]

As long as both channels of the amp have the same "uneven gain" for any given input voltage, what's the advantage of swapping one type of cheap op-amp for another? Given that (e.g.) the NE5532 and TL072 are equally elderly designs, why is the NE5532 the overwhelming favorite for inclusion in mixing desks and other professional recording gear? I'm pretty certain the engineers of enormously pricey studio gear from the likes of Studer and Neve could have designed in any op-amp chip they wanted, so there must be something about the NE5532 that made it preferable, right? Then there's TI's decision to equip their own TPA3255 eval. board with NE5532s -- if the RC4558 is better for that application, then why that op-amp choice when TI makes both ICs?

[@davidfuller581]

@@Douglas_Blake_579The NE5532 and TL072 are pin compatible, but _not_ interchangeable. The 5532 is a bipolar input dual op amp, the TL072 is FET input. Wildly different input impedance.

[@Douglas_Blake_579]

@@editorjuno If I feed in 1 volt on a gain of 10 circuit... I expect 10 volts out. Not 7 or 13... 10. it's not rocket science. The advantage is that the 4558s and tl072s are linear gain, their input impedances are higher and their inputs are not diode clamped at 1.2 volts peak to peak.

[@richh650]

I have watched over a dozen of the ZA3 amp reviews and your review is the best review by far. You actually understand how to test and then associate that with how it really sounds and preforms. Your efforts are MUCH appreciated as you have become my go-to reviewer!!!

[@litzy7892]

After watching other reviews on this amp, my biggest question was how much power does it really have. And I knew if I were to ask in the comments they would just feed me the vague garbage that was provided by the manufacturer. But Erin came thru with the real measurements I wanted to see! I was impressed. Thanks you very much Erin.

[@JosephTongret]

This isn't really about the amp, but I'd LOVE to see you get in one of the Minidsp models (Flex/SHD/2x4) and do a few videos covering procedures to improve system performance. Time align, fir, crossovers, etc. There's a lot of guides already out there, but the way you explain what's happening and why ALWAYS leaves me with a better understanding than what I initially had. You have a way of making complicated topics so much easier to digest than most sources. Thanks for the content Erin, you're an invaluable source!

[@ErinsAudioCorner]

On it!!!! I am talking with MiniDSP about getting a FlexHT to do a video (or series) on how to use them to tune your system. I'm excited about it!

[@JosephTongret]

@@ErinsAudioCorner Wow, that's incredible news! I find their software a high value addition to a system. I'm looking forward to gaining a deeper understanding of any aspects you decide to cover. Thank you sincerely Erin!

[@peterdesmidt8742]

I"m really happy with my SHD. Dirac made a huge difference in my system. I have tso of the ZA3s. I biamp with them using my SHD. They work fine in my system.

[@kadajawi2]

​@@ErinsAudioCornerOh, awesome. I love my Flex HT, but I'm sure you can help me extract much more from it.

[@markalot]

Thanks for an actual review, much appreciated. As an old guy getting back into amplifiers "hifi" I was surprised to see many golden ear reviews but few if any actual objective reviews. I probably shouldn't have been surprised. Soundstage, imaging, lol, different op amps, oh my! I've got the WiiM amp, very happy with it for what it is.

[@ErinsAudioCorner]

Thanks for watching! Glad to have you back (in the hobby!)!

[@Clobercow1]

Late or not, I'd rather hear it from you than most of those jokers out there.

[@GoggleAX]

100%

[@TheLevitatingChin]

Pull them bum cheeks apart.

[@2kdesignstudios72]

Facts

[@tacofortgens3471]

So for the most in depth reviews on youtube. Love this channel

[@ck17350]

I own one of Fosi's 2.1 board amplifiers and I agree that some of their design choices are baffling. Mine suffers from exactly the same problem and with the small Paradigm Titans I paired it with, I ended up with noticeable low-end distortion from the Titan's at higher volumes. Now here's what I found most interesting about that Fosi product. I checked Texas Instrument's page for the amplifier chip Fosi chose and found that Fosi copied Texas Instrument's very basic example design schematic and just called it a day. Granted, this is one way to save money since there's no need to hire someone to design an amplifier if you can just crib TI's example. At the end of the day, the amplifier board was cheap, far more powerful than I expected it would actually be and I'm happy enough with the performance for what I paid for it. I picked it up as a neat toy to play with and I absolutely enjoyed it as a toy. With just a little bit more work on Fosi's part, it could be more than just a toy but at the end of the day, that's really all their product's are; cheap toys. Nothing wrong with that as long as reviewers make that clear like you have done. Glad to see this review after doubting some other reviews I've seen recently on this product. Keep up the great work Erin! You are a breath of fresh air in this space.

[@user-xg6zz8qs3q]

I have a geology degree here. It's about as useful as a roll of toilet paper. But I can say that sand is classified under different diameters, usually under 30mm (max range). But salt crystals can grow larger.

[@ErinsAudioCorner]

Listen, this is the best reply I will get on this video. By far.

[@Audio_Simon]

We all appreciate toilet paper and your reply 😂

[@AdamMrozek84]

Hi, you do very good tests. I would love to see the Topping pa7 in the future. Regards​@@ErinsAudioCorner

[@michaelblumfield3933]

But as someone who has been a professional writer for his entire career, I would suggest looking closely at the meaning of the cliche. The grain of salt is to guard against the negative qualities of what is being consumed. So if you really want to distance yourself from an assertion, you use MORE salt, not less. Rather than a grain, you might suggest applying a spoonful of salt or even a salt mountain. Or just say "I'm highly skeptical about ... ."

[@dinoalden568]

I don’t know about anyone else, but I have found the aforementioned roll of toilet paper to being quite useful!😁🧻

[@tugbars4690]

Checking the frequency response of the amp with a reactive load is a genius idea.

[@gurratell7326]

And the conclusion is that load dependency is a thing, but it might just be barely audible. And anyone that's interested in audio should have a DSP anyways so those small frequency response deviation can easily be fixed together with the rest.

[@HunterMikeWI]

Fosi is working on LPF and HPF filters for the next gen models. They are deciding on fixed or variable frequency.

[@ErinsAudioCorner]

That's great! I'd vote variable for sure.

[@mathumphreys]

I'm even later to the party, but I'm just glad to be here. I bought this amp after watching your review, so thanks for taking the time to put together a really useful practical review. In relation to the amp, I'm using it in a studio environment to power (secondary) monitors so I needed balanced inputs, flat response and silent. The performance is just brilliant for me - no fuss, does the job, run at around 75% all day and barely get warm and is transparent. Having the RCA inputs is a bonus because it has given me a couple of auxiliary inputs that I can flick when I want. I love this little thing.

[@Static_Age]

You’re not late. You just took the time to do it right.

[@ErinsAudioCorner]

I appreciate the sentiment. I definitely put a lot of time into this to figure some things out that had me stumped for a bit. LOL

[@ItsElvis56]

An absolutely amazing video, Erin. I will be holding onto my Schiit Rekkr for my desktop setting. Almost 50k! Should be way over 100k at this quality.

[@JukeboxAlley]

I have to say, after many years in the hobby and after probably 300+ amps and receivers, i got 2 of these and a decent preamp and i love em (the right preamp is always the key component to separates, remember that). Keep in mind this is compared to old legends (but often a little noisy in the background due to age, power supply, blah blah blah) adcom gfa-555, nakamichi pa-5 class A, emotiva xpa2, and many more, and i liked these better and had just as much to give in any area as the others and are silent, zero noise. Also you dont need to see a chiropractor after moving them around, i think their the best for the money that one can ask for. Thats just me, you hobbyists gotta do whatever works for you but i will say this, when 2 of these are running together on a decent preamp, they are certainly no joke and tonally great. Also on a 4 ohm load driving the flagship polk sda srs 1.2 (6 ft tall and 180 pds each) and did a VERY good job with plenty of headroom and kept their composure even at 115-120 db levels and on 10, 15 and 20 hz sweeps, no problems for me. Sounded as good or better as nearly anything else ive tried, including "claimed" bigger and badder amps and of different classes, they work for me just fine and id buy a dozen more for backups if i could afford to have them around, anyway, to each their own its just my opinion.

[@MrBonger88]

I really like that you show us your testing methodology in detail. Thank you

[@ErinsAudioCorner]

Thank you!

[@TheTrueVoiceOfReason]

Most of the guys I follow that design/build/repair/improve amps, do tests at different frequencies in order to see, not only max output @ % thd, but also to compare thd @ many levels as well as how flat the amp really is. They also test @ the different impedancesfor everything including power supply sag. So yes, you are on the real comprehensive path. Keep it up and don't let anyone tell you any different.

[@sk9592]

The subwoofer output issue you described is a feature, not a bug. At least as far as 2-channel audiophiles are concerned. For those of us from the home theater or car audio world, high-passing your speakers when you have a sub connected just seems like common sense. However, the "common wisdom" in the audiophile world is that subwoofers are a necessary (or unnecessary) evil. You want your speakers to cover as much of the full range as possible, and for the subwoofer to only come in at the very low-end as the speakers are already rolling off. You don't want to pick a crossover point. Your crossover point is determined by where your speakers bottom out. I personally don't agree with this logic, but if Fosi wants to sell to 2-channel folks, they likely need to cater to this approach.

[@ErinsAudioCorner]

I disagree. The use of a subwoofer alleviates the mains from playing lower frequencies so those speakers then improve in both a) distortion and b) dynamics. (These are objective things, btw.) "You want your speakers to cover as much of the full range as possible, and for the subwoofer to only come in at the very low-end as the speakers are already rolling off. You don't want to pick a crossover point. Your crossover point is determined by where your speakers bottom out.... personally don't agree with this logic" ^ I agree with your disagreement here but maybe for a different reason because this particular item doesn't offer control of where the filter is implemented. Maybe if they did I might be more apt to not be bothered by not having a HPF (i.e., mains roll off naturally at 60Hz so you apply the subwoofer to ~60Hz and below). But that's a moot point here, unfortunately.

[@sk9592]

​@@ErinsAudioCorner You mentioned the -3db point on the sub out as being 180Hz. I'm assuming Fosi picked this as a safe maximum for the highest people want their subwoofers to play. The idea being that you adjust the bandwidth of the signal yourself using the LPF built into your subwoofer. I've seen this same approach on $1000+ integrated amps that have a sub out: Speaker level outputs always play full range, and the sub out is not adjustable on the amp. You're expected to low-pass the signal on your sub. It's not ideal, since as you said, I can't pick a crossover point that minimizes distortion, improves dynamics, etc. It also assumes you're using a subwoofer that has a LPF built-in, and it leaves you SOL if your room is creating a null in the crossover region forced upon you. But for whatever reason, this has become the "common wisdom" that hi-fi amp and speaker manufacturers have settled on. To be clear, the lack of this functionality in this amp annoys me just as much as you. I feel like I'm taking crazy pills when I see integrated amps costing thousands of dollars that lack the most basic of bass management features,

[@michaelsparrow4798]

@@ErinsAudioCorner Not to mention, that although a driver/crossover combo has a point where it will no longer reproduce linearly, the speaker will still be forced to "overlay" frequencies above the crossover point on top of lower frequencies. Unloading the speaker by removing lower frequencies lets the upper-bass/mid frequencies drive the speaker's cone with less "interference".

[@Blewis-Diarrheo]

I found this channel because of the whole speaker controversy. I subbed because the content is really good, and the subject matter is very interesting and very informative.

[@darrylbrueckner2812]

I picked up a ZA3 to match with a pair of Micca RB42'S and Topping E30 I had kicking around. Pull the speakers out a couple of feet from the wall and sit nearfield and this combo delivers some audiophile fun on the cheap. So, a solid value compared to other budget Class D amps? Absolutely. GIANT KILLER like some of the more hyperbolic reviews? No. That being said, I'll probably upgrade to the Sparkos op amps because I can't stop myself 😂

[@Douglas_Blake_579]

I'd be careful with the ZA3/Micca combo ... the amp has enough kick to send those tiny speaker cones flying across the room...

[@kraig7777]

@@Douglas_Blake_579 Are you drunk? Only about a million people have that combo and no RB42s are blowing up. I have a Fosi BT20a Pro with 48v power supply and RB42s and they could use even more power. In fact my old Yamaha receiver finally croaked and I'm using the Fosi in the living room driving vintage Boston Acoustics A100s and the amp and the speakers are doing well.

[@BobbyBass-x6i]

Erin thanks for your measurements and explanation of what they mean. Very helpful. Keep up the great work!

[@williewilson6832]

I own a pair of ZA3 monoblocks but I personally prefer the sound of FOSI TB10D stereo power amplifier. They can easily drive a pair of my 6ft tall Genesis APM-1 speakers. Sounds very detailed with crystal clear highs, excellent mids and very tight bass response.

[@carlatdmf]

I just love the sound of measurements.

[@JoseFerreira-zb7wh]

You should try the smsl a300. Seems like good competition for this. It can be used as monoblocks, has a remote, tone controls and eq presets and a high pass filter (100hz is the only option though) so at least from a feature/ functionality point of view, it kills this.

[@ErinsAudioCorner]

I'll look into it. Thanks for the suggestion.

[@stephenjarzombek2903]

I have one, purchased before any independent reviews came out and am not impressed. I find the interface clunky at best. In stereo mode, the power into 8 ohms is only 50 watts and has worse distortion than average these days. It has a lot of power as a monoblock into 4 ohms--over 400 watts---but at the expense of even more distortion.

[@JoseFerreira-zb7wh]

@@ErinsAudioCorner rsl also has a cheap one with hpf. Sabaj a30a is a digital amp with that option also.

[@JoseFerreira-zb7wh]

@@stephenjarzombek2903 yes, just checked asr's review. He wasn't impressed either. It's a shame because the featureset and price are great!. I don't understand why there aren't more amps with hpf. If one's into streaming only there's always something like bluesound node or the upcoming wiim ultra that have bass management, but other options are much pricier.

[@michaelsparrow4798]

I agree 100% on everything you describe. Suggestion: You mention multiple times that a high-pass would unload the speaker. I'm sure you know that it also would/should unload the amplifier, but unless stated explicitly, some newbies may not perceive this. Cheers!

[@ErinsAudioCorner]

Good point! Thanks!

[@marinoint]

Excellent review as always. Hope Fosi will see this and fix the weird "subwoofer output" problem.

[@NickSim86]

It's not weird. It's the industry standard. Idk what Erin is on about

[@Douglas_Blake_579]

@@NickSim86 Exactly .... you gotta climb the price scale quite a bit before you find high pass filters on the mains.

[@hmurchison8123]

I've got Monitor Audio Radius 45 ( 3" Cubes ) so not only do a need High Pass but it has to be flexible enough to go to 120hz. I get that vendors want to "tick boxes" but just save the Sub out money , if you're not adding a HPF to the mains "you're doing it wrong" I don't care how cheap your product is if it's designed inflexible.

[@NickSim86]

@@hmurchison8123 you have very niche speakers and should use an AVR

[@anthony2010707]

I was using the subwoofer output in mono mode and it was quite weak, i did the same in the stereo mode and it was fine, now i have gone back to the mono mode using two ZA3'S with the P3 Preamplifier, running two subs from my P3 Preamplifier using a 3.5 mm to rca y splitter, lovely results, enjoying the music. 😊

[@forty5cal1911]

Priceless review as always Erin!

[@mikeortiz2139]

I really like the approach of your reviews on these audio components. And more so for the amps than speakers. I guess it's bc speakers easily price themselves out, but budget amps like this, the deep objective dive is something to pay attention to. Thanks.

[@wesleyaardahl7278]

Thanks much for this review. Please review the Fosi V3, which is quite different and does not have a sub out. Many people (self incuded) are buying them and it will be helpful to have a similarly thorough, empirical review of it. Thanks much!

[@andreas_aus_d]

I actually own two of those things and I am happy with them so far. What is really necessary to provided with an equalizer, because in my opinion based delivery is kind of weak. It is a bit weird that your measurements look linear on the frequency graph Remember one thing however. Whenever you suggest something to fosi audio and it makes sense to them, then they will often listen. That's how the V3 Mono got started. Thank you for that review. I was already owning two of them when I watched it. I am however grateful for people who take the time to provide such a service.

[@AbsoluteFidelity]

Honestly, not many amps have a high pass, especially chifi. I know of one or 2 that have a non adjustable high pass, but that just solves part of the problem. I really wish to see chifi amps with fully adjustable high pass and dual sub outs.

[@steveh545]

Agree. Even some of the bigger hifi companies don’t include high pass filters. It seems like that’s something if want, but you have to hunt around to find that. Or buy a WiiM amp 😂.

[@AbsoluteFidelity]

@@steveh545 I mean its not hard to make an amp with a fully adjustable high pass, dual sub outs, load indepedent and hdmi out. Lol! Infinite PEQ would be nice too. 😜

[@steveh545]

@@AbsoluteFidelity no idea how hard it is to make an amp… just many many more expensive amps don’t offer high pass filters, so I wouldn’t ding this amp for that. YMMV.

[@AbsoluteFidelity]

@@steveh545 its like amp manufacturers do not know what the consumer really wants, they keep churning out same ol products. It could be easily done by china, or any manufacturer for that matter.

[@DomSchiavoni]

What you all are talking about is easily and thoroughly solved with a minidsp. Pair a 2x4HD with Any amp (including this fosi) and you have the power to time align your individual subs and mains to maximize amplitude then dial in the frequency response as desired for multiple listening positions. It's a real game changer. Far superior to an amp with "dual sub outs".

[@c0mbat15]

Totally agree on the subwoofer pre out. That's why I went for the Aiyima A07 Max amps at half the price of these. I'm using the line out from the WiiM Pro Plus in to the Kef KC62 and then use the high pass filter on that to feed anything above 85hz to two Aiyima A07 Max amps that then feed above 85hz to the Kef LS50 Meta.

[@Karto86]

Agree, Wiim amp, with its build in streamer, dac, PEQ, subwoofer management is so much better money spent. Also, a wizard is never late, Frodo Baggins...

[@davidperkins4139]

Totally disagree as someone that owns both. Dual ZA3’s with a wiim pro is SO much better than a wiim amp. It just doesn’t have near enough power.

[@Leo-tf7gu]

Thank you for bringing in the HPF issue to light. "Audiophiles" love running their speakers full range with a sub, not realizing that it causes the mains to do unnecessary work at the same time possible cancellation of low frequencies when all 3 speakers are playing them. Not to mention, the unnecessary load on the amp when playing low frequencies.

[@stackoverflow8260]

I had a similar experience with another class D, Purifi 1ET7040SA from Buckeye Amps rated at 950 watts@2ohms. I am using Thiel CS3.6 which are almost a resistive load from 80hz to 22khz at 3.3ohms. Whenever I played "When it's going wrong" by Marta and Tricky, the amp would experience bus pumping and go into protect mode. It happened even when crossed over at 50hz, it doesn't happen when crossed over to subwoofer at 70hz. Dylan from Buckeye will be sending extra capacitor boards to mitigate this issue.

[@ubacow7109]

I'm excited to see the TPA3255 amps with PFFBs be implemented into Home theater amps. Super affordable and high performing and hi powered class D amps.

[@Panslapper]

PFFB is location bias. Depending on where you live in the world. Most electrical circuit feeding homes in Europe won't need PFFB and so the average consumer will not necessarily see any improvement anyway...apparently.

[@ubacow7109]

@@Panslapper measurements show significant improvement

[@Panslapper]

@@ubacow7109 So it appears...I can't sat either way myself as I have no way of testing but if offered with or without I would likely go with...

[@gameswong]

Thank you for the great review!!!!! ❤❤❤

[@ErinsAudioCorner]

No way! Thank YOU!

[@wingertm]

After seeing your reviews, I’m asking myself why I even bother watching anyone else. Your insight on these products is superb and well worth the wait. I will give kudos to Andrew Robinson who does level match when A/B testing. Thank you and looking forward to your next.

[@AzGael]

Not late at all, the sales start tomorrow so this is conveniently and precisely on time. Thanks for the review!

[@alphacapo]

The dummy workaround is if u have good eq software you drop everything below 60hz a few db adjust your sub level at the sub. You get to your sweetspot volume for where u like to sit. And adjust levels from there. This is how you did it in the 90s anyway

[@lionelh152]

Is there many Microsoft audio player that support this? The software should be compatible with something like vst ? A vst name bundle?

[@tycheesey]

Thank you so much for dampening the sensationalism that surrounds these types of amps. It isn't all rainbows and unicorns. The RSL iA255.1 is the only amp that has addressed all of the failures of the Fosi, Douk, Aiyima "giant killers." Hype is a hell of a drug. I don't like how so many reviewers latched on so hard to the V3 clone era we are in currently.

[@ErinsAudioCorner]

A few others have asked about that RSL as well so I will send them an email and see if they are interested in sending me one to review.

[@sasoesasoe]

This is exactly the answer I needed. In a home theater for more directional bass I had the idea to use 3xFosi as an amp for RCL with 3x sub. On the receiver I would define the RCL speakers as large. Ok, is not recommended. We knew that Fosi does not have a manual crossover setting when you connect a sub. Well now we know that it does not have an automatic one either. And also the output power is not a big step up. So, no go.

[@stu-po]

I really would like to see someone test these in Mono mode at 2-ohms. The PBTL should be more stable at 2 and 4 ohms than a single side of these amps.

[@Revenant_Knight]

I've used a bunch of these Chi-Fi amps before. They're fine for just getting music or audio somewhere where you want it in the background or if you don't have great speakers, but they aren't going to be amazing or anything like that. I use one hooked up to an old CRT for old gaming consoles. They're fine for that.

[@Rumburak852]

its a desktop amp for 150 bucks , what do you expect?

[@literalghost929]

You have to be careful and not generalize. There's different chips which make a significant difference. The newer one ones use TI TPA3255 amp chip, whereas older D amps used TPA3116 or others, and the newer ones are by far superior; much more comparable to 'real' amplifiers in terms of performance. But yeah, I agree, older chips weren't quite up to snuff. Latest generation though? Debatable at best, my personal experience; great performance for very little $$. Really not the same as older amps which were audibly inferior.

[@Revenant_Knight]

@@Rumburak852Exactly what I wrote.

[@Revenant_Knight]

@@literalghost929 Newer ones are better, but still not the miracles that a lot of people claim they are. I do enjoy some of them though. I just don't pair them with speakers that need more than what they offer or use them for ultra high fidelity situations. For my retro consoles though they work well. As a little integrated amp for some small speakers in the dining room that will never be more than background, they also work well. As a PC speaker that you're not looking for monitor class audio fidelity, again, they're fine. Attached to a pair of KEF R3 Metas in a properly conditioned listening room? Not so much.

[@Harrisongrey19]

@@literalghost929fosi amps are garbage

[@matiaserp]

Erin, while adding a high-pass filter would be better, ideally with the low/high-pass filters variable, the lack of is clearly a cost saving decision. And to why would anyone add a subwoofer without high-passing the speakers, while I agree that would be ideal for lower strain on the woofers, in the absence a subwoofer would still extend the bass of smaller bookshelf speakers and totally worth it. So while we all agree that having the adjustable low/high-pass filters is ideal, the sub-out is still worth having. Thanks for the great review!

[@Lokaminiac]

A excellent reviews and good catch on the subwoofer output issue. To compare, a low price (powered) Edifier speakers with sub output all have high-pass filters. Damn, I think I will wait for their next generation products. So close…

[@theAbbeBusoni1]

Scientific Audiophile was right! ASR really needs to fix the way the measure these amps because a product that performs in this way seems to be as a result of poor measurement method at ASR (Fosi V3 was hailed there).

[@sk9592]

Any review methodology that chases a single number in its rankings will result in companies trying to game that one spec while ignoring everything else.

[@literalghost929]

@@sk9592 And then the 'measurement' type folks will scoff at people hearing differences which they didn't measure. I'm much more on the measurements side, but sadly, people take it to extremes, and ASR, although I hate to say it, might have gone ab it over the edge in that respect.

[@theAbbeBusoni1]

@@sk9592 I do agree, though most on ASR don't really do the SINAD chasing most people think they do. I would say the biggest problem is that the power numbers are greatly exaggerated on ASR in comparison to what actual music would take.

[@tonyosicka]

You never mentioned the speaker's used...why? Second with my testing so far with surplus speaker's..... PMC OB 1. Quad 11s In a large room. It did a fantastic job. Wiim pro plus as the pre amp for streaming and cd playback and the parametric EQ.to fix your issues. Fantastic job from both units.

[@Douglas_Blake_579]

Because those are laboratory tests done with dummy loads.

[@frequeencytv8130]

It takes 10 years to realize and appreciate 🙏 the CHIP use in these Amplifier, The arduous trial and error and Research and development sleeples nights, changes and alterations made to Improve the Chip, the team that made and designed the Chip is so delighted and very happy HiFi People are talking about the application of the Chip Projects finally making waves, making people enjoy a not too expensive Amplifier. Thanks to the Reviwers

[@chinmeysway]

class D has interesting history. maganovox employee discovered long time ago early 90s ? and was really unrecognized / misrepresented. but i don’t recall the deets very well and if chip was grown up yet n stuff

[@ron5493]

Great Job! It is good to know about the lame sub out. Great information!

[@ErinsAudioCorner]

Glad it was helpful!

[@sensibox]

Very thorough measurement! BTW Fosi is going to release a new monoblock amp with the PFFB implementation to devoid load dependency.

[@ErinsAudioCorner]

That’s good news.

[@lionelh152]

Does O-NOORUS D1 Amplificateur Class D TPA3255 PFFB already do that?

[@buckaroobonsi555]

I agree I thought they should have have an automatic low pass on the sub output of 90hz and a switch to give you the option of high passing the mains!

[@booobtooober]

Great review. So many fanboys rave on this amp its hard to get a fair assessment. I get bashed in the comments when I tell people this amp is not bridged. My thoughts are that this is a great alternative to computer speakers, or perhaps for background music in a small business. But by no means is this a replacement for even your sub-audiophile equipment. And it completely ridiculous to me seeing people stack these up in their monoblock home theater set up. They're spending more for a worse setup. Another point is that the 3255 chip in this amp, (and many other branded amps not just Fosi) is a PRO-sound amp chip and they are woefully underpowered even with their more expensive upgraded power supplies, in addition to the RCA line input voltage is to low to take full advantage of its design. On that note I dare any of you naysayers to hook ur your 3255 amp thru a mixer and crank it up! "You'll see" said the blind man.

[@proffessasvids]

Brilliant. Yeah that sub/main issue is a little silly. It needs a switch on the back really. A number of crossover points to be selected would be wonderful xx

[@acemacgruber6593]

Have you considered reviewing a vintage restored amp every so often and contrasting it with the new ones at a similar price point?

[@dinoalden568]

I bought the Fosi DA2120C with the Nuemi Silk 4 for my mom a few months back for her TV viewing. Awesome little amp and speaker combo!! Fosi makes great stuff!

[@willbrink]

It's a great little desk top amp for my office set up, but not gonna replace my Anthem STR in the main system any time soon :)

[@avnut5517]

Green shirt. Appropriate. Great review as always! Measurements details really separate you from many other reviewers. Thank you!

[@ErinsAudioCorner]

haha. Wasn't intentional. Not for this ginger, at least. :D

[@OnwiththeAction]

Finally some reactive load testing! Excellent. This is something ASR has failed to do.

[@alexw890]

Hey Erin. Can you please review the RSL ia 2.55 mini amp? It has a switchable high pass for the mains.

[@jeffsloane8628]

Most of the low cost amplifiers don't cut the main low frequency when a sub is plugged in. This is why the Wiim amp is such a good deal.

[@paultravisano9403]

I think a great alternative would be the Arylic B50. I would love to see a reciew. It high passes the mains and has a nice app for a good price.

[@ErinsAudioCorner]

Thanks for sharing!

[@paultravisano9403]

@ErinsAudioCorner I've got the B50 SE which is a great value if you don't need the extra features. Awesome review as always 👏

[@jackdanyal4329]

there was a writing from REL Acoustic about a high pass filter where they stated, that it's fine not to have a high pass filter and just let the speakers sound with the whole range they can. Also integrated amps like Yamaha A-S301, 501, or Onkyo/Denon with the same price range don't have a High Pass filter. They just run main speakers with full range (I have a ZA3 for my desktop use and A-S301 for my living room).

[@OlaJustin]

You the best Erin! Super nice review!

[@METAL_SOLAR]

Excellent overview ! That sub pre-out conundrum *is* rather quizzical I was almost going to buy a couple of these, but that is a deal breaker ... :)

[@ValerioCongeduti]

For me HPF is a must. However it seems to me that most amplifiers with sub out do lack HPF, even more costly and acclaimed ones: Hegel, Audiolab, Camdridge Audio, Naim, Rotel, Rega, etc.

[@riccitone]

Very comprehensive, and very much needed. Reactive load is important and widely overlooked. Also not terribly hard to understand, which is strange why it’s not more focused upon. But you do have the equipment so there. This may be too time consuming or even worth messing with, but wondering how swapping opamps could/would change anything? Many seem to like using Sparkos labs discreet amps for “better” sonics - would actually be fun to try 🙏🏼👍🏼

[@NirreFirre]

❤ Great points on people "testing" electronics without level matching or making it blind. I also recommend all HiFi-interested to skip those all subjective and definitely biased videos.

[@991carreras3]

I have 3 sets of speakers and bought 2 of these za3 in December just for the mono function and there is absolutely no sound difference between stereo /mono on any of my speakers. I think all fosi is doing is just shutting off one channel this isn't true mono so theres no need to buy two. I did endup sending both back after a few weeks. I have tried many of these ti3255 amps and I'm done with all of them and I dislike those separate power supplies and they get annoying after a few hours.

[@ErinsAudioCorner]

Kinda... but there is a slight power increase in the mono channel. FWIW, I did measure the other channel in mono and nothing changed, so you have to make sure you are using the channel labeled as "mono" to get the extra power output.

[@991carreras3]

@@ErinsAudioCorner I do have everything set correctly. I was looking for a difference in sound quality and there's none that I noticed on my CSS, ELAC, or my PSB speakers.

[@ErinsAudioCorner]

@@991carreras3it may just be that the difference is so small that it might not sound louder. Otherwise, yeah, I dunno, man. Just weird stuff.

[@Douglas_Blake_579]

The problem here is one of expectations ... many people are stuck on the notion that bridging an AB amplifier does increase output power. Not so with Class D chip-amps. They are in fact, already bridged in the stereo mode. The TPA3255 chip as 4 independent power amplifiers in it. They can be combined in various ways to provide differing system topologies... 4 x SE (single ended) ... giving 4 independent outputs 2 x SE + 1 x BTL (bridge tied load)... your typical 2.1 stereo setup 2 x BTL ... two pairs of bridged SE outputs... the most common use. 1 x PBTL ... parallel connecting the 2 x BTL outputs for more current. They are *not* just turning off one channel. They are switching the chip into PBTL mode which allows it to drive loads as low as 1.5 ohms. The two stereo BTL channels are paralleled so there is no increase in speaker voltage (and thus power), it's all about the current needed to drive low impedance loads. If you're expecting some magical transformation in mono mode, you're clearly at the wrong alter. Above 4 ohm impedance it makes little or no difference at all. The only real advantage to running 2 monoblocks is that each amplifier has it's own power supply.

[@tom6363]

I just bought this Fosi amp to drive only my rear SS (7.2.4). Will arrive this week and hopefully will suffice in this limited role.

[@Artcore103]

That power supply, even though it's 48v, is likely still holding back the amp in terms of max power, perhaps it simply can't provide enough current. But I'd use it in a bi-amp setup to only power a pair of compression drivers so the power capability it has is MORE than enough for me. Would love to have it!

[@1337sim1]

Looooots of integrated amps don't have a high pass filter applied to the mains when using a subwoofer. That's why I like the WiiM Amp so much 😎

[@chinmeysway]

which one do ya mean ?

[@RAW-CAt]

Erin, is this the first public amp review/measurements you have shared? I watch your channel, but never seen you doing amps.

[@ErinsAudioCorner]

Hey, thanks for asking. This is the 3rd electronics review I've done. I just started within the past few weeks. Here are my other two. NAD C 3050: kzbin.info/www/bejne/bpi0c2Vnidp5qKs WiiM Amp: kzbin.info/www/bejne/iJStcpaMl9ZorrM

[@NemoPropaganda]

Hi bro, is high pass common in these little chifi class D units? In my experience it's been pretty rare but I'm usually reiveweing more traditional full size stereo amps. Personally I like to run my speakers full range even when using a sub so its not a big deal to me but everyone has their preference and some speakers certainly may sound better when highpassed.

[@Artcore103]

I could really use that amp and would enjoy it and be grateful!

[@rancosteel]

Criticism or not those Texas Instruments engineers are on to something with that very affordable TPA3255 audio chip. I think for a Class-D it really does the job. No back breaking heavy amps and broken bank accounts. It’s hifi for the masses.

[@literalghost929]

Great review, thanks! Very interesting to see how even other technologically driven review sites have failed to capture what should be at this point I'm guessing industry standards. Psychologically, it's interesting technically driven sites somewhat created a rift of extremes. Like on ASR, what wasn't measured didn't exist, and comments reporting hearing differences were essentially dismissed and removed. The review showed measurements, and if reports didn't match the review's measurements, they were assumed to be wrong or imagining things, and moved to a generic snake-oil thread, so removed from the product discussion thread. Basically the opposite equivalent of audiophile sites blocking terms like measurements, blind tests, etc.

[@ErinsAudioCorner]

My personal take is this: as someone who does have a bit of a following (albeit, quite small) I will always default to not talking about hearing things that I can't at least point to evidence with. In the case of amplifiers, many say they hear different things. A regular ol' dude talking to his buddies is one thing. A fellow with 100k subscribers is entirely different. I want the latter to be able to prove he heard these results. A blind test is an easy way to do this. So just offer up those results and that's that. None of this means that differences can't be a) heard and b) measured. It just means that I take a reserved stance and am cynical of these claims without some sort of backing. If someone can so easily hear differences in amplifiers, opamps, etc then it really should be easy to prove via a simple blind listening comparison. No one is doing that, though. At least not in the review space. So it's just conjecture until then. That doesn't make me a non-believer. It just makes me someone who is careful with what I say and believe. 😉

[@literalghost929]

@@ErinsAudioCorner 100%! BTs, DBTs & measurements are hard and take a lot of effort. In a perfect world, there would be no subjective opinion and non-validated claims, but ASR just went a bit too far in that direction. (No measurements = does not exist = imaginary = censored.). In that particular case, ASR failed as it actively censored individuals who were possibly correctly reporting their experience; they failed to appropriately measure amps, and censored reports of experience with said gear, if the experience didn't match the review's conclusion and measurements. In this case, there were shortcomings in measurements, and ASR effectively censored the truth by moving comments to a generic to snake-oil thread.

[@kartikk127]

Great review as always! Do you review AVRs? I would love to see a review of the Onkyo NR7100. It's the cheapest AVR with Dirac Live.

[@ErinsAudioCorner]

Not currently.

[@techsamurai11]

I’ve never heard these but they are a bit on the lush side with extreme dynamics especially in 4ohms at very high levels using the bias knob😂😂😂. Multi-tones really stand out on these.

[@JoMiTo83]

Agree with the crossover, and they should allow a user to dial where in freq to put that crossover as well. The power I think you’re just saying it’s 15-20% off at lower impedances, but less than 5% at higher impedances; however, it has enough power for most people so maybe not such a big deal. Spectrum I believe you’re saying it has variable output response of up to 0.7 dB above with a complex load that may be audible. How does this stack up against other amplifiers? I think you’ll find most class D amps have reactive load variability because that LP filter on the output of the driver reacts with the output loads where other amp types have high impedance outputs that don’t react as much. Just a hypothesis. Great measurements 👍 subbed

[@samuelferraro5601]

Thanks for the video. I was thinking of getting this amp for my bedroom system but the way the subwoofer out is implimented makes it a no go. Nobody else that reviewed this amp ever mentioned that and that issue is huge for me. Thanks

[@ErinsAudioCorner]

Glad I could help

[@59seank]

If you search for "RSL Amplifier" you will see that they have a $110.00 USD amplifier with a "Selectable 90Hz high-pass filter"

[@kadajawi2]

Awesome review, as always. Makes me wonder how you would review the Benchmark AHB2.

[@jaygannon1095]

Thanks for the review, my sincere hope is that more objective reviews like this gain enough traction and popularity that it forces manufacturers of these products to up their game. :) IMO they've gotten a free pass for far too long, and I say this as someone who watches and enjoys subjective reviews too. I just try to bear in mind what their limitations are. There's a time for entertainment and a time for education. There are many great products out there, and I'd say in a lot of ways more of them can be found for less money than ever before which I'm thankful for, but there are still some super basic expectations (like a high pass filter on a sub out!) that should be met regardless of price! I would consider this product, maybe for desktop applications or something simply because I live where a sub's not an option at present but if that changed I wouldn't even consider buying this.

[@c0mbat15]

@ErinsAudioCorner What are your thoughts on the Sparkos SS3602 Op Amps? Is it snake oil or are they genuinely reasons why it could improve perceived audio quality?

[@AndiGomez-yu2dx]

That’s a really good question, I’d be interested to see him test this.

[@wric01]

Not snake oil, best upgrade you can do to your amp.

[@c0mbat15]

@@wric01 If it really is such a good upgrade then most manufacturers would be preinstalling more expensive op-amps. The fact that they don't tells your something. The reality is that Erin blind tests amps and can't tell the difference in sound between then (when the gains are matched between the two) so how is an op-amp going to change things so materially?

[@d.branch3604]

Erin - Is it normal for one channel to play before another? I noticed with the ZA3 that at very low volumes only one channel plays. I had to turn it up to a certain volume level to have both channels play simultaneously. Maybe it's user error, but I bought two of these and it's the same for both with the 32v and 48v power supply.

[@ErinsAudioCorner]

Definitely not normal.

[@speedking92]

If the volume control in these is a cheap potentiometer it's possible there's a lot of channel imbalance at lower volumes. I've ran across this with multiple products. In this case the channel imbalance decreases as you up the volume.

[@Douglas_Blake_579]

Yes that's pretty normal at extremely low volumes. It's in the way volume control 'pots' are made. They use mechanical wipers on a strip of carbon resist material to divide the input voltage onto their outputs. If those wipers are not tracking perfectly together you can have a noticeable low level balance problem. And no this is not Fosi or even a chifi thing... I've seen it on some pretty high end stuff, over the years.

[@sandyjust]

I had this one. It's extremely loud and almost good. However I did notice dynamics were not enough and it was not responding well to details of music. I have returned

[@pliedtka]

Finally a review and measurement test they way I like it be, with reactive load and multi tone power test to check amp's real behavior. I had Canadian designed Sima amp 2x50 @8ohm which doubled it's power down to 2ohm load. But other than very capable output transistors, the power supply was four 250VA transformers and four 33,000 uF caps to supply needed current. Maybe an overkill but it would definitely deliver what was written in specs.

[@donalddeorio2237]

I bought the Fossi Audio bt20a pro for $80 with the 32v power supply driving a pair of Klipsch sb2 bookshelf speakers for the cost you can't go wrong. The speakers cost me $50 used so for $120 I have a nice desktop system.

[@rene_magritte]

I don’t get it. What’s the problem with running the speakers at full range and add the frequency they are missing by a sub. Why would you want to squeeze the speakers full potential? In stead of replacing a part of the speaker with a sub, you just add a sub for the missing low end.

[@ErinsAudioCorner]

Because they can actually work against each other depending on the rolloff profile of the mains. Aside from that, I answered the question in the video.

[@albe1796]

​@@ErinsAudioCornerSo my big 10 inch woofers would go dormant?

[@ErinsAudioCorner]

@@albe1796 not necessarily. But if you look at my reviews of many budget 10” subwoofers you’ll see they typically don’t have much extension past 40Hz. Most of them are just midbass modules. Obviously there are exceptions. And more expensive 10” subs typically use DSP to extend the bass response (but do run out of excursion at high volume).

[@cbayardelle]

Love your channel, I’m using the Fosi v3 with elac ub5 which go down to 3.7 ohms, the amp never shuts down, even with music louder then I can stand, is the v3 different than the za3 ?

[@ErinsAudioCorner]

I’m honestly not sure about the differences. I haven’t looked into it yet.

[@danielcarlsson615]

Ty again for a great video =) keep it up 👍

[@geevee1045]

Really excellent new information on determining consistent power output across all frequencies. You've used this multitone test for 3 class D amps, but only the NAD C 3050 passed the test. Why is this? Would it be that cheap class D amps have poor power supplies or what would you say is responsible for this discrepancy? Thx.

[@Douglas_Blake_579]

My take on this... The even amplitude multitone test Erin is using is very torturous for an amplifier. It's average power is way higher than single tone. Plus he's feeding in full power signals at 5khz and above. In normal use with music an amplifier would rarely if ever encounter such a signal. First there are no music fundamental tones above 4,200hz, everything above that is harmonic content that decays in amplitude quite rapidly until at 10khz there's almost nothing there. This is normal musical decay as you find in the "Fletcher Munson" body of curves. This is placing an abnormal load on the amplifier. While in average day to day use a 36 or 48 volt 5 amp supply would be adequate, this test is triggering the current limiting protection in the amplifier chip and/or the power brick resulting in artificially low power readings. The TPA3255 has current limiting logic that can be set by a control voltage on pin 7. by manipulating this pin the current limit can be set between 17.5 amps and 12.9 amps and it can be programmed to either self-restart or latch into fault mode. (This is all explained in the "TPA3255 Data Sheet" which you can download for free) A single sine wave at clipping, with a 48 volt power supply of adequate current capacity will produce about 40 peak volts on a speaker. For a 4 ohm speaker that turns into 40 / 4 == 10 amps (200 watts rms)... double that for stereo and you've got 20 amps. Which does exceed the chip's maximum safe current. Increase the average power with a multi-tone signal and you're well beyond the safety limit of the chip and it will limit the output, even at the maximum setting of pin 7. It might even overheat and go into full on protection. I would also not be at all surprised to find out with a 5 amp power supply, the power brick itself is going into an overcurrent panic and reducing voltage to protect itself. In fact, I would suggest that Erin needs to monitor the DC supply during this test. Some bricks just shut off, others will reduce the DC voltage. So... double whammy. But to be honest, I would much prefer it self-limit like this rather than upright burning itself out. There can be multiple reasons why the NAD passed this test where the TPA3255 is driven into protection ... Perhaps the NAD has a higher limit, perhaps it has no protection against over current, etc.

[@Kevin-sh1sx]

The explanation was very clear, great， brother@@Douglas_Blake_579

[@Grommet2007]

Idea for your next video - "How to design a simple and cheap conjugate filter so your speakers present a resistive load at high frequencies to cheap Class D amps" :-).

[@jeffp7038]

would love to see a review of the LOXJIE A40, or the A30. These amps (WIIM, LOXJIE, etc) are trying to take over the established go to's for 'good enough" amps/dac. I still love my Audiolab, NAD, NuPrime, ... for in depth listening, but for every day stuff, they are overkill. Thanks for the reviews, and getting under the hood, which many overlook.

[@ErinsAudioCorner]

Thanks for the feedback. I definitely have some others that I want to look into so I will take those considerations in mind.

[@chevyimp5857]

Very informative... This kind of review works for me..😊

[@ErinsAudioCorner]

Cool. Thanks!

[@Abide]

check out the TPA 3255 with PFFB(post-filter feedback) it apparently jumps the sinad WAY UP. noone is selling a finished cased product yet but these are looking very flat and quiet. they are gonna sell like hotcakes. Honestly though the geologist's sand sizing stuff seems even more interesting.

[@steakhoux]

NAD always refers to Hifi Wats when they mean measured at a certain level of distortion 20-20000Hz. I think we should call this ChiFi watts. Only accomplished under certain measurement protocols. Presumable the once that it was optimized for. Seriously, the IMD distortion at 77W looks horrendous. -20 db. Like with the Wiim amp.

[@Finite-Tuning]

@ 8:50 -- I gotta disagree. This is exactly what I want to see from any amp. Give me full range on the mains and give me sub only on the sub out, this is exactly what I want to see in the measurements. I don't want anything trying to tell me what sounds good, that is for me to decide. Just give me the signal and the power, I'll decide what to do with it from there. In my opinion, this amp is doing it's job very well in the frequency domain. On a side note, bookshelf speakers won't handle subwoofer frequencies regardless, they will naturally roll off down low or the inbuilt crossover will cut them off. Either way, bookshelf speakers just won't play deep sub levels. I just don't see a full range signal being a bad thing, quite the opposite in fact. It's clearly got issues elsewhere, but I think the freq response is right on target. Excellent review as always! Cheers man🍻

[@ErinsAudioCorner]

The purpose of a subwoofer is to alleviate the mains from distortion, bottoming out and providing more dynamic range. That’s how I’ve always used them. I have never - not once in hundreds of tunes - had the mains play full band while using a subwoofer. It just seems silly

[@Finite-Tuning]

@@ErinsAudioCorner: I get where you are coming from, but what about towers with 10" - 15" woofers? You'd want nothing less than full range regardless of subwoofer activity. That's how I'm looking at it anyway. They don't know what you are going to drive with the amp. My home receiver gives me full range on all channels, unless I manually set a crossover and that is how it should be in my opinion. I don't want an amp to decide for me, I just want the power and the full range signal and I'll decide what to do with it from there. Just my nickel worth. Cheers man 🍻

[@kellybest2357]

Erin, so you always recommend using a high pass filter if available on the amp? Even for full range speakers like a Linton with a powerful (100+ watt a/b) amp?

[@ErinsAudioCorner]

If you’re using a subwoofer then you are almost certainly using it to cut the mains from playing too low. This decreases distortion and increases dynamic range. I’ve been able to show this in my speaker testing.

[@bbfoto7248]

@kellybest2357 Not Erin, but if you are already wanting to add a subwoofer to the system in order to improve the low frequency extension and dynamics of your system, in 90% of circumstances, applying a High Pass Filter to the main speakers will lower their distortion, improve dynamic range/reduce power compression, and allow the main speakers to play a bit louder when asked to without experiencing distortion or unpleasant sound. Pay attention to Erin's "Compression/Dynamic Range" graphs combined with the Distortion graphs in his speaker reviews. In addition, to address some of the other comments here, I have rarely, if ever, been able to properly integrate the main speakers to subwoofers in regards to ideal phase alignment/phase coherency if/when the main speakers are still playing full range w/o a HPF so that they are "overlapped" with the subwoofer(s). Ideal phase alignment between the main speakers and the subwoofer(s) is required in order to achieve that "up front bass" experience where ALL of the low frequencies from the subwoofer(s) appear to be locked up front to the main speakers and the bass will actually image properly within the soundstage (if it is in the recording....play the intro to Malia & Boris Blank's "Celestial Echo" from the Convergence album). Sure, at particular frequencies, you will have some constructive interference where both the subwoofer & main speaker sum to provide a boosted response. But it is very unlikely for that constructive interference to be both Phase AND Time aligned. So while that scenario might provide MORE BASS at a certain frequency, it will not be "articulate/detailed/clean/fast" bass, and will not provide that perfect "up front bass" experience. This will be most noticeable with instruments such as an acoustic upright double bass which has lower fundamentals combined with lots of upper harmonics and transients, or fast "slap bass" on an electric bass. So the phase alignment through the crossover region and beyond will rarely ever be ideal between the subwoofer and main speakers when both are "overlapped". As a starting point to achieve the easiest integration between the subwoofer and the main speakers, and as a most-likely scenario example, I would set the main speaker's HPF to 80Hz and the Subwoofer's LPF a half-octave lower to 60Hz, and use 24dB/Octave 4th Order Linkwitz-Riley (LR4) filters for both. While this is not a "set in stone" configuration that is guaranteed work perfectly every time, it is the most likely configuration to get the response and integration of both the subwoofer and the main speakers at least close or "in the ballpark" more so than others, especially if your subwoofer has a "Variable Phase" control knob rather than just a 0°/180° phase swithch or "normal/reverse" polarity switch. What really matters is the actual Acoustic Crossover response of the system and the Phase Alignment between the mains and subwoofer(s) within the crossover region. And to achieve that, your crossover network filters (either passive or active) may need to be asymmetrical with completely different center frequencies and orders/slopes for the HPFs & LPFs that are applied to each driver.

[@DanielNilssonSe]

I'd argue that there are some very good reasons to use sub(s), even if not highpassing the main speakers 1) bass output capabilities (in terms of extension, general frequency response - maybe a house curve?, and volume, and having headroom enough to allow for EQ to attain this), 2) the ability to choose main speakers based on your perception of them over, say, 100Hz, while "fixing"

[@fungt89]

The definitive review

[@D1N02]

You can build a high pass pretty easily. Implementing it in such a basic amp seems hard. ideally it would need to be variable, but that would mean extra electronics and cost. Fosi sells GaNfet powersupplies. Those may be able to take the amp to higher levels.