Fosi Audio BT20A Pro Amplifier Review

[tomtoo]

We are entering the era of cheap amplifiers. Sometimes it’s very difficult to me to accept this kind of reality.
Waiting for the next era of cheap loudspeakers which is also small sized but big sound.

Nothing in view.

 

[sound67]

Just got my Fosi BT 20 A pro from Amazon, but I'm a little surprised:

Mine doesn't have the bronze-colored volume knob which is visible in all the ads, videos, and such. Everything else is as advertised.

 

[JktHifi]

Nothing in view.

Just wait. Probably they’re still busy with DAC, Amp, Headphone amp, etc.

 

[Talisman]

Just got my Fosi BT 20 A pro from Amazon, but I'm a little surprised:

Mine doesn't have the bronze-colored volume knob which is visible in all the ads, videos, and such. Everything else is as advertised.

View attachment 278979

I don't like this thing at all, mine just arrived today but I still have to unwrap it, one of the reasons I ordered it and that I really liked that orange knob

 

[asnow]

Has anyone received the 48 volt PS version? I was wondering about any safety rating on the PS. Is it something we could trust to be plugged in 24/7?

 

[Talisman]

I don't like this thing at all, mine just arrived today but I still have to unwrap it, one of the reasons I ordered it and that I really liked that orange knob

Very disappointed, my unit also only features the gray knob, despite the very distinctive nice bronzed knob being shown in all the pictures.
I almost want to return it, it bothers me a lot.
@Fosi Audio can you explain why it is advertised everywhere with the orange knob and then not supplied?

 

[Deleted member 48726]

You ordered it because of an orange volume knob??

Is this the real life?
Is this just fantasy?

 

[JktHifi]

You ordered it because of an orange volume knob??

Is this the real life?
Is this just fantasy?

I think our friends here are talking about CONFORMITY TO THE BROCHURE.

 

[stunta]

The 32 volt version is currently on sale at Amazon - $20 off. Code: 20APROMKT20

 

[TonyJZX]

well yeah

i mean if this is what's stated in the BOM then why doesnt it have in the box?

 

[kemmler3D]

small sized but big sound.

This one is harder, because voice coil-based speakers have inherent distortion mechanisms that are difficult / impossible to overcome. The smaller the speaker cone, the more IMD (doppler) distortion you get from low frequencies. Also, you get some unavoidable surround distortion if you somehow manage to build a speaker with extremely high excursion. Even if the magnets are perfect beyond reason, there is only so much distortion-free sound you can get out of a 3" cone, for example.

In theory there is no reason you can't have a tiny 1KW amp, but a tiny (say 3") 100dB SPL @20hz speaker cone is physically implausible. You just can't move the cone that much without creating a lot of other problems.

 

[JktHifi]

This one is harder, because voice coil-based speakers have inherent distortion mechanisms that are difficult / impossible to overcome. The smaller the speaker cone, the more IMD (doppler) distortion you get from low frequencies. Also, you get some unavoidable surround distortion if you somehow manage to build a speaker with extremely high excursion. Even if the magnets are perfect beyond reason, there is only so much distortion-free sound you can get out of a 3" cone, for example.

In theory there is no reason you can't have a tiny 1KW amp, but a tiny (say 3") 100dB SPL @20hz speaker cone is physically implausible. You just can't move the cone that much without creating a lot of other problems.

How about if the speaker flexible in size? It become bigger when it needs punchier bass. And back to reduce its size if no audio signal. Similar to design of the new NASA UAV. Probably the material can come from polymer.

 

[kemmler3D]

How about if the speaker flexible in size? It become bigger when it needs punchier bass.

Well, the issue is if you want a small speaker, you need a small device to produce both low and high frequencies. When you try to reproduce loud, low frequencies at the same time as higher frequencies (which always happens with music) then you get distortion.

There is almost such a thing as a "speaker that changes size" though, they are called coaxial drivers. The tweeter is in the middle of the woofer/mid cone. You can see this on some KEF and Kali speakers, for example. These have their own trade-offs, though.

A cone that literally expands and contracts depending on frequency would have to do so many times per second... I am not sure what that would be like!

 

[tomtoo]

How about if the speaker flexible in size? It become bigger when it needs punchier bass. And back to reduce its size if no audio signal. Similar to design of the new NASA UAV. Probably the material can come from polymer.

Looks like speakers are a much harder physical problem than amps. Maybe some 50 years more? Or maybe you get the power for your amp from a fusion reactor before small speaker can do what big speakers can do? Who knows?

 

[Jimi Floyd]

Why the peak power is less than the max power with the 32V PS? Does anybody know?

 

[mhardy6647]

@amirm You say 69 watts but X=98.84? Is this a mistake? I just bought this little amp with some 8 ohm outdoor speakers (B&W AM-1), so 99 or 69 makes a big difference.

1.6 dB (rounded up).
uffda.

 

[mhardy6647]

I think our friends here are talking about CONFORMITY TO THE BROCHURE.

At least back in the good ol' days, the brochure usually featured a disclaimer:
Specifications subject to change without notice.

In fairness, in the hazy days of the (early) '70s, the Zeitgeist, allegedly aided and abetted by a bit of recreational pharmacology, meant at least that many of the small loudspeaker manufacturers that sprouted like mushrooms on a cool, wet morning in those days rarely made any two units exactly the same way for the first few years of their existences. NO ISO-conformity in those times!

Now, I am not suggesting that, e.g., Matthew and his partners were puffin' the magic dragon when they were on the clock -- but their factory staff in the early days just may have been...

 

[Talisman]

@amirm I can't get it right, with the 48V power supply the knee seems to be around -80db which corresponds to 85watt, while your marked point is at around 100watt, so where does the 69watt value come from?

 

[amirm]

That's a typo I need to fix.

 

[amirm]

Why the peak power is less than the max power with the 32V PS? Does anybody know?

That is because of small variability in that measurement.