Fosi Audio BT20A Pro Amplifier Review

[GrayA]

This is such a dealbreaker for me. I live in a student complex, and it would be just a matter of time before someone would start messing with me.

Can people connect even if you disconnect the Antenna?

 

[Jimi Floyd]

Can people connect even if you disconnect the Antenna?

Without antenna it connects to devices in the same room, the next room is out of range.

 

[Joe Smith]

This is one nice thing about the SMSL AO100 amp, it allows you to set/switch source and the BT does not automatically override the RCA input. It remembers which input was the last you connected to so does not need resetting each time you turn it on.

 

[sound67]

I got the 48V power supply from Fosi, together with my complimentary bronze knob.

Maybe I'm naïve, but I expected a considerable increase in power compared to the 32V ps. That didn't happen. With my Q Acoustics 3030i, with the 32V I had to dial up the volume maybe 10% more than with the 48V ps now.

 

[asnow]

I received yesterday my Fosi BT20A Pro with the 32V power supply and I have it connected to a pair of Klipsch Cornwall speakers. It is amusing to hear this very small and very light amplifier push two giant speakers with their 15" woofers to loud sound levels.

View attachment 280067

Anyway, using the RCA input or the Bluetooth input does not give the same sound quality, the Bluetooth being brighter. In wireless mode I am comfortable dialing 60° counterclockwise the treble knob. Out of curiosity I decided to measure the frequency response of both input modes. I don't have refined lab equipment. As a signal generator I used a MacBook pro running Audacity, set to produce sine tones, 31 in total, spaced by 1/3 octave. The computer was connected to the amp 1) through a Topping D10s DAC and RCA cables to the RCA input, and then 2) via Bluetooth. The amp was connected to the speakers and the output voltage of the right channel was measured by a multimeter for each frequency. 0dB was set at 2V amplifier output (1 kHz).

RCA input

View attachment 280070

More or less flat, with a rise in the treble reaching +2dB at 20 kHz. My ears arrive to 12-13 kHz, so I hear a +1dB rise at most, that is perfectly tolerable. Amir measured a +0.5 dB rise at 20 kHz with a 8 ohm load. I don't know the high frequency impedance of the Klipsch Cornwall, It may be higher than 8 ohm or have a phase different from 0, that would explain the difference.

Bluetooth input

View attachment 280071

I didn't expect this result. Bass frequencies are rolled off below 63 Hz, then there is a minimum at 315 Hz, then a rise up to almost +3dB at 10 kHz followed by a drop at 20 kHz. Now I understand why the sound feels light and bright with Bluetooth. I googled "Bluetooth audio frequency response" but I could not find any single plot, so that I don't know if all Bluetooth devices have this problem or it is codec related or it is just my amp. I hope somebody here can help me to better understand what is going on.

I was able to verify the bluetooth on my unit has basically same frequency response into an 8 ohm load as @Jimi Floyd reported, the RCA input is flat as @amirm's tested. I used an inexpensive bluetooth device like this one into the RCA input and the response was flat.

This devalues the device quite a bit in my book and it would be great if @amirm could test the bluetooth input.

And I was hoping @Fosi Audio could comment on this.

 

[neRok]

I was able to verify the bluetooth on my unit has basically same frequency response into an 8 ohm load as @Jimi Floyd reported, the RCA input is flat as @amirm's tested. I used an inexpensive bluetooth device like this one into the RCA input and the response was flat.

This devalues the device quite a bit in my book and it would be great if @amirm could test the bluetooth input.

And I was hoping @Fosi Audio could comment on this.

This might be a symptom of the (presumably) lossy bluetooth audio transmission codec?

This is such a dealbreaker for me. I live in a student complex, and it would be just a matter of time before someone would start messing with me.

Yer but that's all part of the adventure!

 

[trivium]

I got the 48V power supply from Fosi, together with my complimentary bronze knob.

Maybe I'm naïve, but I expected a considerable increase in power compared to the 32V ps. That didn't happen. With my Q Acoustics 3030i, with the 32V I had to dial up the volume maybe 10% more than with the 48V ps now.

Everyone thinks wattage is the best, when the reality is the difference in sound is marginal. If you want loud speakers wattage is not the answer, unless you plan to throw thousands of watts at it. Your speakers have a sensitivity of 88, 50 Watts will get them to 105DB, 100 watts gets them to 108DB. Both ear deafening levels, though my point is a change of 3DB is basically perceived as the smallest increase in loudness that someone can detect. The biggest advantage I would say for the extra wattage is headroom for bass so that it doesn't clip.

 

[Svenimal]

Without antenna it connects to devices in the same room, the next room is out of range.

Without the antenna I can still connect to it from the adjacent rooms of the apartment, so it depends on how thick your walls are I guess

 

[Jimi Floyd]

I was able to verify the bluetooth on my unit has basically same frequency response into an 8 ohm load as @Jimi Floyd reported, the RCA input is flat as @amirm's tested. I used an inexpensive bluetooth device like this one into the RCA input and the response was flat.

This devalues the device quite a bit in my book and it would be great if @amirm could test the bluetooth input.

And I was hoping @Fosi Audio could comment on this.

Yes, I tested 3 ways to transmit Bluetooth audio to the Fosi BT20A Pro.
1) Direct Bluetooth to the Fosi

This is the frequency response 1):

2) Bluetooth to a cheap Aukey receiver/DAC connected to the Fosi RCA in

This is the frequency response 2):

3) Bluetooth to a S.M.S.L. DO100 DAC connected to the Fosi RCA in

This is the frequency response 3):

Differences are NOT due to different codecs because the transmitter was an Apple MacBook pro M1 which uses only the AAC codec.
The rise in the treble is due to the load (a Cornwall Loudspeaker) impedance. Apart from that the Fosi BT20A Pro produces a perfectly flat frequency response when fed through RCA by the S.M.S.L. DO100. The amp shows some 2dB bass drop when fed by a cheap bluetooth receiver/DAC. What is worrying me most, just like @asnow , is the bluetooth frequency response of the Fosi itself with its several dBs of deviation from linearity. Now I know it's not just my sample and I would like to ask @Fosi Audio if some improvement to their BT20A Pro is due sooner or later. I might like to buy a bluetooth-linear improved unit, hopefully with the mythical orange volume knob.
Currently, the need for an external DAC/receiver nullify the convenience of a small self-contained amplifier with Bluetooth connection.

 

[robertospeed]

Fosi Audio BT20A Pro with power supply 48V 7.3A

 

[BeQuietZen]

Andrew's view on cheap 'small' amps. Fosi BT20A Pro not that bad at the and, after all.

 

[DavidM1]

Andrew's view on cheap 'small' amps. Fosi BT20A Pro not that bad at the and, after all.

Perhaps it’s a cultural difference but these two people are quite irritating. I felt like I’d met a guy who‘d just come out of a “trust me, I’m a salesman” course. It did sharpen my appetite for hard data and blunt engineering talk.

 

[Joe Smith]

Yes, I tested 3 ways to transmit Bluetooth audio to the Fosi BT20A Pro.
1) Direct Bluetooth to the Fosi
View attachment 283485
This is the frequency response 1):
View attachment 283486
2) Bluetooth to a cheap Aukey receiver/DAC connected to the Fosi RCA in
View attachment 283487
This is the frequency response 2):
View attachment 283488
3) Bluetooth to a S.M.S.L. DO100 DAC connected to the Fosi RCA in
View attachment 283489
This is the frequency response 3):
View attachment 283490
Differences are NOT due to different codecs because the transmitter was an Apple MacBook pro M1 which uses only the AAC codec.
The rise in the treble is due to the load (a Cornwall Loudspeaker) impedance. Apart from that the Fosi BT20A Pro produces a perfectly flat frequency response when fed through RCA by the S.M.S.L. DO100. The amp shows some 2dB bass drop when fed by a cheap bluetooth receiver/DAC. What is worrying me most, just like @asnow , is the bluetooth frequency response of the Fosi itself with its several dBs of deviation from linearity. Now I know it's not just my sample and I would like to ask @Fosi Audio if some improvement to their BT20A Pro is due sooner or later. I might like to buy a bluetooth-linear improved unit, hopefully with the mythical orange volume knob.
Currently, the need for an external DAC/receiver nullify the convenience of a small self-contained amplifier with Bluetooth connection.

It would be interesting to test some of the other integrated BT amps and see if your first curve is equally bad/similar for all of them...cause, yeah, that's pretty poor. I mostly use external BT receivers but have the SMSL AO100 with the BT built in.

 

[renaudrenaud]

Andrew's view on cheap 'small' amps. Fosi BT20A Pro not that bad at the and, after all.

Sounds the usual bs to me. Spend more, it's better.

I got the FOSI for 57€ and there is a TPA3255 inside. I use the Ti Chips since the TPA3116, bought plenty of cards, use the EVMTPA3255 from Ti, bought the 3eAudio cards etc. I can buy another TPA3116 or TPA3255 card, buy a power unit, build a box etc... At the end it will be more expansive than the FOSI ready to use, well designed, no brainier amp.

I use the FOSI with a 98dB sensistivity system and there is more power than I need. A raspberry Pi, a little DAC and at the end I have an ultra defined utlra impressive ultra powerfull 2 way system for less than 500€: this is the real cost. And the measurements from @amirm show the Fosi in on par with a lot more expansive things (not a surprise because the TPA3255 is a very competent chip, the question is just implementation).

 

[MidnightCarouselRider]

Andrew's view on cheap 'small' amps. Fosi BT20A Pro not that bad at the and, after all.

This dude is full of it. He's just doing damage control because people are waking up to audiophile BS thanks to amps like these and this forum.

 

[SaltyCDogg]

It does run hotter than I'd hoped. Switched on but not playing it gets pretty warm, which makes me wonder what the idle power usage is. My S.M.S.L SA300 doesn't get warm and I'm happy leaving it on 24/7, I wonder if the Infineon MA12070 just runs cooler.

 

[Rick Sykora]

As it is not just about Fosi, there is a whole thread on the Robinson Youtube here:

Cheap class D amps. Andrew Robinsons take.

Has anyone else watched this video on cheap class D amps ? Made me think twice about purchasing one.

www.audiosciencereview.com

 

[Svenimal]

It does run hotter than I'd hoped. Switched on but not playing it gets pretty warm, which makes me wonder what the idle power usage is. My S.M.S.L SA300 doesn't get warm and I'm happy leaving it on 24/7, I wonder if the Infineon MA12070 just runs cooler.

For me it gets a bit lukewarm to the touch when in idle. Why would you want to leave an amplifier switched on 24/7 tho? It just seems like a waste of power?

 

[Maiu95]

I have a pair of elac debut reference. I've tried various class AB around $400 but I prefer the TPA 3255. Unfortunately I find the mid-high frequencies a bit sharp (like drum cymbals in the new Metallica's album, or some electric guitar). With the a07 the problem is more evident, especially with LM4562 (I like more OPA2134 which is more "dark"). With the fosi BT20A Pro I can fix it by keeping the highs at 10 o'clock but never swapped op amps. I have a 36v Mean Well but I was thinking of going down to 24v to get sweeter highs, does that make sense? I listen to low volume music in a small room.

 

[Rick Sykora]

Anyone know if it sounds better than the Aiyama A08Pro and this it's worth the op amp upgrades from Sparkos?

Andrew Robinson sort of reviews it here:

Welcome to ASR!

You are about 10 posts too late on the Robinson video. There is now also a separate thread and may have some upgrade stuff there. The management here likes to keep review threads focused on the review, so best to try this query on the video thread here:

Cheap class D amps. Andrew Robinsons take.

Has anyone else watched this video on cheap class D amps ? Made me think twice about purchasing one.

www.audiosciencereview.com