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Fosi Audio BT20A Pro Amplifier Review

Rate this stereo amplifier

  • 1. Poor (headless panther)

    Votes: 4 1.3%
  • 2. Not terrible (postman panther)

    Votes: 35 11.0%
  • 3. Fine (happy panther)

    Votes: 188 58.9%
  • 4. Great (golfing panther)

    Votes: 92 28.8%

  • Total voters
    319
OK, I'm in my office now...but already I gather that's not how it works. So still run the signal through a traditional amp, and then through this baby on the way to the speakers? Or vice-versa, from my music source through this and into the amp (if my amp can't hack the speaker load on its own)? Anyway, thanks for quick response. I'm going to catch a train, but I hope to hear (in brief) what I'm missing...
How the amp setup is requires more details. Is it a integrated amp or a power block/amp? Does it have a volume control on it? Best is to provide the model number for best results.
 
Thank you Doodski, you are very generous. Its an RX-V475. And I have a 2nd hand rx-V777 in the mail. Both are standard AVRs. In my office, I have a Marantz PM6007. They are all regular integrated amps with volume knobs and the works. I just ordered a Polk ES20 set for my office, and I already have a pair of Mirage OMD-5 speakers and some tiny Nanosats that I use in my office. I am hoping to have 3 pairs hooked up which I can switch between depending on how empty the building is. I was worried that the marantz might not be able to handle two sets of speakers at the same time (it's seems pretty wimpy and my office is big).
 
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Doodski, I'm looking at the Fosi company instructions, and I feel that my initial impession was correct. I shouldn't need any other amp/device. I have an SMSL DAC at home and a Topping in my office. It looks like I should be able to go directly from DAC to FOSI to speakers. Of course, for my hope/dream of having 3 pairs of speakers in my office, this probably will not be helpful for that, but I can easily set up a dual system.
 
Not exactly. What model of Yamaha amp do you have?
Doodski, perhaps you are getting ready to suggest that I could use the Fosi in addition to my main amp, to power an extra "problematic" set of speakers? Would the rear of the Amp need a "pre-out" to do this? (I'm just guessing, I'm not sure what a "pre-out" means, but from the thread I have the impression the Fosi can be used this way...)
 
Doodski, perhaps you are getting ready to suggest that I could use the Fosi in addition to my main amp, to power an extra "problematic" set of speakers? Would the rear of the Amp need a "pre-out" to do this? (I'm just guessing, I'm not sure what a "pre-out" means, but from the thread I have the impression the Fosi can be used this way...)
OK. I am awake now. Sorry I never answered sooner but it was late at night and I had gone to sleep.
A pre-out is a variable low signal strength audio output that is designed for powering a outboard power amp. Like the Fosi if you want that.
The RX-475 does not have a pre-out for main speakers. It does have a subwoofer out which is unrelated to what you want to do here.
RX-475.png

The RX-V777 has front pre-out as well as at the speaker terminals there is the speaker output ZONE 2 and that might be useful. You will need to consult your operators manual and see just how that works and if you can operate a second pair of speakers on that ZONE 2 speaker output.
RX-V777.png

The Marantz PM6007 does not have pre-out but i9t does have a subwoofer pre-out which is not useful at this time for your needs and 2x 45W (8 ohms RMS, 20Hz – 20kHz). That should be enough for a single pair of small speakers. Most of the volume of sound comes from about zero watts output to about 50W/ch or so and after that it is not as substantially noticed for your type of application. So you can run this amp if you like that.
Marantz PM6007.png

The Fosi Audio BT20a has pre-out so you could use that to drive a second amp/receiver too.
Fosi pre-out.png
 
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Thanks, Doodski:p You went all out. And don't apologize for going to bed! I couldn't figure out how you were even up at that time (I'm in Japan, so we are on opposite schedules...).
 
Well... I got it and hooked it up. Still sampling, but it's pretty impressive. It does NOT crank the way I thought it would from the specs. It gets plenty loud, but not near as loud as my 12 year old mid-to-low-level Yamaha. There is no way for me to quickly A/B with another amp, but fidelity seems very good. In fact, my impression is that drum beats and snares are punchier than my other set-up. Its being literally 20X smaller than my regular amp just blows my mind:oops:
 
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Wake me up when they do a 250W/ 8 Ohm at 20HZ with no HF peaking for $100.00. Until that I'll keep zzzzzz, :p
 
Wake me up when they do a 250W/ 8 Ohm at 20HZ with no HF peaking for $100.00. Until that I'll keep zzzzzz, :p
So you're asking for a very cheap 55V (more realistically, 60V) power supply @ 800-1000 Watt output (for decent stereo 4 Ohm performance). Plus a new power driver from Texas and some cheap 63V big electrolytics. You 'll have to wait a bit I guess...
 
OK. It's not the whiskey. These are definitely upping my audio game. Unfortunately, that just ups the ante. I know these aren't meant to be "hi-fi" --just remarkably good for their size and price. If these are giving me this kind of new insight into my favorite tracks, what next? I don't want to devolve into an "audiofool," but presumably an amp a few stages better than my ancient 475 would leap-frog this little gizmo... Damn, this is a frustrating hobby:oops:
 
A garage setup here: I didn't have time to read ALL 30 pages, but wanted to share how I'm using the amp in 2025. I skipped the bluetooth on the Fosi and connected a Bludento Bleutooth Receiver that uses the PCM5102A chip. Using the Bludento, I feel as if the gain increased by 35%, and clarity increased by 25%. Also the bluetooth range is wild. I can be outside the garage about 75ft and my iphone is still connected. I have the volume knob at around 25% on the Fosi, and the volume on my iphone set to 35% and it makes my CSS 1TDX's SING.

For comparison, I used to have a Cambridge Audio EVO 150 integrated (150w x 2) in the garage and decided to move it in the house for ARC usage. I can't say there is much of noticeable, real world, difference between those two. I used Airplay 2/Tidal through the Cambridge, however when comparing side to side with the Fosi, that bludento is really impressive. Hopefully the Fosi lasts! If you are interested in this setup, I'd recommend a USB on/off switch for the bludento otherwise it will stay ON. Or you could use a Smart Strip to trigger on the bludento when you turn on the Fosi.

Bludento

bludento.jpg


fosi 2.jpg
 
Hi,

Fosi does not mentioned below specs in BT20A Pro amp. But Ayima does.

1)BASS boost in dB ( In tone controls)...!
2)Treble Boost in dB ( In tone controls)...!
3)Frequency response graph...!
4)RCA input sensitivity....!

Does anyone knows above specs?

also
How is the BASS performance compared to Ayima A07 Pro...? I am Bass lover.Does it worth to change the amp?
 
Well... I got it and hooked it up. Still sampling, but it's pretty impressive. It does NOT crank the way I thought it would from the specs. It gets plenty loud, but not near as loud as my 12 year old mid-to-low-level Yamaha. There is no way for me to quickly A/B with another amp, but fidelity seems very good. In fact, my impression is that drum beats and snares are punchier than my other set-up. Its being literally 20X smaller than my regular amp just blows my mind:oops:
This is my impression as well. I have been wanting to try one of these for some time now. And since I already gifted my friend a small system for his TV downstairs (it includes a portable CD player as well), I decided to swap this mini amp for the mid 80s Technics SU-V75. Very good and clean sound but not quite the volume level. I need to take a dB meter over there and see what we are actually getting. He has it hooked up to a pair of speakers that I put together from scratch, a two way 8" set. That combination seems to work quite well.

The setup works well. He can sit in the next room with his computer and send music by Bluetooth. The two rooms are separated by a large archway so the sound travels quite easily.

What is the next level up? I feel like experimenting now...
 
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 have a T20 on the way. This is useful info if the same is true on BT.
 
hi, today I received the bt20a pro. I noticed that at very low volume only the left speaker sounds, then when I turn it up the right one starts to sound too until when I turn it up a little more the sound balances out. Does this happen to anyone else? In your opinion, is my specimen like this (I could ask for a return and get another one) or are they all like this? Thank you
 
this happen to anyone else?
Yes, it's completely normal for Potentiometer based volume controls to behave in this way.

At max volume they have perfect channel balance and as you turn down the volume, left and right channels start drifting apart in volume.

The best way to avoid this is to turn down your DAC and turn up the Amp to compensate.

If you want to completely avoid this issue in the future, look for either digital attenuation or stepped, resistor ladder attenuation.
 
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