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Fosi Audio BT20A Pro Amplifier Review

Rate this stereo amplifier

  • 1. Poor (headless panther)

    Votes: 3 1.0%
  • 2. Not terrible (postman panther)

    Votes: 33 10.8%
  • 3. Fine (happy panther)

    Votes: 180 59.0%
  • 4. Great (golfing panther)

    Votes: 89 29.2%

  • Total voters
    305

rj2wells

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Jan 22, 2024
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Any recommendations?

I am using this one


Because it shuts all power to the Fosi off when the trigger fires it takes about 5 seconds to power on and carry the sound. That prevents using it for things where you want immediate audio (ie, if you connected a voice assistant it won't turn on fast enough to carry the first reply)

However for things like music, podcasts, etc, I prefer the 5s delay over the amp running up my electricity bill
 
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rj2wells

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Jan 22, 2024
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I'll speculate he's talking about power strips designed to sense if a PC is on or off (by AC current draw), and turn other outlets on the same strip on or off with the PC. Might work if the upstream equipment draws enough power. There are solutions to rig something sensing audio input on/off, like the Radio Labs ST-ACR2, but that's more than the amp itself, and needs more stuff to make it work for this.

You mentioned ATMOS somewhere, is there a trigger on the receiver/pre amp? That could trip a simple relay switching the power supply brick on and off.

The one I linked has a relay controlled by a 12v trigger. I stream from a wiim which has such a trigger out, with the wiim you need a 2.5mm plug and then connect the other end to the terminal on the side of the power bar.

It works but as mentioned when it turns on you need a few seconds before you will get sound.
 

nathan

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May 24, 2020
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I think my solution so far is one of those "computer power strips" where when my main amp/processor goes on, it makes all the other outlets power on -- including the Fosi. I need that anyway for the turntable and phono pre-amp, both of which have obnoxious power bricks that constantly draw power too.

But this conversation does get me wishing the Fosi included a 12v trigger sensor. Of course every little extra idea costs extra money and adds complexity, a chance for degradation of sound and or lower reliability, so it's a balancing act. For my use case, it doesn't matter much, but I can see it being a factor in other situations.
 
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bobebaugh

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Feb 1, 2024
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The power strip linked to by @rj2wells is a slick solution for the Wiim, the same thing I have feeding my amp. My less elegant solution, the Wiim and the audio amps AC power is connected to the light switch, so if I'm not in the garage and I didn't forget to turn off the lights, both are shut off.

What I don't get is why these need that much power at idle anyway? At normal listening levels, a level where you can have a conversation without shouting, the DC power draw on these amps doesn't budge over idle power requirements.
 

carlo

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Feb 13, 2020
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Hello, just thinking about getting one of these, I have some questions...
1) Leaning towards 48v powering, just wondering if heating would be an issue
2) Bluetooth. It seems that there is a priority of BT over RCA, unless you press and hold the knob and unpair it. Question is: if I leave something not paired through RCA (i.e. a cd player or a TV) AND other gear paired ( i.e. iPad to phone), will it be possible to listen to the unpaired source without issues or the need of switching BT to off?
 

bigpilot

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Feb 14, 2024
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I personally find it difficult to justify morally designing and building a wasteful Class-A or even Class-B (or AB) amplifier when these Class-D amps are about the size of a pack of cigarettes and pump out 2x50W or even 2x100W without breaking a sweat and heating up.

But one question hasn't been answered in this review: "How does it sound?!"
 

rj2wells

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Jan 22, 2024
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The power strip linked to by @rj2wells is a slick solution for the Wiim, the same thing I have feeding my amp. My less elegant solution, the Wiim and the audio amps AC power is connected to the light switch, so if I'm not in the garage and I didn't forget to turn off the lights, both are shut off.

What I don't get is why these need that much power at idle anyway? At normal listening levels, a level where you can have a conversation without shouting, the DC power draw on these amps doesn't budge over idle power requirements.
It seems to me heat somehow builds up when it's idle. After playing for an hour or two it's barely warm whereas it got super hot sitting idle before I got the trigger controlled strip.
 

Talisman

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I personally find it difficult to justify morally designing and building a wasteful Class-A or even Class-B (or AB) amplifier when these Class-D amps are about the size of a pack of cigarettes and pump out 2x50W or even 2x100W without breaking a sweat and heating up.

But one question hasn't been answered in this review: "How does it sound?!"
It doesn't "sound" , it's not a violin, it amplifies an electrical signal, if you want to know how it does it just look at the measurements you'll find at the beginning of this review, you'll have all the answers to your questions.
 

Guddu

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But one question hasn't been answered in this review: "How does it sound?!"
You are right, but the review provides useful measurements to tell how well this amplifier does his job.
Sound or output is resulting factor.
 

John in CH

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Oct 27, 2023
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Since I posted, I bought a Fosi TB10D. Same chip, different implementation. Or perhaps we are comparing knock-off chips to TI. I don't know.

There are significantly different parasitic (no load) current draws. One implementation takes 4W DC power in at 48V. The other example takes 8W DC power in at 48V. This is with no input audio. More or less, reducing the voltage to 24V on both lead to 2W vs 4W. The "parasitic" current measured in mA is more or less stable, but that's less power to dissipate. My conclusion, if you want to save energy (and reduce component temps), use the lowest voltage power supply sufficient to power your speakers to desired volume levels.

I would like to test further, but lack silent loads for the amplifier output. The PA speakers I use are rated 91 db at 1 watt.

Interesting observation. I'm powering a pair of Martin Logan electrostats, which are terribly inefficient. I began my amplifier journey with a Macintosh MC2200. Very nice piece of kit but couldn't drive the ML's loud enough (could still speak to the person next to me at max volume). Then switched to a $$$$ Purifi-based amp with Hypex power supplies (also nice piece of kit) - - plenty of volume. But the sound was antiseptic and cold. Almost shrill. I already had a Fosi BT20a Pro in the basement hobby room and decided to swap it in. Just for fun, really, as I wasn't expecting much. But lo and behold, it sounded way better than both the Mac and the Purifi. I proceeded to buy two more of BT20a Pro's (both with 48v power supplies) and have been quite pleased for the last 1/2 year or so. Given the high idle temps at 48v mentioned in my OP, I ended up running them mono - - one channel per amp per speaker which solved the idle temp problem. At 32v max volume simply isn't enough.

I'd like to bi-amp the ML's at some point but am hesitating to do so because of the high idle temps. And frankly too busy lately to investigate further.

Currently shuffling my hobby room and in the midst of unpacking boxes. Once I dig out my ammeter, load cells, and other gear I will do some more measurements. Try to figure out the root cause for the high idle temps.

Rgds... - John
 

Zimmerframe

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Jan 22, 2024
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France
Hi All,

Great Forum with so much good information and many many many interesting members !

Some questions and observations.

I'm no audiophile and only recently re-discovering things, so most of what I say is subjective.

Someone was asking about the sub out being active or not with the amp off. I was messing with the speaker connections the other day with the amp turned off and also unplugged at one point. On both occasions the sub activated so the "circuit" is open.

Wasn't sure if I should go with a V3/Topping PA5 II and a pre amp for bluetooth or this, in the end went for this as I wasn't sure I'd be able to hear the difference and keeping things simple/reducing boxes and connections seemed sensible ?

Initially went with banana plugs due to numerous posts about that being the way to go. However, each time I touched or moved the amp you could hear variations in the sound so recently swapped to post mounts on the amp, though have left the speakers on banana's. Sound is much more consistent and with improved clarity - though this possibly just points to the lack of quality with the banana plugs ? Though in my head, connecting a wire straight to something, must be better then connecting it to a plug then plugging the plug into a socket ? Just using cheap 2.5mm cable which only just goes in the post mounts.

Speakers, decided to go away from the recommendations (Dali's/Polk's/Elac's) just to try something different and bought the Jamo S7-17B's... thinking they'd have the size to fill the room (6m high ceiling). They actually seem overly bass orientated - or the amp is ? or my amp is .. Presently have the treble turned up and bass down.

Despite being bass orientated, some tracks still felt like they weren't fully presenting the bass, so added a C912 Sub (it was the same colour...) and spent several days fumbling with cross over and volume. Cross over is now almost down at the base 40 and volume is incredibly sensitive on what feels correct.

Have tried the speakers in various positions from 2m apart to 5m apart and presently at 4m apart with seating about 3.5m away. This gives a great sound stage. I keep moving them closer thinking this must be better, but it just seems to squash everything together to a more focused point.

Toed in about 10 degrees. Tried extreme toe in which gave a wider field but just didn't quite sound right and lost a lot of crispness (sorry, don't know all the correct vocabulary :) )

Started on the 32v power supply. Which seemed fine, but upgraded to the 48v as I wanted more depth at lower volumes. In theory, this shouldn't really make any difference except on higher volumes, but to me it sounds like there's much more depth at all volumes - though this could be my subconscious over ruling reality based on desires !

Mainly streaming via bluetooth. It's a great sound. I guess my comparison is a Mission 10.1 setup with technics amp. The Jamo/BT20 sounds much "fuller, warmer,. complete, immersive" - just better ! Surprisingly so as the Mission setup I thought was outstanding until I then compared it back to back. The Mission setup now just sounds empty ?!?

So the main questions. Should I need the treble up so much or is there something quirky ? Have tried the speakers in all sorts of positions and it's the same situation. In the wrong places there's considerably more bass. Even as it is, stand in the wrong place in the house and it feels a bit like someone's put a dustbin over your head. Ultimately, I guess it doesn't matter as I like the sound now. But just concerns me if I consider swapping to a different amp.

Just plugged an old Toshiba DVD player in to run as a CD player, connected via RCA. Playing the same album via youtube/spotify or CD (Fat of the land) on CD it just sounds so much MORE ! Is this the source youtube/spotify ? or the Bluetooth DAC in the BT20 ? I've also tried direct (3.5 to RCA) from the PC (chromebox), which I guess uses the PC's DAC - this really limits the whole sound range and sounds awful.
 

hkrooner

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Mar 4, 2024
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Dumb question: when the the BTA20 Pro's preamp offsets the volume to below the line source, is this done by the TPA3255 itself or by a potentiometer? This looks so good as a power amp that it seems silly to me to buy it just to work as a preamp (let alone a passive preamp).
 

moonthink

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Just like I think it's odd that Fosi claims the "preout" on the BT20A Pro can drive a powered subwoofer through this connection. I have three Sony SACS9 10-Inch Active Subwoofers that prove otherwise. The only way I could get them to work is via line in/out speaker connection.
I have a BT20A Pro, and the subwoofer out has worked fine with 2 different subs (Sony SA-W3800, and Rythmik FV12). Maybe it's the cable you used? I am using a 3.5mm TRS aux to dual RCA cable, with the red RCA going in to the subwoofer (white RCA just dangles).
 

paradoxical3

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Apr 1, 2018
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This little guy with the 48v supply worked amazing on my high sensitivity speakers (old Elemental Designs ECD6). My JBL HDI 1600s, however, drive them into shutoff mode when they hit 87db at about 10ft distance. Still happy with it for the price but time to go Hypex.
 
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