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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: 34 11.1%
  • 3. Fine (happy panther)

    Votes: 180 58.6%
  • 4. Great (golfing panther)

    Votes: 90 29.3%

  • Total voters
    307

russdall

New Member
Joined
Dec 31, 2023
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I’ve one myself, and have used one of a friend, both with a wiggling volume knob

Strange....IMHO

I've purchased two Fosi amps. a TB10A in 2020 and a BT20A Pro in January of this year, neither have wiggly knobs of and kind.
I'm not saying it can't happen....I just think it's odd.......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.

Otherwise....I "totally" love this little amp with the Wiim Pro..........It's makes an excellent near field listening system.

PS; The BT20A Pro does incorporate a detent on the volume knob for on and off of bluetooth.......I guess that could be interpreted as wiggly maybe ?
 
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John in CH

Member
Joined
Oct 27, 2023
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Curious BT20A Pro High Temperature Issues...

I have three BT20A Pro amps. One was purchased with the Fosi provided standard power supply (can't remember if it was 32v or 36v). Two were purchased with Fosi provided 48v power supplies.

When idling or running on the standard power supply the amp is just slightly warm to the touch. But when simply idling (or playing) on 48v they become so hot that I'm reluctant to touch them. I swapped the PS's around to see if the heating problem was due to the amps or due to the PS voltage. The heating issue clearly followed the 48v supplies. Based on this exercise it is clear to me that 48v causes extremely high idling temperatures. Whereas idling or running on the standard (32v or 36v) supply the case temps are just slightly above ambient.

I then noticed that 48v idling temps would drop to ambient if I simply unplugged one of the two RCA input cables. Didn't matter if it was left or right side. With both RCA's connected, idling temps would soar to "do not touch" territory. But disconnecting one RCA would cause idling temps to drop again.

I tried this on all three amps and identical behavior was observed on all of them.

My audio source is a Denon DNP-800NE Music Streamer which passes through a Fosi P1 tube preamp (Denon > P1 > BT20APro).

Same high temp behavior is observed when connected directly to the Denon (ie without the P1 in the loop: Denon > BT20APro) and persists whether the Denon or the P1 are powered on or off.

This got me to thinking that the RCA's from the P1 and/or the Denon may have a shared ring/ground (I have not yet confirmed this as several other probjects are taking the priority at the moment).

Question: Could such shared ring/ground on the RCA cables cause the BT20A Pro to run such high temps when running on 48v PS? But not on the lower 32v/36v PS?

Has anyone else observed similar behavior with their BT20A Pro on 48v? And does unplugging one of the RCA's cause the idling temps to drop?

Rgds...
- John
 
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nathan

Senior Member
Joined
May 24, 2020
Messages
368
Likes
255
I need something like this for a pair of ATMOS speakers. I was just about to pull the trigger on this, thinking that for about a hundred bucks I would not find anything better, when I though "I wonder if Fosi makes something that measures as well, but without the bluetooth." A Quick Look at their product lineup and I think the answer is "no, they don't" but am I missing something?
 

grogi.giant

Active Member
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Apr 30, 2021
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I need something like this for a pair of ATMOS speakers. I was just about to pull the trigger on this, thinking that for about a hundred bucks I would not find anything better, when I though "I wonder if Fosi makes something that measures as well, but without the bluetooth." A Quick Look at their product lineup and I think the answer is "no, they don't" but am I missing something?
Tb10d.
 

Talisman

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I need something like this for a pair of ATMOS speakers. I was just about to pull the trigger on this, thinking that for about a hundred bucks I would not find anything better, when I though "I wonder if Fosi makes something that measures as well, but without the bluetooth." A Quick Look at their product lineup and I think the answer is "no, they don't" but am I missing something?
Yes, you're missing out on the fosi V3, slightly better performance in terms of noise and distortion, more power, no Bluetooth
 

nathan

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May 24, 2020
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Don't forget about the new ZA3 as well.....buy one now for stereo and one later to set up a mono-block system....Jes sayin'
That looks interesting/promising. I'll probably get a v3 with the 48v power supply for now, since I need something "immediately," and if I am happy with it for my Atmos use, that ZA3 (if it performs well based on objective criteria, etc) may also be in my future. The key will be whether they get something more linear this time, because it would need to be neutral with a speaker that is 14ohms in the >5khz region, like the following. (My use for now is with speakers that are less challenging.)

index.php
 
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grogi.giant

Active Member
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Apr 30, 2021
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Don't forget about the new ZA3 as well.....buy one now for stereo and one later to set up a mono-block system....Jes sayin'

The za3 mono mode is utter nonsense... Just don't connect second input in the v3 and you'll get same ...
 

Talisman

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You may have saved me from a mistake. Can you tell me what's going on?
Nothing particularly serious, simply the mono mode of the Za3 provides very little more power than the stereo mode, and the gain can be trivially attributed to the fact that the power supply can concentrate on a single channel, which is why the same effect is obtained by connecting just one channel of the V3 and therefore using all the current that the power supply makes available for just one channel.
I currently believe that even using it only as a power amp, the V3 offers better value than any product out there.
 

nathan

Senior Member
Joined
May 24, 2020
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Nothing particularly serious, simply the mono mode of the Za3 provides very little more power than the stereo mode, and the gain can be trivially attributed to the fact that the power supply can concentrate on a single channel, which is why the same effect is obtained by connecting just one channel of the V3 and therefore using all the current that the power supply makes available for just one channel.
I currently believe that even using it only as a power amp, the V3 offers better value than any product out there.
Fair enough. The V3 with 48v power supply and the variable line output (ie, 2024 version) is about $100 direct from Fosi at present, so its hard to argue about value, that's for sure, and I have one on the way.

I was curious whether something in the measurements show that you can consume almost all the power in a single channel if you want to. I haven't learned where to find that objectively in the measurements amirm posts. I would like to learn how to see that.
 

staticV3

Master Contributor
Joined
Aug 29, 2019
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I was curious whether something in the measurements show that you can consume almost all the power in a single channel if you want to. I haven't learned where to find that objectively in the measurements amirm posts. I would like to learn how to see that.
Amir does not test that I'm afraid.

Amps with an explicit Mono mode are tested in mono, while those without are only ever tested with both channels driven.
 

bobebaugh

New Member
Joined
Feb 1, 2024
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0
Curious BT20A Pro High Temperature Issues...

I have three BT20A Pro amps. One was purchased with the Fosi provided standard power supply (can't remember if it was 32v or 36v). Two were purchased with Fosi provided 48v power supplies.

When idling or running on the standard power supply the amp is just slightly warm to the touch. But when simply idling (or playing) on 48v they become so hot that I'm reluctant to touch them. I swapped the PS's around to see if the heating problem was due to the amps or due to the PS voltage. The heating issue clearly followed the 48v supplies. Based on this exercise it is clear to me that 48v causes extremely high idling temperatures. Whereas idling or running on the standard (32v or 36v) supply the case temps are just slightly above ambient.

I then noticed that 48v idling temps would drop to ambient if I simply unplugged one of the two RCA input cables. Didn't matter if it was left or right side. With both RCA's connected, idling temps would soar to "do not touch" territory. But disconnecting one RCA would cause idling temps to drop again.

I tried this on all three amps and identical behavior was observed on all of them.

My audio source is a Denon DNP-800NE Music Streamer which passes through a Fosi P1 tube preamp (Denon > P1 > BT20APro).

Same high temp behavior is observed when connected directly to the Denon (ie without the P1 in the loop: Denon > BT20APro) and persists whether the Denon or the P1 are powered on or off.

This got me to thinking that the RCA's from the P1 and/or the Denon may have a shared ring/ground (I have not yet confirmed this as several other probjects are taking the priority at the moment).

Question: Could such shared ring/ground on the RCA cables cause the BT20A Pro to run such high temps when running on 48v PS? But not on the lower 32v/36v PS?

Has anyone else observed similar behavior with their BT20A Pro on 48v? And does unplugging one of the RCA's cause the idling temps to drop?

Rgds...
- John

I found this post looking for how warm these Class D amps are supposed to run at idle. I have a BRU5, discussed elsewhere on this forum that I think features the same TPA3255 amp chip. I'm using it with a desktop PC and a pair of bookshelf speakers. It runs very warm, just below where you couldn't keep your fingers on it long. I am using a generic 48V 3A power supply. For what is supposed to be an efficient design, it's more like a small space heater. The small 3116 based amp it replaced, idled at ambient. I'll probably swap back.

So I tried your test, disconnecting the RCA input cables coming from the PC. There was no significant change after a couple hours.

I tried a 24V supply that ran the prior amp, that wasn't enough incentive for the amp to turn on, it only lit the led. I'm curious so when I get a chance I will hook it up to the Wiim/garage speakers where I have a variable lab type 60V power supply close by. And see how it drives some bigger 12" PA speakers out there.
 

rj2wells

Member
Joined
Jan 22, 2024
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I believe that the problem is not the thermal resistance of a small 100 euro domestic amplifier.
I think the problem is that you intend to use this little amp as if it were a professional power amp for outdoor parties, it is not and you shouldn't expect it to for 100 Euros.
I suggest you orient yourself on the excellent Behringer A800, which is certainly much more suitable for your use case.
https://www.audiosciencereview.com/...behringer-a800-stereo-amplifier-review.10499/
To be fair, he says drilling the holes addressed the problems. Holes are cheap to drill. Seems like regardless of use the holes are an improvement so @FosiAudio should consider adding ventilation to future cases, or connecting the heat sink directly to the case so that it's not air gapped (Aiyima does this).

I also would like to understand why the unit isn't going into a low power mode when it's idle. Mine stays hot all the time and while it may not be an issue for the amp it's an issue for my non air conditioned house and for my electricity bill.

I am now using a power bar with a12v trigger to shut it off when not in use, at the cost of 5-10s delay before it plays music when it turns on. That's ok, but it would be better if the unit itself had better power management.
 

bobebaugh

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Feb 1, 2024
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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.
 

bobebaugh

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Feb 1, 2024
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Any recommendations?
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.
 
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