This is a review and detailed measurements of the Fosi Audio BT10A Bluetooth Class D stereo amplifier. It is on kind loan from a member. The B10A seems to have just been updated with tone controls and sells for US $50 from Amazon with free prime shipping.
This is a very small amplifier:
You can't see the depth but it is quite shallow. Other amps like this I have tested have complicated interface to change built-in digital tone controls. There are none here so it was easy to use.
The back connectors is identical to what you see on myriad of such amps:
While the external laptop replacement power supply is large compared to the size of the unit, it is not as huge as some others. Note that it comes with a three-prong, figure-8 AC socket so you have less options to replace the cord.
I did not bother testing the Bluetooth. I am assuming it autoselects it if in range as the priority input as there is no switch to select it.
Speaker terminals are tiny of course but I managed to plug in my heavy and large banana jacks into it.
Amplifier Audio Measurements
As usual, we start with our dashboard view of 1 kHz tone into 4 ohm load with volume adjusted for 29 dB gain (THX recommendation):
Second harmonic dominates to the tune of -65 dB which sets the SINAD to the same value. This says that noise is not a contributor to SINAD and as such, any gain setting would have produced similar SINAD. The 65 dB SINAD value doesn't put a smile on anyone's face but it is a couple of steps above $30 units (e.g. Lepy LP-2020A):
Noise figures are barely adequate:
32-tone test signal shows more or less uniform distortion at all frequencies:
19+20 kHz tones shows 55 dB of distortion-free range:
Their full (and unrealistic) amplitude shows higher distortions than multitone test does.
Frequency response shows truncated low and high frequencies:
Above with a resistor load. Subjective the amp to my new complex speaker model of a 2-way speaker generates:
The speaker model has a its impedance shooting up in high frequencies causing similar issue here. There is a question of whether the model is similar to mass of 2-way speakers so don't run with this yet. What you can conclude is that the speaker choice does impact the speaker response. In audible band though, the impact is minimal and in some way is correcting the dip toward 20 kHz. A good amp would have no speaker load dependency.
Let's see how much power the BT10A can produce into 4 ohm load:
At 33 watts with both channels driven this is good bit of power. Similar story with 8 ohm load:
Lack of power is by far the #1 issue in these amplifiers. You may not be able to hear their noise and distortions but you absolutely will hear them complain when they clip. At these power levels you should have a reasonable experience unlike some other budget amps that stop at 10 watts or so.
Allowing for more distortion (1% THD+N) and testing with a short-term sinewave and long term produces identical numbers:
The input power supply is regulated so there is no reservoir to provide short-term juice.
Thermal Stability
Performance gradually improved by a 2 dB or so after 30 minutes of playing at 5 watts:
Above tests were performed after this warm-up period.
Conclusions
The Fosi Audio BT10A doesn't seem half bad. I am not going to provide a verdict one way or the other. You can decide for yourself if the performance is good enough for your application.
------------
As always, questions, comments, recommendations, etc. are welcome.
You know the money situation is bad when I have to get a $50 amp on load to test! So please, help improve my bank account balance by donating as much as you can using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
This is a very small amplifier:
You can't see the depth but it is quite shallow. Other amps like this I have tested have complicated interface to change built-in digital tone controls. There are none here so it was easy to use.
The back connectors is identical to what you see on myriad of such amps:
I did not bother testing the Bluetooth. I am assuming it autoselects it if in range as the priority input as there is no switch to select it.
Speaker terminals are tiny of course but I managed to plug in my heavy and large banana jacks into it.
Amplifier Audio Measurements
As usual, we start with our dashboard view of 1 kHz tone into 4 ohm load with volume adjusted for 29 dB gain (THX recommendation):
Second harmonic dominates to the tune of -65 dB which sets the SINAD to the same value. This says that noise is not a contributor to SINAD and as such, any gain setting would have produced similar SINAD. The 65 dB SINAD value doesn't put a smile on anyone's face but it is a couple of steps above $30 units (e.g. Lepy LP-2020A):
Noise figures are barely adequate:
32-tone test signal shows more or less uniform distortion at all frequencies:
19+20 kHz tones shows 55 dB of distortion-free range:
Their full (and unrealistic) amplitude shows higher distortions than multitone test does.
Frequency response shows truncated low and high frequencies:
Above with a resistor load. Subjective the amp to my new complex speaker model of a 2-way speaker generates:
The speaker model has a its impedance shooting up in high frequencies causing similar issue here. There is a question of whether the model is similar to mass of 2-way speakers so don't run with this yet. What you can conclude is that the speaker choice does impact the speaker response. In audible band though, the impact is minimal and in some way is correcting the dip toward 20 kHz. A good amp would have no speaker load dependency.
Let's see how much power the BT10A can produce into 4 ohm load:
At 33 watts with both channels driven this is good bit of power. Similar story with 8 ohm load:
Lack of power is by far the #1 issue in these amplifiers. You may not be able to hear their noise and distortions but you absolutely will hear them complain when they clip. At these power levels you should have a reasonable experience unlike some other budget amps that stop at 10 watts or so.
Allowing for more distortion (1% THD+N) and testing with a short-term sinewave and long term produces identical numbers:
The input power supply is regulated so there is no reservoir to provide short-term juice.
Thermal Stability
Performance gradually improved by a 2 dB or so after 30 minutes of playing at 5 watts:
Above tests were performed after this warm-up period.
Conclusions
The Fosi Audio BT10A doesn't seem half bad. I am not going to provide a verdict one way or the other. You can decide for yourself if the performance is good enough for your application.
------------
As always, questions, comments, recommendations, etc. are welcome.
You know the money situation is bad when I have to get a $50 amp on load to test! So please, help improve my bank account balance by donating as much as you can using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/