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.
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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
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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
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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.