L.RA1 AC headphone amplifier General parameters: Applicable headphone resistance range: 16-120 Euro Frequency range: 12Hz-15kHz (-1dB) Distortion: 0.001%@1kHz Output noise level: -96dB Output impedance: less than 1 ohm; Rated output power: 150mW/16Ω; 290mW/32Ω; 350mW/64Ω; Input sensitivity: 1000mV rms(-7dB) for rated output; Gain: 1 (low gain); 4 (high gain) Power supply voltage: 220V AC Volume and weight: 160*116*48mm (length, width, height), 800 gramsThe RA1 amp has been sprayed countless times by fans with its shabby and unsightly textbook op amp circuit and a price of nearly 4,000 yuan, but it will have a special loose feeling with Goethe’s low-impedance headphones. The circuit has no voltage gain, but only reduces the output impedance, and uses a large-impedance negative feedback circuit to offset the back-EMF of the low-impedance headphones as much as possible. This design brings extremely high signal-to-noise ratio and control, and the sense of hearing is very delicate and loose. When I pair Goethe and Philips low-impedance high-sensitivity headphones, they do have their own characteristics. Do you like it? I hope users try it by themselves. This amp, when paired with our L1852DAC and L4490DAC, is used in small vocals and chamber music. When compiling the work, there will be a very unique sense of air and soft high frequency. Finally, when I borrowed a friend’s new version of the original RA1 amp, I was surprised to find that there was an amplification gain. So at the end of the design, I made the following decision:1. The use of AC power supply saves the trouble of changing the battery and avoids the danger of burning the headset after the battery is depleted.2. Add gain control. When the two red switches are on, the gain is about 1; when off, the gain is the same as the new version, about 4.3. Add the earphone protection circuit, although the original version does not, but for safety, this design is still retained.