OTL headphone amplifier circuit composed of double triode ECC822 tube

As shown in the figure, a double triode ECC82 (equivalent to E802C, E82CC, and North American 12AU7, domestic 6N10 type) is used as an amplifier. This type of pipe l has the characteristics of excellent index and long service life. The preamplifier generates enough signal amplitude to drive the headphones. The triode portions of the pins 1, 2, and 3 amplify the signal. The input signal is passed through a 50kΩ volume control log potentiometer P1 (P1 is not shown in Figure 1) to the board, and then directly to the preamplifier stage via C1, R1, while R1, C1l provide the necessary negative gate bias. The gain is essentially determined by R8. The maximum input voltage is determined by R2. R9 is determined this way. That is, the static anode current is selected in the largest possible linear portion of the characteristic curve. The input signal that is inverted and amplified on the anode is coupled to the second stage gate through C2. The second stage cathode resistance is divided into two parts, R5 and R6. R5 and R6 are connected in series to form a load resistor. Its voltage division selects a correct value for the gate bias. The gate bias is decoupled and stabilized by R4 and C3 before being applied to the gate. The anode current, determined by the gate bias and the corresponding characteristic, creates a voltage drop across R5 and R6 that is strictly proportional to the current. This voltage is then fed to the earphone via coupling capacitor C4. In order to avoid karaoke noise when the headphones are inserted, the access R7 keeps the output at the DC signal ground potential. The above only describes the working principle of the L (left) channel circuit. The right channel is the same.

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