The α24 fully-differential line amplifier

Schematic diagram

The following is the schematic diagram of the α24 line amplifier. Some part values are not shown. Please see the Parts list section for more details.

Circuit description

U1a and U1b (OPA1612) form a differential input buffer, its voltage gain is set by the value of R7. R1, R2, C1 and C2 is a 1st order input LPF to prevent RF "junk" from affecting the performance of the circuit. By default, the corner frequency is about 3MHz as a line stage amplifier, or 700KHz as a voltage DAC output stage.

U2 (OPA1632) is a fully-differential opamp set to have unity gain. R12, R13, R14, R15, C3, C4, C5 and C6 forms a differential 3rd order LPF. When configured as a DAC output stage, the corner frequency is about 100KHz to remove unwanted ultraonic artifacts. As a line amplifier, only small-value capacitors are populated in C4 and C5. They serve as compensation capacitors to flatten the high frequency response and remove squarewave overshoot and ringing. The output of this stage is used for balanced XLR output.

U3 (OPA1611) is a differential amplifier acting as a balanced-to-unbalanced converter. Its voltage gain can be set by the value of R18 and R19. C7 and C8 are compensation capacitors to optimize the high frequency response and stability. The output of this stage is used for unbalanced RCA output.

Each opamp has its own pair of organic polymer electrolytic bulk capacitors on its supply rails, as well as a multilayer ceramic decoupling capacitor across the rails.

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