The Line Level Amplifier

The listening tests of the single ended class A output stage of the phono preamplifer were very promising, so I did not test different circuit variations of the line level amplifier. I just copied the circuit of the second stage of the phono preamplifier and did the following changes (see the schematics in figure 3.1 on page ):

1. Instead of the active filter we set a linear amplification^3.1 of approximately 10 dB using resistors $R_{45}$ and $R_{11}$. The operational amplifier must be compensated for an amplification of 1 (0 dB), as the resulting input capacitance of the Mosfet and the open loop output impedance of the operational amplifier form a lowpass filter which turns the phase and decreases the loop stability. Counting on earlier experience I chose OPA134-its sound is very good and it has high impedance FET inputs with very low input quiescent current, which allows us to use a high impedance volume pot.
2. The quiescent current running through the Mosfet is increased to 45,5 mA to be able to drive the head phone. The dissipated power of $Q_{4}$ and $R_{43}$ is 680 mW each.
3. The heat sinks for the Mosfet and the voltage regulators are bigger due to increased power dissipation.
4. The headphone is connected directly, while the output to the power amplifier is connected via a 50 $\Omega$ resistor.
5. A high quality volume pot is connected to the input of the line level stage.

   Abbildung 3.1: Schematic of the Line Level Amplifier Stage

   

A top view of the printed circuit board is in figure 3.2 on page .

Abbildung 3.2: Top View of the Line Level PCB