Vacuum Tube Audio Projects, Part 2

Part 2 of the Vacuum Tube Audio Projects consists of Preamps 3 and 4. Preamp 3 uses the 12AU7 or its equivalent ECC82, 5963, or 6189 miniature twin triode tube, which has two identical triode sections within a single 9-pin Noval B9A socket base. The triodes are configured as a common cathode amplifier, which means the cathode is shared by both the input and output circuits.
The gain of this circuit is about 2, which means that the input signal is amplified by a factor of two. This is a low-gain preamp that aims to reproduce a high-quality sound with a tube character. The prototype with a 5963 tube was connected to a pair of TDA8932 Class D 35-watt power amplifier modules, and what you hear is a warm, powerful, and rich tube sound.
The Preamp 4 project is a two-stage preamplifier that uses twin triode tubes to amplify the signal from a low-level output source. It is inspired by the "Buffer Line Amplifier" project described in the book "Vacuum Tube Amplifier Basics," pp. 163-166, by EJ Jurich, but with some modifications to the cathode follower output stage based on the "RCA Receiving Tube Manual RC-30," p. 706.
The Preamp 4 project involved testing several dual triode tubes, namely 6189, 5963 12AV7, ECC82 (12AU7), and ECC83 (12AX7). The results of the objective tests did not show much difference among the twin triodes. However, the subjective listening test revealed that the combination of 5963 as the first stage and 6189 as the second stage produced the best sound quality across low, mid, and high frequency ranges compared to the other triodes.