Designing a Cathode Follower Preamp: A Step-by-Step Guide

When it comes to designing a high-quality amplifier, one crucial component is the preamplifier stage. In this article, we'll delve into the world of cathode followers and explore how they can be used to create a preamp that drives your KT88 output valves with ease.

At its heart, our 12AU7 driver increases the input voltage from 5V RMS to approximately 50V RMS, allowing for a significant boost in power. This increased voltage is then sent through the second half of the 12AX7, which acts as an inverter, creating two 5V RMS signals that are precisely out of phase by 180 degrees.

The first half of the 12AX7 takes care of amplifying the input signal from 100mV RMS to a more respectable 5V RMS. This increased signal is then sent through the output transformer, which steps down the high voltage generated by the KT88 output valves to a lower voltage that's suitable for driving your speaker.

But here's where things get interesting. The KT88 output valves don't just amplify the input signal – they also add current to the mix. This means that the amplifier is capable of delivering significant amounts of power, making it perfect for demanding audio applications.

So how does it all work together? Simply put, our cathode follower preamp uses a combination of inverting and non-inverting stages to create a powerful driving signal for the KT88 output valves. The 12AX7 acts as an inverter, creating a mirror image of the input signal that's precisely out of phase by 180 degrees. This allows the KT88 output valves to amplify both halves of the audio waveform, resulting in maximum power delivery and smooth operation.

One of the key benefits of this design is its ability to transition seamlessly from Class A push-pull to Class AB operation. This means that the amplifier can operate at high levels of efficiency while still delivering precise control over the audio signal.

So what does it all mean for you, the audiophile? In short, our cathode follower preamp offers a level of performance and flexibility that's hard to find in other amplifiers. Whether you're looking for a powerful driving stage or simply want to upgrade your existing amp, this design is sure to impress.

Stay tuned for part two of this series, where we'll explore the output stage and transformer design in more detail!

Cathode follower preamp