Vacuum Tube Gain Controlled Amplifier

A project book for the do it yourself builder, includes circuit diagrams and circuit descriptions to build a gain controlled amplifier also referred to as a compressor. This type of amplifier is used to prevent over-driving audio systems by maintaining an average output and keep levels in a more comfortable listening range.
This particular design uses all triode vacuum tubes including a triode gain control stage, all stages are built around the popular 12AX7 tube. Triodes are used because they produce primarily second harmonic content when harmonic distortion occurs, the addition of low level second harmonics enhances the sound.
The design utilizes a variable compression ratio, the ratio becomes tighter as the input level increases past the compression threshold. This allows soft passages to retain high dynamics while imposing more control on loud passages. The amplifier uses broadband compression to maintain the original sound balance. A slow gain control is used as an average level reference on which a faster gain control rides, ratio between the two is adjustable. Gain control is non-intrusive and dynamics are maintained, this is not a brick wall processor. Useful gain control is over a 30DB input range.
The original design was created in 1990 for use in broadcasting at a FM radio station in Baltimore, Maryland. The design concept was to produce a compressor that would maintain an average level but still allow dynamics without artifacts common with many compressors such as pumping when bass notes vary AGC gain or ducking from sudden high input levels.