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Two resistors and one capacitor are all that is needed to create a low-frequency shelving network. At low frequencies, the capacitor effectively isolates resistor R1 from resistor R2 and ground, so no attenuation occurs. At high frequencies, the capacitor effectively bridges resistors R1 and R2, creating a voltage divider and reducing the signal by the resistor ratio.
The usual approach is to use three gain stages separated by two passive equalization networks, for example, the circuit below. This scheme greatly reduces the gain requirement from each gain stage. For example, you would be hard pressed to find triodes that could realize a gain higher than 70 in the two gain stage scheme, which would yield only a final gain of 50 dB after equalization.
But that same 50 dB of gain could be had by cascading three stages with a gain of 15 times the input (23 dB). In the circuit below, each 6DJ8-based grounded-cathode amplifier achieves a gain of 25 (+28 dB) in this circuit, which brings the total gain to +64 dB after equalization!
If the values of the shelving network and the low-pass filter do not look correct, for example, 98.7k instead of 90k and 7.55k instead of 7.5k, the reason is that these values have been adjusted to compensate for the 1M grid resistors that follow and are effectively in parallel with the resistors. Thus, 1M in parallel with 100k yields 90k and 1M in parallel with 7.55k yields 7.5k. (Actually, we should also include each cathode follower's output impedance in the mix; be sure to read the last page)
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