A Simpler Active- RIAA- Equalization Preamp

By using two amplifiers we can greatly simplify the design of the preamp. Each amplifier can be made to handle one half of the equalization task. The circuit below imposes the 3180µS and the 318µS time constants on the signal leaving the first amplifier and the 75µS time constant on the second amplifier’s output. In other words, the first amplifier handles the 50Hz to 500Hz shelving portion of the curve, while the following amplifier handles the 2122Hz low-pass filter portion, without any high-frequency curve flattening.

 The first amplifier must have a DC gain of +20dB for the shelving to work correctly. The second amplifier gain can be set to any value (within reason). Notice that the two stages work independently of each other, which makes tuning the preamp’s frequency response a much easier task. ( Active RIAA OP-Amp preamp simple style.ckt Ver 4.2) ( Active RIAA OP-Amp preamp simple style.ckt Ver 2001)

As can be seen in the graph above, this new phono preamp topology works quite well. Furthermore, the formulas for the setting the equalization curve are much simpler than the previous case:

            R₁C₁    = 3180µS

            R₂C₂    = 75µS

(The values in schematic above have been already adjusted to compensate for parasitic capacitances.)