frequency response. But if the bypass capacitor's value is reduced greatly, it introduces a shelving function, wherein the high-frequency gain is greater than the low-frequency gain, with the ratio being equal to the bypassed gain over the unbypassed gain or
Ratio = 1 + (mu + 1)Rk
Ra + rp
The transition frequencies are based on the capacitor's value and the time constants it forms with the triode's rp and its plate and cathode resistor values:
RkCk = 75µS
[(Ra + rp)/(mu + 1) || Rk]Ck = 318µS
Of course, one triode is not going to provide enough gain in most cases, so additional gain stages will be needed.
The overarching liability of this different path to RIAA equalization is the -12dB insertion loss beyond the expected losses, which brings it 1kHz insertion loss down to -32dB. With this equalization network, the near DC and subsonic frequencies are reduced by -12dB, whereas the traditional path retained all of the gain at the bottom of range. Now, -12dB is just too much to pay in most two-gain-stage phono preamps, but it is almost nothing in a three-gain-stage preamp, where the problem is usually having too much gain. Still the advantage of using a portion of the equalization network in double duty either, as a pseudo coupling capacitor or a partial cathode-bypass capacitor, is tempting.