RIAA responses of the MK I and MK II phono stage versions

Theoretical and measured RIAA responses of the MK I and MK II phono stages. For a better illustration, 1 db were added to or subtracted from measured plots (blue / red).

    RIAA response of the MK III phono stage

Theoretical and measured RIAA response of the MK III phono stage. For a better illustration, 1db was added to elevate the measured plot (red).

Theoretical and measured RIAA response of the revised MK III phono stage. For a better illustration, 1db was subtracted to lower the measured plot (red).

Error plot of the measured RIAA response of the revised MK III phono stage. The plots show the differences from the theoretical RIAA rsponse.

Red plot: RIAA error within the frequency band from 2Hz to 200kHz (ordinate scale +/- 15dB),

Blue plot: RIAA error within the frequency band from 20Hz to 20kHz  (ordinate scale +/- 1,5dB),

The schematic illustrates that the resistors and capacitors of the RIAA network are virtually in parallel with the drain resistors.

 

 

 

With a constant drain resistor and a voltage dividing passive RIAA network following, the amplification of the input stage must be linear, i.e. independent from the frequency. High frequencies plus high amplitudes (inverted RIAA on the LP !) given, the circuit has to be able to perform high voltages changes dU (green) within a short time dt (red). Slew rate (dU/dt) limitations might degrade the sonic performance at high frequencies.

With the RIAA network itself as a variable drain load, amplification decreases with increasing frequency according to the RIAA curve. Hence less slew rate (dU/dt) is required for a full performance of the circuit at high frequencies.