After the Calvin/PlatINA/RP1-projects I was driven to design a Phono stage built with discrete devices. I aim to use a low number of active single-ended stages and power supplies.
JFETs were chosen as amplifying devices again.
They don´t require a base current like bipolar transistors and that allows to directly couple the pickup to the input without requiring dc-blocking capacitors.
Due to their highly-inductive generator system MM-pickups react sensitively on the capacitance of the input stage and the attached cables. The more so with lownoise JFETs that´s large sized Gate structures leads to increased capacitance.
That capacitane is even multipiled by the gain figure, the so called Miller-effect.
Cascode connection of the JFETs omits with the Miller-effect. The nearly constant drain-source-voltage of the steering JFET within the cascode results in low capacitance and low distortion values at the same. In the interest of a low number of active gain-stages ´steep´ JFETs with high gm values are required.
In principle the circuit comprises of a simple cascoded source circuit (J1, J2) loaded by a active bootstrapped current source (J3, J4), quite similar to a SRPP gain stage. The modulated curent source as load instead of the common drain resistor maximises the gain at still low distortion. This is required the more so for the MC-variant where the gm of a single JFET is not sufficient any more to supply for the needed gain.
Paralleling of the JFETs is the key to the problem and is also inevitably needed to achieve the required low noise behaviour. After all at 20Hz where the maximum gain is asked for, whopping 80-90dB are needed. Fortunately due to the low coil-inductance values of MC-pickups the increased capacitance doesn´t play a role here.
A high overload margin requires the RIAA-network to be located as close to the input as possible. The RIAA-equalization is done in quasi-passive style in form of a shunt network at the drain of the input stage cascode J1/J2.
The signal is then taken from the source of the current source J3/J4and coupled to the output through a beefy buffer stage. Instead of the JFET cascode a beefier hybrid CFP-buffer or the Calvin Buffer may be used also.