Thanks to Tim’s suggestion to implement Allen Wright’s Super Linear Cathode Follower (SLCF) which forces the valve to operate in constant voltage (in addition to the constant current) by bootstrapping the cathode to the anode using an extra follower, distortion is reduced further. In my case, it is halved!
The “extra follower” on top of the cathode follower could be another valve. But to avoid elevating heating, I just went for a straight sanded follower using a depletion FET, the famous DN2540. You can use the MOSFET of your choice here:
The stage gets a tad more complex, albeit not much. We can keep one single supply rail here but we need to elevate HT up to 180V minimum to provide the headroom required by the FET to operate well and minimise its output capacitances to ensure HF response is good. M1 is the top follower which provides a fixed DC voltage using the resistor divider formed by R7 and R8. The higher these are in value the smaller C4 is. C4 provides the bootstrapping between cathode and anode to force constant voltage operation at all times. This minimises the distortion of the valve. I found that 100nF was enough to halve the distortion down to 0.0035% at 2Vrms!
PSR is about 60 to 70dB across the audio band so great addition to have a top active device here!
I think this could be a fantastic follower to use in multiple designs. Worth breadboarding it, hopefully shortly.
