Long time ago I used this SLCF circuit as an output buffer with excellent results. It sounds as clean as it can get in my opinion. Recently, I made up a PCB to hold this circuit with a varied of options:
Tag: Allen Wright SLCF
Testing the line stage
Introduction
I couldn’t resist the temptation to try and build quickly the SLCF design proposed here. It was question of building a simple PCB for the tail CCS and the top MOSFET follower. Wiring it then point-to-point could be done in a matter of minutes and a “rat nest” was built fast enough to enjoy this learning experience.
The usual challenges we face when breadboarding circuits
One of the challenges we face when building a cathode follower with a high-gain / transconductance valve is that it can easily oscillate widely into VHF. So we then are a bit more precocious when building the test jig and “try” to have short connections (something which I didn’t do), add some ferrite beads to anode, grid and screen. Also some grid/screen stopper resistors (e.g. 300Ω) are always very useful. If you pay attention to this and check with an oscilloscope with sufficient bandwidth (e.g. 200MHz) you can spot out any nasty oscillation from the valve. I didn’t, thanks to the ferrite beads and stoppers.
The clear challenge of the SLCF is establishing the correct bias point on the top follower due to the high value of the resistor divider and the high-variance we typically get on the VGS(th) of the MOSFETs.
High-value resistors are available on 1% but the variance on the FET defeat the purpose of accurately building the resistor divider.
A (not that simple) line stage
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.