Bootstrapped CF (SLCF) PCB tests

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:

The circuit is actually much simpler than it looks. Let’s start with the main objective of this board. The MOSFET M5 is there to keep the anode-cathode voltages as constant as possible, therefore reducing the modulation distortion produced by the anode resistance of the valve. Please read the previous posts on SLCF to understand how this circuit works. There are 2 options for biasing M5. The simplest one is by using the resistor divider formed by R4 and R6. Alternatively, you can use the CCS formed by M1 and M3 for better PSRR. The CCS will provide a tiny current (circa 300uA) to create the right bias voltage across R6. R7 and D7 provide a peak current indicator. R10 is the cathode bias resistor when you’re bootstrapping the grid via R12. There is an option to DC-couple the input (see next diagram). The tail CCS is formed by T1 and T2. Options are provided to use different transistors depending on the sink current requirements. The array of LEDs will generate the right bias for the CCS. The LED current is sourced by R5 which can be connected to +B or GND depending on the supply arrangements. The current of the CCS is fixed by R13 or a trimpot (P1).

This board provide all flexibility needed when looking to implement a bootstrapped-CF (SLCF) circuit.

If you want to connect the SLCF directly coupled to previous stage then you can do this:

Testing the board

Well, I built the previously used circuit using the beloved 6J52P/6Z52P Russian pentode.  A D3a will do a similar job here. There are multiple high-gm pentode/triodes to use otherwise.

Here is my somehow abused board:

The performance is superb, have a look below. I used the simple bias method first and a MOSFET which has the b2b zeners included in the package. Here’s the circuit:

Let’s look at the frequency response performance first:

Flat from beginning to end. Ignore the capacitors used as it’s what I have at hand. You need to tune them for a LF response. Interestingly, the stage is flat up to 4MHz.

The distortion is extremely low, here is a snapshot at 3Vrms:

Harmonics are down from 80dB so very clean profile. It sounds as good as it looks though.

I need to make some changes to the PCB (doh) but what can I say. Great device to use at the output of a DAC or to drive TVCs or low impedance SS amplifiers!