Hybrid Mu-follower (aka Gyrator) Rev08 PCB Update

It’s been far too long since I last posted on this blog. With the limited spare time I’ve got these days, I concentrated in setting up the new workshop and system since we moved back to our place. I’m nearly there, so now it’s time to get back to work

I made some updates to the “gyrator” PCB. I’ll stop referencing it gyrator from now on, since the name is misleading. However, it got popular that way. Nevertheless, it’s a hybrid mu-follower circuit but if only if you take the output from the anode, it behaves like a “gyrator” from a frequency response perspective. If you’re interested in this circuit in more detail, please read the lecture I gave last year at ETF.18. You can download it from here.

Back to the board, here are the few changes made:

1. Moved the trimpot P1 for easier fixing of the board with the M3 standoffs.
2. Added a gate stopper resistor (R9) to avoid oscillation at low anode current (<10mA) when using high-gm MOSFETs in the lower position J4. This was evident with devices like BSH111BNK
3. Added an LED (D4) indicator and a series resistor (R8) at the drain of M3. This enables indication of:
   1. Normal operation subject to  value of R8
   2. Source current into load (e.g. like in A2 operation) subject to value of R8.
   3. Short output to ground. Depending on duration and current limitation of power supply, this may prevent damaging M3 MOSFET. Not guaranteed, but in some scenarios will work.

Here is the circuit diagram for reference:

Of course you can continue using jFETs in the board. However, I found that the BSH111BK/BSN20BK and other high-gm perform really well in this circuit. They are still plenty and available out there, not like the BF862. Unfortunately this one is EOL (see lecture for replacement options).

Here is a sample test of the 01a preamp stage with this new board:

In this case with the use of an IXTP3N100D2 and BSH111BK operating at 3-4mA the frequency response is flat to nearly 380kHz. Same combination of FETs in a Rev07 board will oscillate at LF due to the lack of gate stopper.

With a Rev07 IXTP08N100D and BF862 board, the response only gets up to 150-180kHz depending on the valve. This is not a limitation whatsoever in audio band, let’s make this point clear.

A snapshot of a completed board:

in the following picture you will see the three new components added so you can compare to the previous Rev07 version. Also note that this is a prototype version so it doesn’t have an ENIG Gold board finish:

The new boards were tested on my 01a preamp. They sound amazing, as good as the previous ones. Interestingly enough, I made a mistake during breadboarding and the LED turn on really brightly with 100mA shunted through the output. The FETs survived this time thanks to the limiting resistor and the current protection I have on my bench power supply.