300B SE Amp: build part V (Fixed Bias)

Last year I developed a voltage reference using an HV LED. Unfortunately these devices from OSRAM seem to be discontinued. I managed to buy enough parts for my own use though. What is interesting from these LEDs is that the dynamic resistance is low. About 150Ω with good current, or between 300Ω to 500Ω.  Tempco is very low and with such a low dynamic resistance, they are great for creating a voltage reference with a stable CCS:

The LEDs are extremely bright and found that with a 1mA current are dim enough whilst retaining the stability needed. I have a cap multiplier arrangement and the LED array is fed by a stable CCS. Jumpers on the board allow bypassing LEDs and there is also the option to use a trimmer for variable voltage adjustment. Very handy for screen grid supply and phono stages. The reference voltage is extremely quiet with more than 100dB PSR.

My latest version of the 300B amplifier will have fixed bias. Yes, adds complexity, but for me that’s not a problem. Also I was keen to DC couple the 300B to a source follower so that already gives me the voltage supply to implement fixed bias easily.

To make my life simpler here, I designed a few PCBs to support the build process. First one is the HV LED voltage reference which I already covered. It delivers a 150V fixed voltage reference and enough current thanks to the MOSFET pass device. I built a pair of source follower PCBs which you can see in my previous post here. Each SF is biased at 20mA. A bit of an overkill, but enough headroom to discharge the 300B grid capacitance as well as the SF MOSFET Cds one. Also will provide grid current and potential excursion to A2 thanks to the positive supply which can be wired to +25 or +50V. I tested the SF boards and with large finned clip-on heatsinks work perfectly fine.

The second PCB is the fixed bias splitter and AC coupling circuit to the driver. To ensure I provide a constant impedance and keep the RC constant with any bias setting variation, I added a simple emitter follower (T1/3 – PMBTA42) with a tail CCS (T1 – BSS126). All SMD devices to minimise footprint. The board has 2 voltage dividers (R9/11, P1/2 and R10/12) with a trimer for fine voltage setting. There is a couple of connectors for external multi-turn potentiometers. The board has space for a nice and large film/PIO capacitor (C1/2) and bias provided via R4/8:

Here is a snapshot of the bench test:

This worked superbly and is rock solid. Left it for an hour to get warm and couldn’t see any bias variation. This looks promising. I will mount the 4 boards (2 SF boards needed in stereo) in a mounting PCB. Love this modular building 🙂

Author: Ale Moglia

"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp) View all posts by Ale Moglia