This is the first revision of the 21st Century Maida Regulator. It has been widely adopted among DIY Tube enthusiasts. It has been superseded by the 21st Century Maida Regulator Rev. 2.0.
The voltage regulator design below deals with high voltages — LETHAL voltages. If you are not comfortable or qualified to deal with these potentially deadly voltages, please do not attempt to build this circuit. Proceed at your own risk, expense, and responsibility.
- True floating regulator design
- Phenomenal ripple rejection (20 µV output ripple+noise in my setup!)
- 15 % adjustment range on output voltage
- Soft start
- Capable of start-up into a purely capacitive load (up to 47 µF verified)
- Start-up into harsh load (2.2 A in-rush light bulb load)
- Handles high di/dt current pulses, e.g. the ones caused by loose connections in tube sockets
- No need for expensive and bulky high-power resistors
- Approx. 2.5×2.5 inch (65×65 mm) board footprint
Design Motivation & Considerations
If you want the ultimate in ripple rejection and ease of use, this regulator is for you.
It has been 32 years since Mike Maida authored National Semiconductor Linear Brief 47, describing a high voltage regulator based on the LM317. Lots have happened since then. National Semiconductor was acquired by Texas Instruments for one; and semiconductors have improved tremendously since the 1970’ies. Given that it’s been 32 years to the month since LB-47 was published, I figured I’d do an update of Mike Maida’s original regulator.
The main drawback of the original Maida regulator is that it requires at least 5 mA (10 mA worst case) to flow in the regulator for it to regulate properly. Typically, this current flows in the feedback network. For lower output voltages, this is no big deal. But for higher output voltages – such as the ones typically used in tube circuits – the power dissipated in the feedback network becomes quite significant, necessitating the use of 5~10 W rated resistors.
Certain single-ended tube amplifier topologies, such as the Loftin-White, present an almost purely capacitive load during start-up. In order for the regulator to survive start-up into such a load, a soft-start feature is needed. On the original Maida regulator, implementing soft-start is actually a bit of a challenge as it requires the use of high-voltage PNP or PMOS devices. These are becoming increasingly hard to source. However, on the 21st Century Maida Regulator, soft-start is implemented by one low-voltage capacitor. Simple. Inexpensive. Done.
Furthermore, modern voltage regulators have much lower drop-out voltages than the LM317, hence, less power is dissipated in the regulator. As a result, the only heatsink needed is for the cascode device.
My 21st Century Maida Regulator is based on the same topology as the original Maida Regulator; a low voltage regulator with a cascode in front to drop the voltage. I chose the LT3080 as it has a low drop-out voltage and needs only 300 uA (typ; 500 uA worst case) to operate. As described above, this minimizes the amount of power dissipated in the feedback network. Hence, only 2~3 W rated resistor types are needed.
The LT3080 is a low dropout regulator and only needs 1.6 V (worst case) across it to regulate. This minimizes the power dissipated in the LT3080. It doesn’t even need a heat sink.
For the cascode I use a beefy NMOS – STW12NK95 (10 A, 950 V). I’ve used these in my other regulators and they work well. They’re also capable of surviving the conditions present at regulator start-up without running into SOA limits.
The implementation shown below is what I use in the prototype of my DG300B amplifier. The amplifier draws approximately 225 mA at idle and up to 310 mA peak. The heat sink is from an old Pentium Pro CPU. It’s actually a bit small for this application and reaches about 70 deg C under normal operation. In my final configuration, I’ll use a 1.3 K/W heat sink from Assmann. A 3″ piece of the 5.375″ profile from Heatsink USA would work as well.
After running for a few hours feeding 225 mA into my 300B amplifier, the LT3080 has reached 40 deg C. Clearly, no heat sink is needed for the LT3080. The board uses both the top and bottom layer for heat sinking the LT3080. The cascode still needs a heat sink, obviously.
This regulator replaced the HV Regulator Rev. 3.2 – a quite good regulator I designed previously – in my DG300B. I honestly didn’t expect it to cause any appreciable difference in the sound quality. Hence, I was quite pleasantly surprised to experience an increased sense of space in the sound stage. I don’t have measurements to back this up, but I’m guessing the perceived difference has to do with the dramatic reduction in output hum/ripple of my new regulator. The old regulator had about 1 mV of hum/ripple. This new regulator reduces this to 20 µV. I’ll take the improvement!