The circuit 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. The design is provided as-is with no warranties or service agreements whatsoever. It is provided in the spirit of DIY and may only be reproduced for non-profit purposes. Proceed at your own risk, expense, and responsibility.
- Schematic & Bill of Materials (.pdf): MaidaReg_1p0_SchematicBOM.pdf.
- 21st Century Maida Regulator Spreadsheet (MS Excel): MaidaReg_1R0_Calculator.xlsx
- Mechanical dimensions: 2.50 × 2.55 inches (64 × 65 mm). Assembled board height: Approx. 1.10 inches (28 mm) excluding heat sink.
If you want the ultimate in ripple rejection and ease of use, this regulator is for you.
- 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
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. So 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.
For more detailed information and justification for parts choices, please see the 21st Century Maida Regulator thread on DIY Audio.
The schematic for the 21st Century Maida Regulator is shown below. The target output voltage and current is 400 V @ 225 mA. An Adobe PDF copy of the schematic along with the Bill Of Materials is available here: MaidaReg_1p0_SchematicBOM.pdf. Also available is a Maida Regulator Calculator spreadsheet, MaidaReg_1R0_Calculator.xlsx for calculating R3, R9, as well as a few other sundry items. For those wanting a quicker approach, tables of commonly used values are provided below.
My prototype regulator was adjusted to 400 V out @ 200 mA. There is no measurable ripple on its output. With 16 V RMS (50 Vpp) ripple in, I measure 20 µV (yes, micro volts) RMS of ripple and noise on the output of the regulator.
The start-up time comes in at about 10 seconds. This does, however, require a resistive load. Without load, the start-up time is about one second as the output capacitor is charged through zener diode D2. The start-up is smooth without tendency to overshoot.
The output voltage is set by R9. Simply divide the desired output voltage by two and you have R9 in kOhm. The table below shows the output voltage with R4 at approximately the center of its travel for various values of R9. Figure R4 gives an adjustment range of approx. ±7.5 %.
|480 V||240 kOhm, 3 W|
|440 V||220 kOhm, 3 W|
|400 V||200 kOhm, 3 W|
|360 V||180 kOhm, 3 W|
|300 V||150 kOhm, 3 W|
|240 V||120 kOhm, 2 W|
|200 V||100 kOhm, 2 W|
|150 V||75 kOhm, 2 W|
R3 provides a very crude current limiting feature. It is not intended to provide protection against short circuits, but merely serves to help Q1 survive the worst transient currents. R3 needs to be selected according to the current drawn by the load. For best results, use the spreadsheet linked to above. The table below provides some examples.
|Peak Load Current||R3|
|500 mA||6.8 Ohm|
|400 mA||8.2 Ohm|
|300 mA||12 Ohm|
|200 mA||18 Ohm|
|100 mA||33 Ohm|
|50 mA||68 Ohm|
The power rating of R3 depends on the average output current. For using this regulator to drive vacuum tube power amps, either measure the peak and average currents or use the following rule of thumb: For Single-Ended amplifiers, Peak Current = 2 × Average Current. The current draw of Push-Pull amplifiers is more complex. The easiest way to figure the current draw is to consult the datasheet for the output tubes used and add the idle current for the input and driver tubes.
From the average current and the resistance of R3, the power dissipated in R3 may be calculated as: P(R3) = Iavg2 × R3. Iavg is the average load current in Ampere, R3 in Ohm, P(R3) in Watt. Choose a resistor with a power rating of at least 3~4 times larger than this calculated value. For the values listed in the table above, a power rating of 2~3 W is to be expected.
I do not recommend using this regulator for continuous (average) output currents of more than 300 mA as this dissipates significant power in R3 as well as the cascode device, Q1. In that event, it might be possible to modify the amplifier such that the load can be distributed across several voltage regulators. It is also likely possible to modify the regulator design to accomodate higher currents. However, I will leave these options as an exercise for the advanced experimenter.
Assembled prototype. 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 200~210 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.
Listening test: As this regulator replaced a quite good one I designed previously, I honestly didn’t expect it to cause any appreciable difference in the sound quality of my 300B SET amp. 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!