Many tube circuits benefit greatly from having a regulated B+ supply. The most obvious advantage is that the regulator provides a constant voltage. This makes it much easier to ensure that the DC operating point of the tube amplifier remain within their specified ranges. In addition, a well-designed regulator provides on the order of 80 dB (10000x) ripple reduction, which, greatly reduces and ideally eliminates any hum caused by power supply ripple. In addition, a voltage regulator ensures that the output impedance of the power supply is as low as possible – and ideally remains low throughout the entire audio band. In short, a well-designed voltage regulator will make the power supply act as closely as possible to an ideal voltage source.
As outlined in this DIY Audio thread, I’ve tried quite a few different regulator topologies. I found I got the best performance from a floating regulator. Functionally, this is very similar to how the LM117/LM317 works (graphics courtesy National Semiconductor AN-178).
Naturally, the supply for the op-amp will have to be derived such that it never exceeds the ABS MAX voltage for the op-amp. Furthermore, the op-amp supply will have to float up/down with variations in the output voltage – hence the name ‘Floating Regulator’.
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.
The schematic of the regulator is shown below. A .pdf schematic including bill-of-materials can be downloaded here: HVRegulator_3p2_Doc.pdf. PCBs are available for sale on the PCB Sales page.
Q1 is the pass device. The difference between the input voltage, Vin, and the output voltage, Vout, is dropped across this device. U1 is the error amplifier. U2 provides the 2.5 V reference voltage. U1 operates with negative feedback, thus, will drive its output voltage to make the voltages on its inverting (-) input and non-inverting (+) input equal. This results in a reference voltage of 2.5 V being set up across the combination of R5, R6, and R7. This in turn results in a reference current of approx. 1 mA being set up through R8, which, establishes the output voltage, Vout. Diodes D2, D3 protect the op-amp during power-up, power-down, and transients caused by a change in load current. D4 protects the gate oxide of the pass device against over-voltage.With the specified pass device, STW12NK95Z, D4 is optional as the STW… has a built-in zener diode between gate and source. Q2 and R4 forms a current limiter. With the values shown, the output current is limited to just above 100 mA. By replacing R4 with a 1.5 ohm resistor, the current limit can be raised to 200 mA. The regulator is designed to survive a momentary short circuit. It will NOT survive a indefinite short circuit.
The power supply for the op-amp is provided by a voltage doubler running off of a 6.3 V transformer winding. The op-amp supply is zener stabilized to 12 V. The total current draw is less than 10 mA.
Assembly & Test
Populate the SMD components first. Start with resistors and capacitors then move on to U1, U2, D1, D2. Then follows the leaded parts. Resistors first, then diodes, zeners, capacitors, and finally semiconductors, and connectors. The pass device needs to be mounted on a sizable heat sink. Attach the device to the heat sink placing a thermal pad or mica washer (with thermal goop) between the device and the heat sink. Then solder the device to the board.
An inherent problem with the initial testing of high-voltage regulators is that they deal with high voltage, hence high energy. This means risk of parts exploding not to mention electric chock and the like. With this regulator there is a little trick that can be employed to make the first test a little safer. On the first test, connect the regulator to the 6.3 VAC heater winding ONLY. Do NOT connect the high voltage. Short out R8 using a small length of wire. Connect a voltmeter to the output of the regulator and power up the circuit – preferably slowly using a variac. With 6.3 VAC applied, the output voltage should measure 2.5 V. The voltage across C9 should be approx. 12 V. If the regulator works this far, turn the power off, wait for the capacitors to fully discharge, remove the jumper across R8, connect the high voltage, and re-apply power (again preferably slowly with a variac). The output voltage should slowly rise to somewhere in the range of 450~500 V. It will take a few minutes for the regulator to reach the final output voltage. Adjust R5 for the desired output voltage. This takes a bit of patience as the regulator responds slowly to changes in R5. This is the result of the filtering circuit C4, R2.
The heatsink used for the module pictured above is a 5.375″ wide, 3″ long extrusion from HeatsinkUSA. The screws thread into blind holes threaded with M3x0.5 threads. #4-40 would work just as well.
With the component values indicated in above schematic, the nominal output voltage is 475 V. The potentiometer, R5, allows for an adjustment range of approx. 450~500 V. Resistors R5 through R8 can be changed to accommodate different output voltages. A few examples are provided in the table below.
|Output Voltage||Adjustment Range||R5||R6||R7||R8|
|475 V||450~500 V||5 kOhm||3.3 kOhm||8.2 kOhm||470 kOhm|
|400 V||380~420 V||5 kOhm||3.9 kOhm||10 kOhm||470 kOhm|
|350 V||325~370 V||5 kOhm||3.9 kOhm||8.2 kOhm||390 kOhm|
|300 V||280~325 V||5 kOhm||3.9 kOhm||7.5 kOhm||330 kOhm|
|250 V||230~275 V||5 kOhm||3.9 kOhm||6.8 kOhm||270 kOhm|
The ends of the adjustment range may be calculated as follows:
Vout_max = 2.5*(1+(R8/(R6*R7/(R6+R7)))),
Vout_min = 2.5*(1+(R8/(R6*(R5+R7)/(R5+R6+R7)))),
where Vout_max is the highest obtainable output voltage, Vout_min is the lowest output voltage of the adjustment range. All resistances should be in Ohm.
The regulator is quite rugged. However, it does require that the load isn’t completely disconnected from the output while power is applied. Should it be desired that the regulator is to function in an environment where it is likely that the load is periodically disconnected from the regulator while power is applied, it is strongly advised that a capacitor in the range of 100 nF ~ 10 uF is added to the output of the regulator (from Vout to GND). In addition, a snubber consisting of a 10 Ohm resistor in series with a 10~22 nF capacitor should be connected from the drain to the source of the pass device, Q1. This 10~22 nF capacitor should be rated for operation at the input voltage, Vin. The 10 Ohm resistor should preferably be a low inductance type.
The PCBs are no longer being sold as this regulator has been superseded by the 21st Century Maida Regulator which offers a significant performance improvement over this regulator.
The boards are professionally made. Double-sided, FR-4, with plated-through holes and white top silk screen. They measure 2.2 x 2.7 inches.