2/24/99 M. Rehorst
markATrehorst.com (replace AT with @ to email)
I have always disliked the idea of having a high voltage transformer in the bias supply. Even a low current
transformer can be quite dangerous, even for experienced people. Some recent, unpleasant experience has
prompted me to come up with a design for a safer bias supply that is very small and easy to build. The cost
is a little higher than using a transformer and voltage multiplier but if it saves you from an unpleasant
shock, it is cheap at 10x the price. As an added bonus the output of the new circuit is adjustable over a very
wide range.
The new circuit consists of a 12-15V, >100 mA wall wart type power supply, a low voltage adjustable
regulator, and a DC to high voltage DC converter module made by a company called Emco High Voltage.
The heart of the circuit is the DC to high voltage DC converter module. It is very small, and fully
encapsulated with pins for circuit board mounting. I used a G-40 module which cost about $60. This
module is rated at 4000VDC out (at 250 uA) when 12VDC, 100 mA are applied to the input. Emco has
modules in this series that go as high as 6000VDC out. Specs for this series of modules can be found at
http://www.emcohighvoltage.com/emcoindex.htm
The output voltage is approximately proportional to the input voltage so by using an adjustable regulator
circuit ahead of the module you can make a bias supply that can be varied from about 500V to 4000V.
Adjustable bias can be useful if you find that you didn’t quite put enough tension on the diaphragm of your
speakers or if you have problems with corona discharge (I mean the whining sound that can sometimes
come from the speakers, not excessive urination after drinking too many Mexican beers).
The adjustable regulator is made from common components including an LM317T 3 pin adjustable voltage
regulator IC. The complete schematic of the supply appears below, along with a PC board layout. The
dimensions of the board are 1.75" x 4".
You still need a HIGH VOLTAGE RESISTOR at the output of the supply to ensure constant charge (thus
low distortion) operation of the speakers. High voltage resistors are not the same as your standard, run of
the mill carbon or metal film resistors. Standard resistors will slowly self-destruct (open) if you apply too
high a voltage to them (anything over 500V should be considered too high). High voltage resistors are
available from Caddock (try MX-431-20M-10%; go to www.caddock.com) and Micro Ohm (try RG-3H-U-20M-M; go to www.micro-ohm.com). Surplus dealers may occasionally have them also.
The 1 uF capacitors are tantalum electrolytic type. The 10 uF is an aluminum electrolytic. Watch the
polarities when you install them. The negative side connects to ground.
The 590 Ohm resistor is a 1% metal film unit, but feel free to substitute a lower tolerance carbon resistor.
590 Ohms is about what is needed for 12V maximum output. If you use a little bigger resistor, the
maximum output voltage will go down slightly. The approximate maximum output voltage can be
calculated from the following formula: Vout = 1.25 x (1 + R2 / R1) + 0.00005 x R2
R1 is the resistor connected to the regulator output pin. R2 is the potentiometer.
When you wire the pot, be sure to short one end of the pot to the wiper contact. If the contact fails, you
will still have 5k in the circuit and everything will still work.
You can apply any voltage from about +20 to +10V to the adjustable regulator and get some output from
the bias supply. The 15V shown on the schematic is a nominal value and will allow you to swing the bias
supply output voltage over a range of about 300 to the maximum output for the module you are using. The
regulator won’t spit out more than about 12V so it will not damage the DC to DC converter module no
matter what voltage you apply to the regulator input. Don’t apply more than about 20V to the regulator
input unless you provide a heatsink for the regulator chip.
I have set the board up so that you can use two high voltage resistors so you can run two speakers from one
bias supply. You can also just build one supply for each speaker if you don’t want to run high voltage
wires all over the room.
MR