6) ESL Bias Power Supply Construction
by Mark Rehorst
updated November 2006
Many DIY ESL sites will show you schematics of voltage multiplier power supplies for the bias supply. I even had some of those schematics on one of my pages, but I decided that such power supplies are a little too dangerous for most audio hobbyists. A lot of the people making these speakers are not very experienced with electronics and don't realize the danger inherent in line powered circuits. I can't tell you how many times I've been asked if it would be OK to use a neon sign transformer to make the bias supply. If you have to ask that question, the answer is NO! DON'T DO IT! Neon sign and microwave oven transformers are capable of supplying enough power to kill you.
I designed this supply to eliminate the danger. It costs a little more
to make than a supply made from scratch but it is far more versatile because the
output voltage is variable. It is very small- it was designed to fit
inside the 2" PVC pipe frames that I was making for my speakers.
The heart of the circuit is the Emco G-series DC to HVDC 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 when I bought mine 10 years ago. They are probably cheaper now. 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.
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 300V to 4000V. Adjustable bias can be useful if you find that you didnít 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 inexpensive 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 occasionally have HV resistors.
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.
R1 is a 1% metal film part 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). 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 +10 to +20V 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 wonít produce 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. If you turn the output voltage down low the regulator has to dissipate more power and it could get pretty hot so it's a good idea to put a small heat sink on it.
Here's a PCB layout (sorry- the tools I had for layout when I did this were
pretty crude) but the circuit is so simple there's no reason you can't built it
on a piece of perf board. Don't use prototyping board with a lot of metal
traces all over the bottom side, though. That will make it too easy for
the HV to go places where you don't want it. I have set the board up so
that you can use two high voltage resistors to bias two speakers from one bias
1) Principles of ESL operation
2) Materials Required
3) Diaphragm Stretcher
4) ESL Driver Construction
5) Support Frame Construction
6) Bias Power Supply Construction
7) ESL Bibliography and Links