by Mark Rehorst
[Editor: Electrostatic headphones are high voltage systems. Please observe extreme caution and follow high-voltage safety procedures when working with these circuits!]
Most of my experience is with electrostatic loudspeakers (Quad ESL63 and my own designs). I have some Koss ESP9/50 phones, but on the two occasions when they have had problems, I took advantage of the lifetime warranty and let Koss fix them. Electrostatic headphones are basically the same as electrostatic loudspeakers speakers – same operating principles, same basic circuits.
To understand ES headphones, start by studying the readily available material on ES speakers. Be very careful when working on the interface/adapter box and headphones, because the ac voltage that drives the phones can go as high as 2,000V. The internal circuit should consist of a high voltage power source (some phones derive the bias power from the audio signal) and probably a couple of step-up transformers that raise the output voltage from the amplifier to the drivers. [Editor: Modern electrostatic headphones often use high voltage amplifiers instead of step-up transformers. The author advises to first check power supply voltages, then check circuit voltages when troubleshooting these headphone systems.]
The high voltage source connects between the diaphragms in the earpieces and the center taps of the transformer secondary windings. The two end leads from each secondary go to perforated, conductive plates on either side of the diaphragm in each earpiece. The diaphragm is charged to a high voltage level, typically 500-750VDC in headphones, and the high voltage ac signal (audio) is applied to the plates on either side of the diaphragm. The resulting electric field pushes/pulls the diaphragm back and forth producing sound.
If the headphones are dead, and you’ve tried the obvious stuff like checking the headphone cord, then the next thing to check is the fuse in the interface box. If the fuse is blown, you probably have a short in the bias supply and need to fix it or the box will keep eating fuses. If the fuse isn’t blown, you may still have a problem in the bias supply.
If both channels are dead in the phones, then the bias supply is probably dead. If only one channel is dead, then there are a couple possibilities: one of the earpieces has developed a short, one of the resistors between the bias supply and the earpieces has opened up, or least likely, one of the audio transformers has failed.
You can check the earpieces with a multimeter. There should be three wires going to each earpiece. Check the resistance between all the leads. They should all read as open circuits. If they don’t, you’ve found the problem.
The most likely thing to die in the interface box is the diaphragm bias supply. The bias supply consists of three types of components. There will be a voltage multiplier made up of a string of diodes and capacitors, and there should be one or two resistors that connect between the output of the supply and the diaphragms in the earpieces. The bias supply puts out so little current that it will be difficult to test. The easiest thing to do is usually to just start replacing diodes until the thing starts working OK. If that doesn’t work, start replacing caps. If that doesn’t work replace the resistor(s).
For further info on the operation of electrostatic speakers/phones, check out How To Build Electrostatic Loudspeakers.
c. 1999, Mark Rehorst.