The headphones also require huge voltage swings on the order of 100 to 500 volts peak-to-peak this makes plugging them into the headphone jack of stereo or a walkman impossible. The usual approach is to use a step-up transformer with a winding ratio of something like 40:1. But even this transformer cannot be plugged into the headphone jack of stereo, but must be driven by a power amplifier instead. This is not ideal. Where conventional dynamic headphones can be plugged into a CD player directly, the electrostatic headphones demand two extra layers of intermediary devices.  It is easy to see why these headphones are not more popular.

voltage swing at the highest frequency being reproduced must first be calculated:
   Slew Rate = (2FVp) / 1,000,000.
In this case, where 20 kHz is highest frequency and 500 volts is the peak voltage swing,
   Slew Rate = (6.28 x 20,000 x 500)/1,000,000
   Slew Rate = 62.8 volts per microsecond.
Think of slew rate as a measure of the steepness of the waveform at its sharpest slope.
   The next step is to measure the total capacitive load represented by the electrostatic headphone's stators and its cable. The Stax Lambda Pros come in at 130 pF stator to stator. Then this quantity is multiplied against the slew rate to reveal the needed amount of current to charge and discharge the capacitance at the desired frequency and voltage swing.
  Current = SR x Capacitance.
In this case,
   Current = 62.8 x 0.00013
   Current  = 8.16 mA.
This may seem like a fairly trivial amount of current, but do not forget the 250 volt voltage swing and the high voltage power supply, which when multiplied against the current equals a fair amount of power.
  Once we have determined the peak current requirement, we can begin designing the amplifier.

Single-Ended Amplifier
  Driving an intrinsically push-pull transducer with an SE amplifier has a perverse attraction, I must admit. This will require some thought, as the signal must be presented to the stators in a push-pull format. This could be accomplished by using an output transformer with a center-tapped secondary or by using a center-tapped choke as a load. The first configuration is obvious: a conventional push-pull output transformer is wired backwards so that the triode's plate works into the primary and the secondary feeds the stators, with the center tap going to ground.

The usual electrostatic headphone setup: a power amplifier driving a high-ratio step-up transformer 

Vacuum Tubes & ES Headphones
   The drive requirements for an electrostatic headphone are lots of voltage but not that much current, a set of requirements that the vacuum tube meets readily. A power tube is not even required, as a 6SN7 or a 6BX7 can easily put out 250 volt peak voltage swings.
  But voltage is only half of the complete equation. Current is needed as well. This requirement is often overlooked, as a charged capacitor requires no current to maintain its charge. AC signals are altogether different. The higher the frequency, the more current will be needed, as the reactive impedance of the capacitance falls off with frequency. For example, a 1 mF capacitor represents about an 8,000 ohm impedance at 20 Hz, an 800 ohm impedance at 2,000 Hz, and an 8 ohm impedance at 20 kHz.
  To determine how much current will be needed to drive a set of electrostatic headphones, the required slew rate of the peak

< PREVIOUS

pg. 2

NEXT >

www.tubecad.com   Copyright © 1999 GlassWare   All Rights Reserved