Air Speaker
A website devoted to massless loudspeakers  

plasma speaker chain

   

 

Early Days

The early documents I had contained no practical details about the voltage, current, electrode design, air gaps or materials involved in creating this type of loudspeaker.  My previous analogue amplifier designs had always tried to achieve the end results using the simplest circuit topography possible.  I had designed and built several single ended class-A amplifiers so because of that I though of using that design to start with.  It also suited the simplistic nature of the plasma loudspeaker or corona wind principles.

The amplifier couldn't be more simple - a single 1kV FET, no buffer amplifier and the arc set as the load resistor.  The total circuit was 1 FET, two resistors for biasing and an input decoupling capacitor.

My father being an industrial vacuum engineer had encountered some high voltage design work as it is used in penning gauges for the detection of the level of vacuum in a vacuum vessel.  My first power supply design utilised a penning gauge transformer driving a simple cockroft-walton multiplier to create a rough 2kV DC with very low current - probably below 1mA.

With the basic elements in place the last thing to design was the speaker itself.  Although most of my reading had been about corona wind (ion wind) type speakers, most using many points to plane, I was just trying to make a noise and the plasma tweeter type of speaker seemed a simpler proposition to start.  As many people who try this type of loudspeaker have found, two pieces of copper wire with a gap is all that is needed and that's all that mine was.

So that was it and having designed many circuits I was very surprised when this one didn't actually explode first time.  An arc of around 2mm long was produced, very thin and light blue.  Not only did it produce an arc but I could hear the sine wave input of a signal generator which encouraged me to attached the output of a CD player.  I recall the first test track as being Madonna's Ray of Light and I still use that to this day as it has a good mix of vocals, bass and beats.  As that had just come out it places my first attempt to be 1998.  I could just make out the rhythm, it was very quiet and very tinny but incredibly encouraging.  Using the horn from a cheap tweeter I could just make out the vocal with my ear as close as I dare.

Plasma tweeters are great.  If you show someone who has only heard cone speakers up until now (most people!) a working plasma tweeter then the fact it produces sound at all is a revelation.  A tiny dancing spark with a beat tapping out of it never fails to entertain.

5kv psu 2kv amp plasma tweeterAnnoyingly I failed to photograph that very earliest attempt and only have pictures of mark 2.  Most early photography was with a Polaroid camera - a bad choice as most pictures are  a bit fuzzy and colour rendering quite bad, but it is what it is.  The mark two developed the power supply from 2kV to 5kV, more stages on the multiplier and large capacitors (at the bottom of the photo) - but still using the penning gauge transformer (bottom right).  The amplifier (top) has improved swing with 2 1kV FETS in an arrangement which has lasted me throughout the rest of my developments. 

I had also started to play with various arrangements of electrodes (barely visible on the circuit board).  Sticking with the plasma type I tried many multi-point to single point arrangements.  I had upped the electrode materials to use brass screws - sounds very basic but it turns out that these are great.  The brass conducts heat away from the tip well, the thread of the screw provides a cooling vane to further lower the temperature and their mass gives them at least a few hours life before oxide deposits and erosion make them too noisy.  Each high voltage electrode was supplied through a current balancing resistor - an essential component when more than one point is used otherwise the arc just jumps to the easiest point and none of the others.

It is still fascinating to have these high voltage circuits running with the low voltage output of a CD player connected.  However much your electronics knowledge says it should work, and it does of course, there's always part of you that finds it hard to accept that it is only a couple of transistors separating your valuable Arcam Alpha CD player from destruction.

During these experiments I had a large number of pops, bangs, destroyed amplifier components, burnt out resistors and blown capacitors.  Even so I managed to never destroy a single audio source - largely thanks to the failure mode of the amplifier usually being to go short circuit and therefore ground anything before it got to the source.

Most of these early experiments allowed me to start to work out what were the most important things about the design.  Due to the use of the single ended amplifier design the most important thing in my design was the power supply.