I've seen multiple designs for a plasma speaker online, and quite frankly most of them suck. Some problems I noticed were constantly blowing up MOSFETs, distorted audio, excessive heating of the MOSFET(s) etc.
So, In this instructable I'll show you how to build the speaker that's on my website. Properly heat-sinked it'll be able to run continuously; I have run mine for a length of about 6 hours with no problems.
Step 1: Gather the parts
4x UF4007 diodes
4x 12 volt zener diodes
2x IRFP250 mosfets. You can also use some better fets, the lower the Rds On the better. Just make sure they can handle at least 200V, flybacks make some nasty back EMF.
1x SG3525 IC
2x 22 ohm resistors
1x 2.2k resistor
1x 10k pot
2x 0.1uF (104) capacitors
1x 3.3nF (332) capacitor
1x 1uF (105) MKP capacitor
1x 2.2uF electrolytic capacitor
2x 10,000uF electrolytic capacitors If you use 40v 8000uF caps instead you can apply 36V and make the arc even bigger and louder. Just make sure to replace the 7812 with a 7815 or a 7818.
A flyback transformer. You can get these out of old computer monitors, TVs etc.
A ferrite toroid. These may be inside computer monitors, but if you can't find one get it here.
Some 18ga wire.
Some 24 ga wire, the wire from inside of a telephone cable works great.
2 heat sinks, you can get them from a computer monitor. You'll need to use your scavenging abilities here. If you use 1 heat sink make sure you use some insulating pads.