SSTC AUDIO MODULATION
February 18, 2003
After seeing Richie's Audio modulated SSTC on his website I was very much inspired to try out this for myself.
I spent all day at work thinking about modulation and the TL494. My first ideal was very simple and to use the interrupter to turn on and off the drive frequency. This was easy to do since all I had to do was bypass the TL494 output and feed the audio signal direct into the interrupter transistor. Auto tune was used in this first tests. This proved almost to work. I got a nice VU meter in sparks and I could actually here the sound. Though it was that distorted I couldn't actually make out what song was even playing. Sparks were very erratic as the drive frequency was been "messed with".
Having been bored a fair bit at work that day I also decided to build up a small adapter board to house the TL494 IC. This small board plugged into the control board and the TL494 was taken out and placed into the adapter board instead. I tried to use one of the error amps in the TL494 to try and modulate the oscillator frequency. The results I go were just about the same as my first attempt. Distorted sound and unstable sparks. I tried a louder audio signal and tried to boost it up with a voltage divider network though nothing really seemed to help. It still sounded very distorted. Brings back fond memories of my first home-brew audio amplifier too.....
A few trackcuts and changes latter I tried to used the deadtime control pin. On some occasions it didn't even matter if it was connected or not. Other times I just couldn't get any spark output at all. I went back to my previous setup to think up something new. I tried it and couldn't for the life of me work out why I couldn't get it to work again. After about a hour I assumed I had fried the tape player I was using. I wound the tape back and forth a fair bit and couldn't get anything even on some headphones. So I spent the next 20 mins taking the thing apart to try and find out what had gone wrong. Baffled by the look that everything seemed fine I decided to try another tape. To my surprise I had sound! Further investigation found that I only had actually 2 songs on my initial tape and just spent half a hour dismantling the tape player when in fact the tape had ran out :o)
Latter that day I went back to richie's site and found he had used the duty cycle to modulate the frequency. I thought this was kinda a cool idea so took to my signal generator and scope and set to work on my prototype control board.. Half a hour latter and 3 TL494 blowouts I realized I had my power supply set to AC and not DC, which really didn't help matters much. After a lot of track edits and putting them all back again, I found that I could feed the audio signal direct into pin 4 of the TL494. A higher volume made the duty cycle grow or shrink. One thing I noticed that it didn't seem to have any effect until a moderate volume was present. Too high a volume and the waveform turned into a fuzzy mess.
In practice this worked very well. I setup a small 2" arc with a 30VDC smoothed input voltage. This gave me a constant arc which was totally silent, pretty amazing too. At a low volume from my tape player I could hear very clear audio. I turned the volume up a bit and it got louder but very distorted and the arc started to have have problems. I altered the duty cycle via the 10K pot and I found a point near one end of it which gave much better results. It was still fairly quiet but audible nevertheless. I could run at a higher volume input which goes pretty loud though was very distorted. I increase the voltage to about 60VDC input and draw around a 4" constant arc to see if I could get more volume out of it.
It didn't seem to make it any louder and I couldn't seem to stop it from distorting even though the volume on the player was still prettymuch low. Still for my first attempts it did work even with limited results. Latter that night the 13amp variac fuse blew for some unknown reason so after all that I decided to call it a night :) I could sleep well knowing that I have made some limited progress and melted the end off my screwdriver...
More to come soon...