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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I have been experienting with PWM and FM modulation, and PWM really does sound better (though the FM was just an audio input to the Rt pin of a SMPS IC ).
My controller is a TL494 in classic PWM configuration, the error amp is set up as an inverting amplifier with a gain of ca. 4 (22k:5k). The outputs go to a pair of gate drivers and a GDT. The GDT outputs are in phase and the FETs are connected in the "half-controlled fullbridge" fashion (hope you know what I mean).
But I am getting some weird feedback or what. My audio input is a tape player. When I press play with no tape inserted and turn the volume up, the gate drive signal starts getting asymmetric(??) (to the best of my understanding, I can't connect a scope up because it messes the thing up even more) and the arc power decreases. It decreases even more when I touch an ungrounded metallic part on my tape player (?????).
This happens also when I connect the ground scope probe to one GD output and the signal probe to the other GD output (the driver is ungrounded) and touch the input of the error amp, the output is one huge asymmetric mess. The weird thing is that when I swap the ground and signal probes, the "asymmetricity" of the signal shows in the same polarity on the scope (if that makes any sense).
I guess the output signal gets back to the error amp and is responsible for the mess on the output.
How this can be avoided except of putting the controller in a metal box?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I bet the problem is pickup at the switching frequency feeding back into the audio. I've met this issue while trying to make a Class-D guitar amp, it causes complete havoc.
Try low-pass filtering the audio input as aggressively as you can. As in, use the strongest filter you can make, not yell at it and hit it with a hammer.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I'd guess it was the replay amp on your tape player, picking up the electric field from the topload. The symptoms seem consistent with that. Maybe you should look at modifying the tape player for better EMC?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Well it is a cored transformer speaker, not a Tesla coil.
I'll try putting a wire mesh around the arc, hopefully this will solve the interference problem.
But now I have another problem. The sound quality is pretty good now, like a small speaker. But I still can't call it "hi-fi" because it fails on a sinewave distortion test.
I've described the topology in my first post, the running frequency is 40kHz, using an unrectified gapped transformer.
Here is a sample of how a 220Hz sinewave played through the speaker sounds with varying volume. The warbling effect near full volume isn't real, but the "resonant filter-like" distortion is.
Is the error amp in TL494 suitable for audio? Anyone guess how to decrease the distortion?
Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
Dr. Kilovolt wrote ...
Well it is a cored transformer speaker, not a Tesla coil.
I'll try putting a wire mesh around the arc, hopefully this will solve the interference problem.
But now I have another problem. The sound quality is pretty good now, like a small speaker. But I still can't call it "hi-fi" because it fails on a sinewave distortion test.
I've described the topology in my first post, the running frequency is 40kHz, using an unrectified gapped transformer.
Here is a sample of how a 220Hz sinewave played through the speaker sounds with varying volume. The warbling effect near full volume isn't real, but the "resonant filter-like" distortion is.
Is the error amp in TL494 suitable for audio? Anyone guess how to decrease the distortion?
Here's the sample:
I think you can hardly expect hi-fi like sound at 40Khz... For a decent quality reproduction you need to drive your fly-back at least at 100Khz. Otherwise I noticed that the distortion is also very much dependent on the discharge pattern. Generally is better to have rather low power (low amps) blue (not white) discharge between electrodes. Also the electrodes needs to be in proper distance and need to have a proper shape and cooling.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Mates wrote ...
I think you can hardly expect hi-fi like sound at 40Khz... For a decent quality reproduction you need to drive your fly-back at least at 100Khz. Otherwise I noticed that the distortion is also very much dependent on the discharge pattern. Generally is better to have rather low power (low amps) blue (not white) discharge between electrodes. Also the electrodes needs to be in proper distance and need to have a proper shape and cooling.
BTW: what is your circuit?
Hi Mates, the distortion at 200Hz is not due to low switching frequency, and 40kHz should theoretically reproduce all the frequencies up to 20kHz. Of course there is no interpolation so it will distort a bit, but my distortion of concern is of a different nature.
If you use the right driver, increasing power only increases volume and shouldn't increase distortion. My speaker fills the room with a comfortable listening level just fine (I think the arc is what you could call "white", maybe around 100 watts in the arc?)
I described my circuit in the first post: It's a TL494 PWM, the outputs are amplified with gate drivers (2x UCC3732x) and go into a GDT. The GDT drives two transistors in phase (so only one pulse of the two PWM outputs is really used), it is a "half-controlled fullbridge", the other 2 transistors are replaced with diodes, like D) diagram in this picture: If the transformer is unrectified, it must be gapped.
I found this sounds way much better than a standard half-bridge when using PWM.
Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
Dr. Kilovolt wrote ...
I described my circuit in the first post: It's a TL494 PWM, the outputs are amplified with gate drivers (2x UCC3732x) and go into a GDT.
Ahoj Honzo, I'm not very experienced in the GDT design but I always had troubles to drive GDT in such low frequencies. Do you have proper ferrite material for such a low freq? Did you check your wave forms behind the GDT, ideally using two channels osciloscope? I think that using a GDT in audio modulated plasma devices is in general big source of distortion in case you use PWM. Can't you try driving it without GDT? I mean what about to try extra power source (e.g. 12V battery works great as a testing approach) and optically coupled transistor (optoÄlen) for one of the gates? For such a low freq. this could be very nice solution. You don’t need to change so much in your driver and at least you will see if the GDT is involved in the distortion. Just an idea...
Cheers and don't forget to send us some pictures/video of the setup!
Registered Member #1956
Joined: Wed Feb 04 2009, 01:22PM
Location: Jersey City
Posts: 172
Dr. Kilovolt Can you tell me what are you using for electrodes and the distance between them? Switching to tungsten electrodes that I picked up from a TIG welder (3 mm dia.) helped me a lot. Can you hear any hiss from your system with no audio input?
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Singing arc/plasma speaker weird problems
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