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Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
I recently constructed a circuit using the skeleton of this schematic as a guide. I used the same IC, except, with a 2.2k resistor alone, and a .0015 uF capacitor instead of the values shown. I also changed the audio cap to a 4.7 uF capacitor. I also removed the 4 UF diodes, and removed any associated GDT parts including the zener diodes. The square wave I measured coming out was 333 kHZ.
As for the gate drive transformer, I did away with it and replaced it with a FAN7382, an easy to use high-low side IC. I used a 30 ohm resistor, a Schottky diode, and a 10 uF 16 V tantalum capacitor for all of the components shown on the datasheet. i used 10 ohm gate resistors, (from output to gate) and 10k gate to source resistors.
I used a 12 volt regulator for all the logic needs, and used TO-220 mosfets for the half bridge. I left out the 10,000 uF capacitors, (I'm not sure if they're needed or not?) and used a low pass filter consisting of two .1 uF caps in parallel in parallel with the speaker, and a 30 ohm resistor in series with the speaker.
Scoping around, I noticed the wave going to the speaker was a triangle wave. It worked, for the most part, but, the issue that arose was audio quality and loudness. Adding more voltage just distorted the sound even more, and drowned out any music. The actual sound quality at a certain volume is actually pretty decent; but, it's also really quiet.
Is there any way to increase the loudness and quality of the music coming out? Is there anything I possibly left out, or need to change? I appreciate the help a lot!
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Bipolar PWM has three voltage levels: plus, minus and 0. the waveform goes +,0,-,0, etc. The plus and minus on-times are equal, and the control voltage modulates them equally. The result is a waveform symmetric about zero, with no DC component, so it passes through GDTs and SMPS transformers. When rectified in the secondary rectifier, the negative pulses are made positive, so it reverts to regular PWM, and the supply's output voltage ends up proportional to the control voltage.
But since the waveform has no DC component, connecting it straight to a speaker will do nothing. That's why you need unipolar PWM, which just goes +,0,+,0, etc, with the control voltage modulating the ratio of + to 0. This won't pass through a GDT, so you have to use an IR21xx type driver chip.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Steve McConner wrote ...
Bipolar PWM has three voltage levels: plus, minus and 0. the waveform goes +,0,-,0, etc. The plus and minus on-times are equal, and the control voltage modulates them equally. The result is a waveform symmetric about zero, with no DC component, so it passes through GDTs and SMPS transformers. When rectified in the secondary rectifier, the negative pulses are made positive, so it reverts to regular PWM, and the supply's output voltage ends up proportional to the control voltage.
But since the waveform has no DC component, connecting it straight to a speaker will do nothing. That's why you need unipolar PWM, which just goes +,0,+,0, etc, with the control voltage modulating the ratio of + to 0. This won't pass through a GDT, so you have to use an IR21xx type driver chip.
Does that mean a 555 timer based version would work?
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Class D amplifier help
