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Class D Switching Power Amplifiers and Power D/A conversion

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Old 27th October 2005, 06:46 AM   #1
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Default My LM386 Speaks "D" ?

With or without output LPF, it's loud n clear.
Most of all, it's cool.

Do you guys consider it is class-d ?
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Old 27th October 2005, 08:04 AM   #2
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Yeah class D -hysteresis modulator, why not?
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Old 27th October 2005, 08:19 AM   #3
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That's one of the coolest missuses of an IC that I have seen lately (although the IC might be too sluggish for anything but subwoofer amplifiers) !

I wonder how well this IC could deal with the freewheeling current of an output filter.
And the output cap should be moved out of the loop (or even better use symmetrical PSU).

Regards

Charles
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Old 27th October 2005, 12:10 PM   #4
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An LM3886 is as fast approx as this one and more beefy. This might be a cheap solution for a self-oscillating small subwoofer amp (switching @ 20 kHz or below) using hysteresis modulation or even UcD.
One would have to try out in order to know if it works. And one would have to run it for quite some time in order to find out how reliable it is.

Regards

Charles
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Old 27th October 2005, 01:51 PM   #5
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Thank you for accepting it as one of the class-d members.

I just want to keep it as simple as I can ! ( I hate board with over 100 componets )

Since I never have a chance to experience the sound of the class-d, so this is my first test drive.

Make no mistake Charles, not an LM3886, it's LM386. Maybe it is older then you . The LM386 is only capable of 500mW (not 500W for the subwoofer), 12v single supply, 8-pin DIP.

At first, I wanted it to operate above 100KHz (LM386 BW 300KHz), but my junkbox was very limited, so it came out 48KHz as a result.

It, with a large enough unknown value junk inductor on output, outperformed the LM386 in class-ab at same output level by 24dB !! (3rd harmonic of 100Hz).

My next attempt will try more decent designs , most likely an open-loop PWM stage using my old friend NE5532, into a power-amp IC from a broken TV, then into a pair of power BJT from
a retired amplifier. All the components are re-cycle junkparts.

I've enjoyed making stuffs out of junkparts very much. I don't think buying new chips or chip sets from NS, TI... and solder them on a PCB suggested by the Ap note have any fun at all.

Have fun DIY.

Regards

Chen
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Old 27th October 2005, 01:56 PM   #6
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Quote:
Make no mistake Charles, not an LM3886, it's LM386. Maybe it is older then you
I am not mistaking it. I just thought that an 3886 would offer a little more "grunt" than the 386.

Regards

Charles
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Old 27th October 2005, 03:06 PM   #7
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Forgive my ignorance but how is that circuit class d? It just seems like a misuse of an opamp

How does it work?



Lawrence
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Old 27th October 2005, 03:19 PM   #8
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It is the basic op-amp circuit of a multivibrator with duty-cycle control.
If you use a power op-amp instead then it is a power-multivibrator with duty-cycle control ! Simple as that !

We have once used this exact circuit with a 714 for a simple battery-powered dimmer. So it is already a classic basically. But using a power op-amp for this purpose is quite novel and cool.

There are still questions about reliability and there should also be an output filter in place but some precautions may be necessary or the inductor might kill the output devices probably.

Regards

Charles
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Old 27th October 2005, 09:22 PM   #9
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I must confess being lazy. But as new comer to class-d, it only takes less then 5 minutes breadboarding this toy.

Charles is absolutely correct. It is nothing but adding a resistor to a multivibrator.

Take a closer look.

The inverting input of the amp is charged/discharged producing waveform similar to a triangular wave.

Except for some bias current, the amp is overdriven and works like a pair of switching transistor (I hope so).

It generates PWM output just like class-d do.

Not perfect, but it works.

I, a lazy guy, want to take the advantage of the power op having futures like overvolt, overcurrent, overtempˇKˇK Most important, the small size, easy to construct, and cheap.

As a new comer to class-d, I think this kind of open-loop circuit maybe the easier path to learn class-d IMHO.

Thank you for your advice Charles. I will stick to small supply voltage as low as 5V that why LM386 comes to play. After some experience is gained, I may try the beefy LM3886. Right now, Iˇ¦m thinking about the measurement. Without a proper output filter, I have problem measure the performance of the amp. So my next step is to see if I can build an inductance meter out of my junkparts.

Regards

Chen
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Old 27th October 2005, 10:53 PM   #10
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sychen0301, way cool!

this litle ****** seems so easy i might give it a shot myself, using the lm4700 or some junk I have lying around..

-Marius

edit: is "b u g g e r" a "bad" word?
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