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Class D Switching Power Amplifiers and Power D/A conversion

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Old 2nd June 2013, 02:17 AM   #1
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Default attempting to prototype an amp

Ive repaired class D amplifiers for a few years now, but I decided to attempt at building one. I eventually want to build one thats 1000w+ but for now, I have to start somewhere.

I found a PDF on the web from a university/student that used delta sigma modulation.

I tried building his modulator, and I couldnt get it to work at all. Ill post a gif I ripped out of the PDF. I used LF356N and LM311s instead of what he has listed, Also using SN74AS74 as thats what I have on hand.

any ideas as of why? the 100nf caps on the + pins of the op-amp I know arnt right, I tried biasing these to 2.5V. and it sorta works, but it really doesnt.

any ideas? or does someone have a better schematic for a crystal based modulator.

As far as what I am building, I am building a single supply rail bridge amplifier. using a pair of IRS21844 driver ICs, driven from the sigma-delta modulator. But the modulator isnt working. lol.

I dont want a self-oscillating design, I want an external crystal/oscillator based design so I can sync it up with the power supply, and MCU I will be using to control the protection, etc... But ill worry about that later. Thats why I chose his method at first as it uses a crystal oscillator.

Thanks.
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Last edited by mbates14; 2nd June 2013 at 02:19 AM.
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Old 2nd June 2013, 05:16 AM   #2
gootee is offline gootee  United States
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I didn't look at the circuit design but I noticed that there are no decoupling capacitors shown. If you want it to have a chance of working, you would need to add decoupling caps from each chip power supply pin to ground (preferably directly to the chip's load's ground). if you don't want to calculate the capacitances needed, you should be able to try a 10 uF electrolytic in parallel with a 0.1 uF X7R ceramic, from each power pin to ground.

Briefly looked at the 100nF caps from opamp +input pins to ground, per your comment. It is "POSSIBLE" that they are meant to be that way. I haven't analyzed the circuit, even in my mind, really, but, those two (U1A and U1B) opamps remind me of differential integrator opamp circuits, except for the unequal capacitances.
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Old 2nd June 2013, 07:27 AM   #3
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I built that uni delta sigma amp once, it was the noisiest pos ever.

Then i discovered UcD and i was hooked, no other class d can compare to UcD in sound quality.

Pre filter feedback class d is designed to run into one load impedance only, ie you have to change thew output filter capacitor to get a flat frequency response for a given load impedance.

Post filter feedback self oscillating class d(UcD) does not have this problem.
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Old 2nd June 2013, 02:13 PM   #4
gootee is offline gootee  United States
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Not being a class d expert, the questions arise:

What would it buy you, to be able to synchronize the oscillation with the power supply and mcu? And if there would be some significant benefits from doing that, then why couldn't you still synchronize them, without a crystal-driven oscillation? Or, assuming that you could, what is nature of the trade-off space?
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Old 2nd June 2013, 02:56 PM   #5
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I recomment starting simple, take a single channel gate driver like the TC4422, add a LM311 comparator and the LC output filter, then add the feedback loop around that, either pre or post filter feedback. This will make a simple low power class d amplifier but demonstrate the basics of a self oscillating class d amplifier.

Here is a circuit i am working on, it is based on the Philips UcD concept along with snippets of technology by Anaview in Helsingborg Sweden.

Note that this circuit is not for beginners.
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Old 2nd June 2013, 04:21 PM   #6
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See thats just it, I dont want to do a self oscillating one. Maybe later, but I dont want to for now. I come from a repair background, not an engineering background and its possible that you cant cross breed those, but from my experience the self oscillating ones are the WORST ones to repair when things go awry.

the fixed frequency ones tend to work much better and dont give issues when repaired.

But I assume the self-oscillating one is required under multiple load conditions? and a fixed-frequency amplifier only works under a single load?

Lets say I did do self-oscillating, How can I make this work in a full bridge setup? I built my output board already setup for full bridge using the IRS21844, driving a pair IRFB4127s on each side, and its setup for single rail. So i would need a blocking cap for a single channel?

Last edited by mbates14; 2nd June 2013 at 04:23 PM.
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Old 2nd June 2013, 06:41 PM   #7
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For fixed switching freq (clocked) and for Pre filter feedback self oscillating, you need to change the output filter capacitor if you change load from 4 ohms to 8 ohms to keep the frq response flat.

Post filter feedback self oscillating remedies this, you can apply 2-16 ohms and have the frq response remain as flat as on a class AB analog amp, if anything the switching frequency might change alittle.

I've built a clocked class d, ran it at about 250kHz and it sounded ok, clocked class d has its drawbacks that self oscillating doesent have, something with distortions and stuff.

A video: Improved feedbackless Class D amplifier - YouTube
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Old 2nd June 2013, 09:48 PM   #8
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Ok, I built a quicky circuit just goofing off.

I built a R/C triangle oscillator, and used another op-amp to feed that into along with the audio signal. I then take this output into a buffer, then into the IRS21844, using 15V as the voltage for this IC.

I take the low-side output streight into a filter and into my 32 ohm headphones, both headphones are in parallel-mono, so thats 16 ohms.

It is working, sounds ok, but I am getting a crapload of carrier in my filter.

So what is the filter calculation for Fc of 150khz, load of 16 ohms? I can actually feel the headphones heating up on my head because of the carrier that I obviously cant hear. hehe.

Thanks.
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Old 2nd June 2013, 10:09 PM   #9
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At that low switching i'd say 50-100H inductor and a around 1F cap.

However since you are also using a single 15V psu, you need a dc blocking capacitor in series with the headphones, otherwise the 50% duty cycle makes it so that your 16 ohm load always see 7.5V DC, hence the heating you experience.
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Old 2nd June 2013, 11:05 PM   #10
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Yea, I am using a blocking capacitor, I have them on the return side of the load, 1 between the load and +15v, and the other one between load and ground, basically to bias the headphones at 7.5V. the other end of the load is going to my inductor.

I am using +/-15v for the op-amps, +5V for the buffer in between the op-amps and the IRS21844. Then I am running the IRS21844 at 15V. Using a 1K ohm resistor between the PWM output op-amp, and the buffer so i dont overdrive the buffer and blow the logic IC. I am using the buffer inverter so I can chain into another driver at inverted signal for full bridge setup. Otherwise i would just route the PWM direct to the driver IC.

I am not targetting a full range setup, just a subwoofer amplifier so full range isnt a necessity, hence why I am using a lower switching frequency.
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