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Class D Switching Power Amplifiers and Power D/A conversion

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Old 10th July 2006, 11:35 PM   #1
BWRX is offline BWRX  United States
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Default My 10W Mono Single-Ended modules - D10.1

In keeping with the whole low power Tripath amp craze, I decided to design some modules that can compete with the Tripath amps but have a few key differences that make them more DIY friendly.

This is not a discrete amp. It uses a chip, however the chip comes in an SOIC8 package which is much easier to solder than the 36 pin surface mount Tripath jobbies. Since this is a single ended amp it requires +/- rails. This particular chip uses up to 12V rails. Being single ended with split supply rails also gives it the advantage of not needing any input or output coupling caps. The amp is self oscillating, which is why the chip comes in such a small package, but it does not use quite the same scheme as UCD.

The number of parts needed to build a working amp of this type is quite low. I've added some extra parts to the basic design so the modules you see in the pictures could probably be reduced to half the size if I did away with the stuff I added.

The on-board supply section is similar to what was implemented on the Ref-T. There are two 680uF Panasonic FM caps per rail, with an optional ferrite bead in between for filtering. It might sound better without it but I have yet to try that.

There are indicator LEDs for each power rail to show that each rail is powered up. They're also there to supply a constant load for the rails and to drain the supply caps at a constant current when the power is turned off. Right now I have them configured to draw about 5mA and the LEDs are still really bright, so I may decrease that a mA or two!

I used the same toroid inductor that comes with the 41Hz amp3 and amp6 kits because it's what I have available and I can wind them in my sleep by now (I still hate doing it though...).

There is no turn on thump with this circuit but there is a small turn off pop that's nothing to worry about.
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Old 10th July 2006, 11:47 PM   #2
BWRX is offline BWRX  United States
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The parts values I chose for the circuit make the amp oscillate at approximately 660kHz. Using such a small chip has its advantages when it comes to optimizing the PCB layout. The switching node distance between the chip's output pin and the inductor is only 8mm long, and the high current switching paths were kept as short as possible. The power supply and output ground points were kept as close as possible to each other. As such, the film output cap is hard to see because it is nestled in between the supply caps

The board itself is about 54mm x 35mm and is a two layer design with 1oz copper.
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Old 10th July 2006, 11:51 PM   #3
BWRX is offline BWRX  United States
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And how does it sound? So far I'm liking it just as much as my Ref-T. The channel separation is very good because of the dual mono implementation even though both modules in my setup share the same supply. The detail of string instruments sounds excellent much like with the Tripath amps.

Because of the small size and mono output I will eventually upgrade to an active line level crossover and have one amp module per speaker I just need to find some good 9V AC wallwarts so I can make some separate supplies for these guys.
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Old 11th July 2006, 12:20 AM   #4
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Which chip does it use?
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Old 11th July 2006, 12:30 AM   #5
BWRX is offline BWRX  United States
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Wouldn't you like to know? I want to see if anyone knows what it is before I tell.
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Old 11th July 2006, 12:32 AM   #6
phn is offline phn  Sweden
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Very nice.

Back in the late 1980s I soldered PCB (ICs and the lot) for computers used on oil tankers. I didn't put much thought in it back then. But later I thought that was a big deal. Imagine a single cold joint! It must cost a fortune to have a tanker stuck in some port.

The point to this is that I still am afraid I'm not up to soldering those tiny SMTs. I will have to give it a go eventually. But I'm not rushing it.
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Old 11th July 2006, 12:36 AM   #7
BWRX is offline BWRX  United States
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Don't be afraid of them. They're much easier to solder than through hole parts once you get the hang of it. The smallest parts on these modules are 0805.

I dread having to solder on the large caps, the inductor, and the wires more than any of the surface mount parts.
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Old 11th July 2006, 01:11 AM   #8
davidcw is offline davidcw  New Zealand
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Default 10w Class D Amp

Hi,
Is this the MP7720 from Monolithic Power?
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Old 11th July 2006, 02:58 AM   #9
BWRX is offline BWRX  United States
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Default Re: 10w Class D Amp

Quote:
Originally posted by davidcw
Hi,
Is this the MP7720 from Monolithic Power?
davidcw
Well done sir! I'd give you an amp as a prize but I had a hard enough time getting samples of the chip.
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Old 11th July 2006, 05:40 AM   #10
Pano is offline Pano  United States
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Hey Brian,
I see you've been up to no good.

Cool board! I don't know that chip. How do you keep it cool?

As for toroids, you might try the 2100 series from Jameco. Price is good, so is the sound. Too big to fit the Sonic boards, but should work well for you.
They don't kill the high RF harmonics like some cores, but they aren't bad. Only problem is wire thickness. You'll want bigger holes on the board. But that big wire and short windings will give very low DCR. And those inductors are NOT going to saturate!

Good work on the new board. Keep us informed.
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