Class D HPA - Xmas Project #2

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Ray P
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#1 Class D HPA - Xmas Project #2

Post by Ray P »

Here's another project I've had lurking on my desk too long. It's a headphone amp project that is looking to repurpose a cheap Chinese Class D amplifier board board that uses the STA333Bw chip. The original board has a USB input (44/48KHz) and is rated at about 20WPC.

Today I've been stripping back the board to get it ready for it's new use. I've removed the 'ordinary' electrolytic caps and 5V regulator ready to replace them with decent parts that are on order from Mouser. I've also removed all the loudspeaker output filter stage as this will be replaced with a new stage based on stepdown trandformers.

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If all that works out well I then have the option of using the STA333BW's I2S input with a higher quality/resolution USB module (if I can get my head round the register settings in the datasheet) to feed it better quality data and make use of the STA's higher rate internal processing capability.

Here's an image of an original board;

Image
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Ray P
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#2 Re: Class D HPA - Xmas Project #2

Post by Ray P »

The stepdown transformers are wound and on the way.

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Ray P
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#3 Re: Class D HPA - Xmas Project #2

Post by Ray P »

Some more progress today - installed the replacement power supply caps with (Panasonics, also increased from 470uF to 1200uF) and USB section decoupling electrolytic.

I also fitted the output coupling caps, which are 220uF Nichicon Muse bipolar types. These are installed using the PCB pads from the removed output inductors - I drilled through the board in the centre of the pads and sleeved the leads of the Muse caps with teflon tubing to ensure no contact with the ground pour on the back side of the board.

I just need to install a 5V regulator to replace the cheap one I removed and I can verify that I've not killed the board during surgery. All being well, I just need to wait for the output stepdown transformers to find out how it sounds.

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Ray P
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#4 Re: Class D HPA - Xmas Project #2

Post by Ray P »

Installed the low-noise 5V regulator today;

Image

It appears that the board has survived my butchery, powered up with a 15V SMPS I'm seeing 5V from the regulator and if I use my smartphne as a source (via a OTG connection) the board is seen and music apparently plays.

Image

The stepdown transformers are on the way so just have to be patient now.
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Ray P
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#5 Re: Class D HPA - Xmas Project #2

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I thought I was making good progress with hooking this up but when I gave it a test listen I just heard white noise - thats when I noticed that, when removing the parts on the PCB that aren't required, I got a bit carried away...

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I need to reinstate some 100nF 0603 caps, C18 and C32 in the extract from the STA333 datasheet below. Doh! :oops:

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Ray P
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#6 Re: Class D HPA - Xmas Project #2

Post by Ray P »

I reinstated the capacitors today and I now have music, albeit heavily distorted, so still some work to do.
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Paul Barker
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#7 Re: Class D HPA - Xmas Project #2

Post by Paul Barker »

Well, its always a case of overcoming adapting and eventually succeeding. Glad youve shared what youre doing even though Im not in that method. Im in the time warp I came on board at and glad to stay there. I havent finished experimenting at that level. But I use little boxes of digital audio trickery for my music these days, and my phone accesses a very large mustic resource without drama.

I may build a mofo for the van one day. Probably not though, more likely to build a 6em7 amp for the van. Ive always had a dream to have a valve amp for van entertainment. But noone gave me a round tuit.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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