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For the sake of audio
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PostPosted: Wed Jan 20, 2010 3:03 am 
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Goat

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I'll politely apologize for saying the F word, out of politeness. Engineering makes you curse like a sailor, especially when I've been working 12 hour days/weekends on new products and stuff that everyone wants done by yesterday. But I guess this is the internet, and there's probably a chance a set of young ears may read this...

Anyway, I figure the best explanation of why to avoid building your own class D... is to share a narrative.

I've thrown together a few things built using TI's integrated audio-input class D chips, nothing over 10 watts/ch, with part values stolen directly from the datasheets. Which doesn't really count, and the applications (a voice amplifier which ran off a wheelchair battery for a lady with MS, and a solar powered radio for camping) weren't exactly hi-fi. But they worked fine.

But before that (before I graduated EE, even) I decided to build a class D amplifier using a HIP4080. And I set my ambitions quite low, so I thought - I decided to start off making a subwoofer amplifier, which kept the switching frequencies low. I did my research, ordered over $100 of parts from Digikey, scavenged old power supplies/etc for heatsinks/inductors, etc... and protoboarded up a poor clone of the HIP4080 evaluation board. To power the thing, I used a transformer/full-bridge/capacitor power supply. Didn't have an autotransformer so the power supply was all or nothing.

And the first time I turned it on? It blew the freakin' output FETs, almost instantly. And I didn't order more! So I scavenged some different FETs, and set the delays as high as they would go, thinking the old ones had blown because of shoot-through or something. Turned on this amplifier, everything seemed fine... put a load on it, started applying audio, and popped the FETs again. So I decided to 'scope the thing, started looking at waveforms, and saw that every thing I probed (even the damn grounds) was ugly. At this point I realized that I was clueless and had undertaken a project far beyond what I was capable of. Hauling apart a random E-core inductor and rewinding it with new wire until it measures "right" on a LCR bridge means absolutely frig all, because the LCR bridge doesn't tell you when it saturates. After wasting months of time I gave up, and the thing never was connected to a speaker.

I wouldn't say it was a total waste, because it did humble me a bit.

Since working in the "real world", I've done a few off-line SMPS designs with guidance from other engineers. Learned a ton about things like snubbers, magnetics design, stability testing, proper compensation... and how to properly lay out a power supply so that the switching section doesn't ruin the analog section. Every last one of these things is something I got wrong with the HIP4080 experiment, and class D amplifiers are really not all that different from SMPSes. And in the real world, nobody builds a SMPS and has it work perfectly the first time around - there's always something wrong with the design the first time around that you didn't expect.

Now I'm not saying don't build your own class D... I'm saying budget yourself tons of time, spare parts, multiple iterations, etc... if you plan to do it. By the time you're done you could have easily bought yourself several commercial, decent-quality class D's, but you'll have learned plenty. If that doesn't interest you, go with an "easier to build" thing like the aforementioned TI parts, a Tripath based amplifier or something of the sort. Or throw a couple UcD modules in a box.


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PostPosted: Wed Jan 20, 2010 3:06 am 
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Goat

Joined: Sat Mar 04, 2006 5:26 pm
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Location: Nova Scotia
cheapJack wrote:
PS: what else is not made in China? Walk in Walmart stores, you will find over 90% stuffs, from electronics to hardwares, are all "made in China". That's the realworld we got to cope with

There's two kinds of Chinese things:

- Things which are built in China, usually by non-chinese companies, which are built there because nobody's willing to spend $200 on a union-made, north american alarm clock.

- Stuff that is outright counterfeit garbage, barely works if at all. Every last part of it is created as part of an elaborate lie. Like fake iPods, and the aforementioned class D amplifier.


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PostPosted: Wed Jan 20, 2010 3:44 am 
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Benjamin
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gmarsh wrote:
Now I'm not saying don't build your own class D... I'm saying budget yourself tons of time, spare parts, multiple iterations, etc... if you plan to do it.


Buy lots of good 'scopes and probes. Learn how to use them properly. Read a ton of information first. Maybe that will reduce the number of iterations and save tons of blown parts.

Jocko


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PostPosted: Wed Jan 20, 2010 2:48 pm 
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Jocko Homo wrote:
Buy lots of good 'scopes and probes. Learn how to use them properly. Read a ton of information first. Maybe that will reduce the number of iterations and save tons of blown parts.


Hi.

My bottom-line question for the reason of building class D audio amps as a diy music lover, like yours truly, assuming one gets the time, resources & patience to get ready to build one:-

How musical a class D audio amp would sound ???????

PS: maybe I am a very old school type, tube is still my best bet for most musical sound regardless of price.

_________________
"If the facts don't fit the theory, change the facts" quoted Hans Albert
Einstein.


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PostPosted: Wed Jan 20, 2010 3:02 pm 
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gmarsh wrote:
Stuff that is outright counterfeit garbage, barely works if at all


Hi.

Even passports can be "counterfeit". Why not electronics?

There is no free lunch. One should not expect miracles for paying pennies for it. You should expect to get what you pay. That's why nodays TVs break down so frequently as most most of them are all built like tin boxes off shore, in China or in Mexico by the original manufactuers. In return, one saves a bundle upfront. This is realworld economy.

Don't complain. Just play smart.

c-J

_________________
"If the facts don't fit the theory, change the facts" quoted Hans Albert
Einstein.


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