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Class-D audio amps

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Steve Conner

Sun Mar 04 2007, 08:19PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hi all,

I always wanted a Class-D amp but I'm far too lazy to actually build one. They are easy to get "sort of" working, but getting low distortion, unconditional stability, and flattish frequency response is far more difficult. So I ended up buying a BP4078 400 watt power amp module and the matching SPS30 switched mode power supply from these guys

I could go on for ages about these things, but in short, I'm amazed by the sound quality from it. It's not a subwoofer amp: it covers the full audio range with no problems. Having listened to it on hi-fi speakers, I doubt I could tell the difference against any other solid-state amp in an ABX test, and it gives crazy amounts of power from a very small and light package with very modest cooling requirements. It would have taken me months to make something this good myself, if I could manage it at all.

For the geeks among you, the power supply contains a half-bridge of IRFP460s running at 85kHz, and the amp module has a half-bridge of some TO-220 MOSFETs (that I can't identify without tearing it completely to pieces) driven at 320kHz by an IR2110 half bridge driver chip.

Anybody else play with these things?

uzzors2k

Sun Mar 04 2007, 10:30PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Not yet, but I've had a class-D subwoofer amplifier on my to-do list for a while. I have built a smps for my class AB amplifier though, +/- 18 volts and +/- 36 volts. It shoud be good for some 250W, given the transformer lives up to its ATX PSU rating. Its unregulated so far, as I'm still playing with the PWM, various GDTs and drivers.

Steve Ward

Mon Mar 05 2007, 03:59AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Cool! Ive seen some other ones like that too. How much does it cost? And how does it do as and instrument amp (how does your bass sound through it? and is it loud enough for playing out?). So far the only amp ive built is a little stereo 60W class AB solid state... i use it for hours every day (possibly my most useful project ive ever built). But it gets hot, and is really big for its power.

Im still tempted to just build my own class D amp... perhaps if i had more time and money i would.

Dr. Slack

Mon Mar 05 2007, 08:27AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

I'm impressed, with price as well as performance.

A question. I've always threatened to build myself an "agile power supply", and obviously the least effort way to do it is to use a commercial DC coupled amplifier. Will these modules go down to DC with the judicous shorting of any coupling caps in the input and feedback networks, or is the 8Hz corner immutably built into the switching algorithm and then buried in an ASIC?

Steve Conner

Mon Mar 05 2007, 10:15AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hi guys,

Steve: Both modules together cost about 200 GBP. It seems expensive, but it would have taken me a lot more than Â£200 worth of my time to make one... My plan is to build a tube preamp for it, or maybe even try to use JFETs and do without tubes altogether. I've no doubt that it will be loud enough given that I currently use a 60 watt tube amp and the band still ask me to turn down

Neil: I'm sure it'll be easy to modify for DC coupling, but I don't feel like voiding the warranty to find out just yet. As far as I know, it's just a regular PWM modulator with no fancy ASICs a la Tripath etc. Three of these things modified for DC coupling (or at least very low frequencies) would make a great three phase power source for experiments.

Wolfram

Mon Mar 05 2007, 12:19PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

I've played with some smaller class-D amplifiers (5-50W per channel) from LCD tv mainboards. The audio amplifier part is usually quite easy to seperate from the rest of the board with a hacksaw. I'm amazed at how powerful they are for their size, and they usually only get slightly hotter than room temperature at full output power.

Marko

Tue Mar 06 2007, 01:07PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Maxim produces some pretty class D amp ASIC's for up to 50Watts, and will happily send some samples of new IC's.

This is a little SOIC wich can give out 50W continuous power with outputs paralelled!

Maybe you couls stack some additional power stages onto outputs but I don't think anybody would want more unless he is a real power freak!

I don't know about the sound quality (it's a filterless amp) but I feel it would work much better than if we used some IRFZ44's and TL494...

Steve: i'm just curious, what is the 'brain' PWM IC used in your beast amp? Surely there must be something inscribed on it?

Mike

Tue Mar 06 2007, 11:00PM

Registered Member #58 Joined: Thu Feb 09 2006, 05:40AM
Location: Tri-Cities, Washington, US
Posts: 317

haha, funny this thread should be made, just today i received the maxim engineering journal and the first artical is on Class D amplifiers: fundamentals of operation and recent developments. Looks like a good read and it also explains the basics which is nice for people like me :P

Steve Conner

Tue Mar 06 2007, 11:57PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hi Firkragg

I looked at the board and all the ICs on it are:

LM6172 (high speed low-distortion dual op-amp)
LM319 (high speed dual comparator)
N5532 (dual audio op-amp similar to NE5532, I think)
NE555 (You'll never guess what this is)
IR2110S (High speed half-bridge driver)

I guess the PWM brain is based around the LM319.

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