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Buck Converter and Audio Amp Design Advice

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Turkey9

Sun Feb 27 2011, 02:59AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Hello. I'm thinking about building a buck converter to regulate a 53V input to a 35V output. This is for a audio amp and I have a nice transformer that happens to output too much voltage. I don't want to add another transformer to step down the voltage as it would take up too much space.

Should I try and use one of Linear's Buck controllers or would it be better to try to design my own. I have plenty of MOSFETs to use so that's not really an issue. I need the converter to output at least 4.5 amps as that is the max output from my transformer.

As far as the inductor, would it be better to use a toroid core than anything else? I have some of those yellow cores out of ATX supplies that I could use. I also have some 1/2 in by 2 in cylindrical cores from micro-metals. I imagine the inductor is going to have to be pretty large in order to stand up to the about 150W output.

Are there any other clever ways to decrease the transformer output without using other bulky transformers and without compromising the output power? Thanks!

genious 7

Sun Feb 27 2011, 03:31AM

Registered Member #2887 Joined: Sat May 29 2010, 11:10PM
Location: Panama City, Panama
Posts: 107

Transformers aren't bulky at high freq. You could use those ATX supply transformers at their inteneded frequency; altough you will probably have to rewind them. Don't quote me on the number of turns, as I have no idea of the formulas. Just think, those transformers are supposed to stand min 300W the computer uses, it will surely tolerate your load.

Bear in mind, though, driving them would require a more complicated setup than just plugging in. It would be kinda:

53V Trans ------> diode bridge --------->inverter --------->Atx Trans ------->Fast Diodes (Atx Supply Aggain) -----> Filter cap & Inductor -------->Your Amp
.............................................. ............ / \................................................. .................................................. ....................|
................................................. ...........|.................................................. .................................................. ....................|
................................................. ...........|.................................................. .................................................. ...................\/
................................................. ............-------------------------------------- ------Regulation (IF a stable output is wanted)

IF you decided to use this topology, forget the 53V trans and go directly from mains, which would use a design almost identical to the power supply, changing the transformer (rewinding), and the resistive divider for error correction (As well as overcurrent detector if you want it)
Perhaps a more experienced member will have a simpler method to suit your needs.

IamSmooth

Sun Feb 27 2011, 05:18AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I used a ferrite E core for my buck converter. It worked very well for the 30-50khz frequencies I tried. If you are interested I can dig up the source that I purchased it from.

Turkey9

Sun Feb 27 2011, 06:08AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

That would be nice if you could find that info. Maybe not just for this project.

As far as the omission of the main transformer, I kinda like that idea. Didn't occur to me at all that a converter would kind of render the transformer unnecessary. I can use the transformer for a sub amp later on.

Can a buck converter be ran straight off mains level and still have good efficiency? Would a normal SMPS be better in this application?

If I do go for a SMPS, what kind of design differences would there be for a split supply? Can I just replicate the windings for the negative supply and return the voltage for the positive supply for the voltage feedback?

I know this isn't an audio forum, but what kind of noise or interference could I expect on the output of a SMPS? Sorry for all the questions, I get excited when I see a new project coming on.

Thanks!

Steve Conner

Sun Feb 27 2011, 08:37AM

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

Well, it might not be an audio forum, but I'm building more amps than Tesla coils nowadays.

The simplest solution would be to get a different amp circuit that can handle 53v.

Second simplest would be to get another transformer that puts out 35v. I have a heap of transformers of not quite the right voltages, but I still had to spend 75 bucks on one for my latest project. :(

Once you introduce a buck converter, you've opened a whole can of worms with switching noise getting into the audio. You might as well run the whole thing off a SMPS or two. Note that you can not run a buck converter straight off the mains, because audio amps, like most circuits, need to be isolated from the mains. You have to use some sort of transformer, whether HF or iron cored, to get the galvanic isolation.

Many successful SMPS based amps have been made. If you do make your own buck, my tip would be, use an iron powder toroid for the buck inductor, and mount the whole converter in a shielded box with filters on the inputs and outputs.

IamSmooth

Sun Feb 27 2011, 05:55PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Why don't you rewind a transformer with the proper current for the voltage that you need?

Turkey9

Sun Feb 27 2011, 08:32PM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

the main reason that I don't want to get another transformer is the expense. I got a nice 340VA toroidal transformer for Christmas but wouldn't be able to spend that much money on my own. I have all the parts needed to build a SMPS and more so I figured that would be a good alternative.

@ IamSmooth, I can't rewind the transformer easily because it is epoxy filled in the center to allow for an easy mounting point.

What are the major factors determining a max voltage on an amp? Is it the rating of the transistors, or the resistors that determine the quiescent current draw? The design I want to use is really simple but says it has a max of 42V rails.

Ash Small

Sun Feb 27 2011, 09:15PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I don't suppose you have three small 12V transformers that you could connect with outputs in series?

They may be easier/cheaper to obtain than one 35V transformer, but they'd need to be identical.

I'm obviously thinking re-cycling here.

EDIT: post corrected at 22.25 GMT

Steve Conner

Sun Feb 27 2011, 09:31PM

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

Can you post the amp design here for a critique? :)

My latest one runs happily off +/-48V

and the one I built before that ran off +/-55.

Voltage rating of the transistors is the main thing, they have to be rated equal to the sum of the two rails, plus some headroom for safety. You can usually uprate an existing design by swapping the trannies for higher voltage ones.

But there are some more subtle things, if you crank too much voltage, the amp may try to deliver more power to a 4 ohm load than its heatsinks can handle, so it will overheat and blow its power trannies. Also if it has a protection circuit, higher voltage may make the protection trigger prematurely and be unable to drive a 4 ohm load at all.

That's what dictated the rail voltage for the amp I linked above: the electronics themselves can stand up to +/-70V, but at that voltage they'd want to put out something like 400W, which would need 4 pairs of output devices with forced air cooling.

Turkey9

Mon Feb 28 2011, 02:23AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

I'm using the design on Eliot Sound Products... the info and construction phase is here

I'm not using the main output transistors that are listed in the schematic, i'm using these:

and

I'm doing a rebuild of this amp so these transistors have run in the circuit for three years without any issues. There are a few differences in the rebuild and my original, the original is the old version on ESP and I'm rebuilding with the updated one.

I also changed the name of the thread to cover the tangent of amp design.

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