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currentkills91

Thu Nov 03 2011, 09:41AM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
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Posts: 265

Okay so this whole thread has been changed, but here goes. (It originally asked for PCB help on a TDA7293 amplifier, but it was just too much money for the parts)

So, I'm trying to find a decent amplifier I can build for use in my car, I don't wanna take the easy way out and just buy one. This amp needs to supply anywhere between 100-200 watts of power...but no less. I was thinking something like a MOSFET amplifier or something along those lines maybe, but I'm not sure...

I've done a fair share of google searches and can't really find anything. Anybody have any ideas?

Oh yeah, this amp will be powering a small 10 inch sub, and wont see more than around 125Hz, just in case that info is needed.

Experimentonomen

Thu Nov 03 2011, 11:43AM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

At those power levels and no knowlege what so ever i would NOT suggest building yourself. Also its cheaper to just buy one since in a car you need a DC/DC converter that converts the 12VDC in the car to atleast 2x50VDC split rail for 100-200 watts into a 4 ohm load.

I have need designing and building amplifiers for a while so i can tell you that diying an amp at these power levels is neither cheap, nor easy, and that voltage booster needed in a car application complicates things even further.

So save yourself the headache and just buy a car subwoofer amplifier in a car audio shop.

currentkills91

Thu Nov 03 2011, 05:37PM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
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I've built small transistor amplifiers before, but nothing really over 20 watts. I also already know about the DC/DC converter and I am in the process of building one now. I'm not gonna just buy an amplifier, because buying one doesn't allow me to have the fun and excitement of building my own. Plus, if I'm ever gonna learn don't you think I should build one myself? (P.S. - money is no option...i have more than enough)

Experimentonomen

Thu Nov 03 2011, 07:44PM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

If you insist.

Heres a design for a high power subamp i am working on:

do NOT NOT NOT build this one!!! it is not yet tested!

Heres one that is tested and working:

I also have another one that uses bipolar outputs, these tested ones are only 50 watts output per channel but you get the idea.

The trick to a high power amplifier is using manu output transistors to stay within their safe operating area or SOA for short while still allowing for high enough rail voltages for the desired power output.

Heres a mosfet amp i actually measured to 214 unclipped watts into 4 ohms one channel driven, with 2x50V rails:

This is the schematic for the version of that mosfet amp in these videos:

and

And yes that heatsink was way way way undersized for a amp of that power, but it was only a test for fun.

currentkills91

Thu Nov 03 2011, 08:34PM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
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Thank you very much!! I have a few questions though, why use IRFP450's? They have an Rds of 400 mohms... Also, do the MOSFETs have to be 500V FETs, or could lower voltage FETs be used?

Or, could I use 2 of -These- instead of 6 of the IRFP450's, as these are 500V 60A FETs and only have an Rds of 100 mohms? I dont know...I'm far from an expert x.x

Okay so that link isn't working for some reason, the part number is IXFH60N50P3...

Experimentonomen

Fri Nov 04 2011, 06:56AM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

You can NEVER use a single pair for power above 50 watts at supply rails above 35V, check the SOA curve in the datasheet for the max current you can draw through the device at a specific voltage between drain and source.

A fets current rating is when its used as a switch, that is fully on or saturated as its called, where the Rds(on) is low enough that the Vds is only a volt or less. In the linear region you have to pay attention to the safe operating area for DC operation.

I used IRFP450 because those were what i had at hand at the time, the design is originally intended for the same number of pairs of IRFP240.

Steve Conner

Fri Nov 04 2011, 10:44AM

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

The DC-DC converter and the power amp are two separate modules. The power amp is no different to an ordinary one for mains powered use.

Do yourself a favour and design them that way: test the DC-DC with a dummy load, and test the amp with an ordinary 60Hz transformer and rectifier power supply.

Here is a DC-DC converter design by Rod Elliott and Sergio Sanchez-Moreno of Coldamp.

Experimentonomen

Fri Nov 04 2011, 11:59AM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

The DC/DC converter beeing a switching circuit requires very careful attention to layout and a properly made board, this is VERY important!!! Otherwise you will end up with a EMI and RFI emitting mess that will not only disrupt all the electronic equipment in your car, but also i worst case, land you a reservation in jail for illegal radio transmissions, or atleast have all your electronics taken away.

Steve Conner

Fri Nov 04 2011, 12:53PM

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

Hmm, I guess that is a valid worry now that cars are full of electronic engine management and braking systems. But then again, people install CB and ham equipment in cars all the time.

An advantage of a DIY setup is that you can put the converter next to the battery, and run +/-50V to the amps inside the car, or whatever. Hell, if you have a Prius you can probably connect a rack of PA amps with switchmode power supplies straight to the 300V battery.

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