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Help desiging Class A/B Amplifier

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I've had a bit of a 'challenge' from a friend. I've got very minimal experience with electronics, but am a very quick learner.

First things first, I need help reading schematics. Any links to basic sites would be much appreciated. I can read very basic diagrams, but once they start getting more advanced I have issues. Again I'm a quick learner so just a few links would be more than adequate.:)

Secondly, I need either links, or a quick crash-course in amplifier design. This will be intended for car audio, so I need it to be moderately efficient, and I want it to be able to do between 80W and 125W RMS @ 4Ω. There's one catch to this project, however. It has to be small; no more than 8.25 x 4.8" x 1.7", to be precise. Any schematics for something similar would be helpful if there is also an explanation as to what to change to get the desired effects.

Third, I need to find an electronic component dealer that either is located within Canada, or ships to Canada. I have one bookmarked, but A) I'm not sure if they ship to Canada, and B) I'm out of town for another week and a half, therefore don't have access to my computer.

I know this is asking a lot, but I'll be very grateful for any information provided. If I missed anything let me know and I'll do my best to fill in the details. Thanks in advance.

- Jesse

Nigel Goodwin

Super Moderator
Most Helpful Member
Buy one - you're not going to build anything as cheap, or as small, as a commercial in-car amplifier.

It's not just a question of building an amplifier, it's a question of building a custom switch-mode power supply as well.




What is the minimum power requirement; is it 60W or 80W?

Over on Electronics Lab you said 60W.

This will be intended for car audio, so I need it to be moderately efficient, and I want it to be able to do between 60W and 125W RMS @ 2-4Ω.
Is that per channel or for all the channels combined?

If it's 60W per channel then you can't do it straight from 12V, you need a boost converter.

It isn't possible to put 60W RMS into even a 2Ω load, let alone a 4Ω load.

The peak power can be calculated using Ohm's law.

P = V²/R = 12²/2 = 144/2 = 72W.

The RMS power is half the peak power so 72W peak becomes just 31W.

This is at clipping, assuming negligible loss in the driver transistors, with it sounding terrible.

The only way you can put more than 31W RMS into a 2Ω load is to use a boost converter to increase the supply voltage. To put 60W RMS into a 2Ω load, the peak voltage must be 15.5V. There's always a few Volts loss in the driver transistors so the power supply voltage needs to be higher than the peak output voltage. For a 60W amplifier, you'll need an 18V supply.

I advise scaling it down to 12W RMS per channel into 4Ω and it'll need to be bridged otherwise you can knock that down to just 3W.

Many car amplifier companies lie about the power output of their equipment. They might specify it at clipping with a supply voltage of 15V and give the peak power for all channels added together, rather than the RMS power per channel with a supply voltage of 12V. Quite often they don't tell you this, they might just say "power output 100W" which is really meaningless.
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