Yeah, ok I'm bored......
I'm thinking I want to try building a car audio amplifier, a simple (hehe) multistage BJT power amp, any suggestions on a particular transistor for the application? (hoping one that has good tech support docs).
Building in speaker protection might be a wee bit tricky (DC protection, etc..), but I figure I'll have a crack at it.
Any suggestions would be appreciated! (ideally its gotta run on 12VDC and have power ratings near 125W+/4 ohms)
-- Myron Samila Toronto, ON Canada Samila Racing http://204.101.251.229/myronx19
multistage BJT
(hoping one
etc..), but I
have power
You need quite a bit more than 12 volts to get 125 watts from 4 ohms. If you just connect the 4 ohm speaker across the battery, the power will only be 144/4 = 36 watts. And a class B amp will have the output set in the middle of the supply range so it can move both ways which means you only get +/- 6 volts to the speaker, which would be about
4.5 watts or less.So, you probably want to start with a switching power supply that delivers about +/- 35 volts from the 12 volt battery.
-Bill
Yes, the power supply portion shouldn't be too much of a problem. I've worked with 12VDC switching power supplies for other applications, and they may just work in my favour (decent current capability).
Typically, the amplifiers I work on (professional audio power amps) are supplied
45+/- VDC down the rails.Since this amp would be used for subs, the transistors will be biased near cutoff, I'd like to keep distortion to a minimum. It's more starting with the ideal transistor for the job, I guess if I can get the voltage I require, I could just use some of the professional audio stuff I use now?!?
-- Myron Samila Toronto, ON Canada Samila Racing http://204.101.251.229/myronx19
multistage BJT
(hoping one
etc..), but I
have power
Actually, the power supply is the most important part!! If it can't deliver, I'm not going to see much output....
Maintaining a constant average voltage is going to be tough.
-- Myron Samila Toronto, ON Canada Samila Racing http://204.101.251.229/myronx19
with
12VDCfavour
supplied 45+/- VDC
cutoff, I'd
transistor for
the
multistage
(hoping one
etc..), but I
have power
If you use an op-amp to control the output transistors, the power supply voltage won't matter as long as it stays above the output voltage. And since the op-amp will control the output voltage, you also don't have to worry about the transistors, any old transistors will work that are rated for the current and voltage. The output will be linear (low distortion) because the negative feedback from output to the op-amp input will cancel out any non-linearities in the transistors.
But, like you said, a power supply to deliver the required power will be the hard part.
-Bill
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