Designing a car audio amplifier... ?

[Myron Samila]

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)

 

[Bill Bowden]

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)

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

 

[Myron Samila]

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]

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.

 

[Bill Bowden]

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

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