The simplest audio amplifier

[Kevin Weddle]

A guy in the chat room wanted to auto stereo amplifier. I suggested he use a 3A transistor but needs the negative supply. I noticed the 555 timer can be configured to give the -12 volts output. But of course the current is too low. Could somebody post the simple solution.

 

[audioguru2]

Hi Kevin,
1) What is an "auto stereo amplifier"? Car amp? Automatic amp?
2) What does he want to do to it?

 

[audioguru2]

Sorry Kevin,
Many factory car radios have 4 bridged amps that (with a 12V supply) can deliver 14WRMS per amp at low distortion into 4 ohm speakers. Most have 4 amps for a total of 56W. They operate cooly and fairly efficiently.

How much power do you expect your "2 PNP transistor attenuator followed by a 63W NPN heater" to deliver to a 4 ohm speaker?

Since you have 12V across the NPN transistor's collector resistor and you say a 3A current, its value must be 4 ohms. With the transistor at cutoff, the maximum peak voltage across a 4 ohm speaker will be only 6V. Therefore your amp will clip at 12Vp-p. That calculates to an output power of only 4.5WRMS, and probably with very high distortion since it doesn't have any negative feedback.

Let's calculate your amp's efficiency. The NPN transistor and its collector resistor dissipate 63W. The PT6640 voltage converter dissipates 4W and the PNP transistors will probably dissipate 3W.
Your output power of only 4.5W divided by your amp's dissipation total of 70W equals an efficiency of only 6.4 percent!

Sorry again but your 4.5W amp can simply be replaced by a 3 ohm series resistor and it will have the same attenuation as your amp and the same poor damping, but with much less distortion. The 3 ohm resistor will dissipate only 3.4W when the original 14W amp is at clipping (and the 4 ohm speaker is at 4.5W) and dissipates nothing when there isn't sound output.

Real car amps efficiently deliver hundreds of watts at low distortion by using many parts, complementary-symmetry class-AB output stages and lots of negative feedback.

 

[Kevin Weddle]

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[audioguru2]

Hi Kevin,
You need more than just push-pull. Todays ordinary car radios use a cheap and simple bridged-amps IC that puts out 21V p-p (with a 12V supply) into each 4 ohm speaker, without using a negative supply nor output coupling capacitor. But the crappy 20 ohm flimsy speakers with tiny magnets that cars come with are lousy.
When the alternator is violently charging at 14.4V, todays ordinary car radios deliver a low-distortion 20WRMS per channel into proper 4 ohm speakers, and they usually have 4 channels. Try boosting that.

Philips (and probably other IC manufacturers) has a bridged-amps car radio amp IC with a built-in voltage-doubler and automatic switching of the supply voltage with input level to stay cool! Something like 60W per channel. Try it.
Don't forget that doubling the power sounds only a little louder, 10 times the power sounds twice as loud (logarithmic).

 

[audioguru2]

Hi Kevin,
I didn't see your last amp until now 'cause it took too long to load.

What are you doing? You can't use inverted and non-inverting drive to a complimentary pair, they just fight each other modulating the supply current and blow the fuse without swinging the output up and down. You need complimentary emitter-followers, biased a little for low crossover distortion and a single drive to both.

 

[Kevin Weddle]

I hope this circuit is better. Sorry.

View attachment 36370

 

[audioguru2]

Hi Kevin,
Now the transistors don't have any regulation of their idle current so when they heat-up, or when the supply voltage goes up above 12V (when the alternator charges the battery to 13.8V) then again the transistors will fight each other and draw a massive current into each other, causing more heating which makes them conduct more..... Thermal Runaway! Another fuse popping circuit.

Use bridged amps instead of a dual-polarity supply and use amps with complimentary emitter-follower outputs.

 

[Kevin Weddle]

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[Kevin Weddle]

Okay. Here is the charge pump that will give us the negative supply.

View attachment 36385

 

[audioguru2]

Hi Kevin,
Your 3 left side transistors will work well making -12V pulses, when it is driven with a square-wave oscillator like a 555.
I don't know what the right side transistors will do. I think that the PNP transistor at the extreme right side will do nothing but breakdown since it base is much more positive than its emitter!

Here is a real car radio amp IC that provides 176W!
http://www.semiconductors.philips.com/pip/TDA8591.html