Audio Amplifier

[wmmullaney]

Hi all, I want to amplify audio signals.
I have looked at schematics but all seem to have soooo many parts.
I was wondering why you coundn't use 1 transistor and 1 rehostat, like they did when the transistor first came out, "They attatched a microphone to one end and a loudspeaker to the other end. Their wispers came booming out of the speaker."

thanks

 

[Speakerguy]

It will work, it will just be very high distortion and low power output. Back in the day they might have been using a horn loudspeaker, which are usually somewhere between 10x and 100x as efficient as today's modern speakers.

Look at chips like the LM3886 from National Semiconductor. It is essentially a high power op amp for audio.

 

[audioguru]

Maybe he doesn't need much output power.

 

[Leftyretro]

Hi all, I want to amplify audio signals.
I have looked at schematics but all seem to have soooo many parts.
I was wondering why you coundn't use 1 transistor and 1 rehostat, like they did when the transistor first came out, "They attatched a microphone to one end and a loudspeaker to the other end. Their wispers came booming out of the speaker."

thanks

For a useful answer you need to tell us what the signal voltage level and impedance is of the device needing the amplification and what you want to drive from the output of the amplifier. Then it can be determined how much gain and power will be required. If it's a speaker we need to know it's impedance and power rating and ideally it's SPL (efficiency) rating. Also we need to know what power source you want to utilize.

Lefty

 

[k7elp60]

I can remember building a single stage class A amplifier with a power mosfet, putting an 8 ohm speaker between the drain and the + supply rail. Biasing the mosfet to class A and coupling the audio to the gate.

 

[audioguru]

DC in a speaker causes its cone to be off center creating distortion and heat.

 

[Speakerguy]

He didn't say it sounded good But I'm certain there's a market out there for it (looks over at audioasylum... hmmm....).

 

[wmmullaney]

Thanks all, I wanted to amplify something like an ipod, to an 8 ohm speaker at 5v.

 

[wmmullaney]

Or a crystal radio...

 

[audioguru]

With only a 5V supply, a single-ended audio power amplifier like an LM386 will produce only 0.11W at clipping into an 8 ohm speaker. A very low volume like a little toy.
A TDA2822 stereo audio power amplifier connected as a bridged amp will produce 0.6W. Much louder, like a clock radio.

 

[wmmullaney]

I could do 9v, where can I get an LM386 cheapest?

Thanks

 

[wmmullaney]

Also, should I use a PNP or NPN transistor for the 1 transistor amp?

 

[audioguru]

With a 9V supply, an LM386 produces 0.45W at clipping into an 8 ohm speaker. Less than a cheap clock radio.

An LM386 costs less than $1.00CAN. About the same as one litre of gasoline in my part of Canada today.

 

[wmmullaney]

Thanks so much!!!

 

[wmmullaney]

Will it work with a crystal radio?
Please answer transistor q.

 

[Nigel Goodwin]

Will it work with a crystal radio?

Yes it will.

Please answer transistor q.

Either, but it would be totally rubbish!, and not suitable for battery use.

 

[audioguru]

A real radio has many tuned circuits for selectivity (to separate all the stations). A crystal radio picks up many AM stations at the same time.
A real radio has RF amplifiers for good sensitivity. A crystal radio needs a very long antenna and an earth ground to pickup local AM stations.
A real AM radio has automatic-gain-control so that a weak distant station has the same volume as a strong local station.

The LM386 little power amplifier IC can have a gain of 20 for a normal tuner, CD player or MP3 player or a gain of 200 for a crystal radio or microphone.

A single transistor is not an audio power amplifier. Its input impedance is too low for being driven from a crystal radio, its gain is too low, it has high distortion and it puts DC into the speaker.

A PNP transistor has the same performance as an NPN transistor.

 

[wmmullaney]

Thanks all. I'm ordering 5 368s.

 

[audioguru]

What is a "368"?
I think it is a voltage reference IC, not an audio power amplifier IC.

 

[wmmullaney]

sorry, LM368. My bad.

wmmullaney