Simple single transistor audio amp

Thread Starter

CTRL+X

Joined Jul 1, 2009
27
I'm looking for a schematic for a single transister NPN audio amp. It is will be used to amplify the beeping signal from a radar detector. Input signal will be coming from a headphone jack. I would like it to power off of a 12vdc supply, so that I can hardwire it into the vehicle, but if it's simpler to use a smaller power source, batteries are fine.

This is not for music, so I don't care about frequencies/clarity, I just need to amplify the beeping alerts from the RD. Looking for the simplest possible.

I tried building this below on a breadboard, but it didn't work. Of course it was probably a builder error :rolleyes:
 

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Wendy

Joined Mar 24, 2008
23,415
I was thinking both actually. The resistor can be small, but you never know what kind of DC component the output of the previous device has. A cap is cheap insurance. A lot like the resistor actually.
 

eblc1388

Joined Nov 28, 2008
1,542
Your ckt has a big connection mistake.
I can't see any big mistake in the original circuit.

Can you tell us where do you think the big mistake is?

On the contrary, your proposed circuit have the possibilities of burning up the loudspeaker and/or the darlington. :(
 

jj_alukkas

Joined Jan 8, 2009
753
How could it be that the battery and speaker all in one line??

Pardon me if im wrong but I havent seen any circuit like that with the supply going to collector through the speaker. The circuit I drew up is the most commonly used types( I think !!, Im not good wit transistors) . Could you explain what makes the mistakes??

Ive removed my ckt if you think its wrong.
 
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ELECTRONERD

Joined May 26, 2009
1,147
All of the single transistor circuits I've designed never seem to have enough amplification to drive a speaker. I usually use one of those ceramic earphones you find in crystal radio sets. I would recommend that you use a transformer on the output to match the impedance. I'm not sure, but Is there a formula out there so I can calculate the impedance?
 

jj_alukkas

Joined Jan 8, 2009
753
All of the single transistor circuits I've designed never seem to have enough amplification to drive a speaker.
Haven't you tried the above ckt i have posted in any project??

It provides a minimum amplification. I tried replacing them with several other transistors and found that types like BEL provided very much better amplification. I can say that 1 transistor configurations can provide enough amplification to drive a speaker, a small one, ok.

And how would you like to calculate the impedance? A transformers formula shpuld work right??
 

Søren

Joined Sep 2, 2006
472
Hi,

How could it be that the battery and speaker all in one line??

Pardon me if im wrong but I havent seen any circuit like that with the supply going to collector through the speaker. The circuit I drew up is the most commonly used types( I think !!, Im not good wit transistors) . Could you explain what makes the mistakes??
Just open your mind a bit ;)
It's just how it's drawn that's confusing you. Pull the +9V point to the top and rearrange the rest and you'll see that it's a totally standard circuit.
 

Audioguru

Joined Dec 20, 2007
11,248
The darlington transistor is almost saturated all the time and therefore is a poor amplifier. The speaker with DC in it might not work either.
 

Thread Starter

CTRL+X

Joined Jul 1, 2009
27
Can I just substitute the darlington transistor w/ two seperate transistors? With the emiter of the first T going into the base of the second one?

Also, what resistor values would I need to change to run this off of 12vdc instead of 9v?

In the below crt diagram, what would I change to run it off of 12vdc? And it was suggested that I should put a cap across the imput? What value?

And a stupid question. Input is only one wire. Where does the other input wire connect to?

Thanks,
 

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Thread Starter

CTRL+X

Joined Jul 1, 2009
27
OK, went to Radio Shack to get some stuff, picked up an LM386 audio amp IC, and some connectors.

However I also grabbed a 102db 12V piezo siren. So, slight change in direction...
I would like to use the output from the headphone jack to trigger the 12v crt to the siren. The siren is self contained, and just needs a constant 12v supply to operate.

I believe that headphone outputs are in the 1-2v range. So I'm basicaly looking for a crt where a 1-2v input signal will trigger a 12v crt.
 
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Thread Starter

CTRL+X

Joined Jul 1, 2009
27
Will this work? So I attach one side of the headphone output to R1, but where does the other wire from the headphone output go. Do I just Grd one side? Would that place the headphone crt on the radar detector into the 12v crt and damage it?
 

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Thread Starter

CTRL+X

Joined Jul 1, 2009
27
The crt below works as I want it too. When the radar detector beeps, the piezo siren goes off with equivalent intervals, but much louder. :D

With just the transistor, the intervals were too short to actually activate the siren, however adding a 220uf cap across the load fixed that problem.

Any additional resistors or diodes that I should add to protect the radar detector headphone jack crt, or does this crt not pose any risk to the RD?

Thanks,
 

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Thread Starter

CTRL+X

Joined Jul 1, 2009
27
Not sure if I should make a new thread since this no longer involves an amp, but....

In the crt in the post above, it works fine when the power is a 9v batt, however as soon as I connect to 12vdc auto power, it just goes off constantly as soon as the input wire is connected, whether there is a beep coming thru or not. With 9vdc it only goes off at intervals corresponding to the beeps from the imput.

The transistor is rated to 25v, so the increased current shouldn't just push thru, should it? Do I need to put a resistor in here somewhere to limit the current?

Thanks,

Edit: Nevermind, put a 330k resistor in at the grd connection. Works fine, a little too quite though so just need to find the right resistor value.
 
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