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Hi,

I used this suggestion http://blog.bryanduxbury.com/2012/01/one-transistor-audio-amplifier-for.html to amplify a small, 8 ohm speaker that is part of an Arduino project.

What I am finding is that the transistor is getting very hot. Is it a question of wiring (as per the diagram) or the transistor that is recommended (2N706)?

I am using an Uno for testing but will be using a Nano for the final project.

Any suggestions would be greatly appreciated.
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Hi arduinokov,

You aren't alone...  At the bottom of that blog is a  similar comment:  ( unless it's you  smiley )

Quote
Georgia said...
Hi,

I am running a little 8 ohm speaker with the transistor as you suggested. It works great but it is getting quite hot. Do you have this problem?
January 20, 2013 at 7:17 PM

Without thinking too much about it, I suspect that there isn't much resistance in the circuit to limit the current through the transistor... Just the speaker coil.

Probably not a very robust design.
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That's a real brute-force and inefficient design, it's going to try to drive the speaker with over half an amp of current if the transistor is reasonably saturated. I looked up the 2N706 (ancient part number!) on Mouser and it is only rated for 10mA collector current. I'm surprised it doesn't melt down altogether. Maybe the power supply cannot provide quite enough current to destroy it. I'd try a proper audio amplifier instead. There are quite a few of small inexpensive ones out there. I don't have great luck with the LM386 personally, but I guess a lot of folks do. Slightly more esoteric, there are some interesting class-D chips out there that are very efficient and have very low quiescent current.
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Thanks for the help. I'll check out the LM386.

 smiley
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If its getting hot, it has a lot of current going thru it.
With a speaker in series, that's like having an 8 ohm resistor from +5 to Gnd.
With some voltage drop across the transistor, you could have something like 500mA going from Collector to emitter.

Is 2N706 the correct P/N? Like this?
http://www.abra-electronics.com/products/2N706-Transistor-General-Purpose-NPN-10mA-20V.html
http://www.digchip.com/datasheets/parts/datasheet/417/2N706A-pdf.php

That part is only rated for 10mA.  You're gonna smoke it.

A one transistor amplifier will work much better like this
http://electroschematics.com/6910/single-transistor-amplifier-revisited-part-1/
or here
http://www.fastquake.com/2012/02/simple-transistor-amplifier-circuit-schematic-and-explanation/
taking your output from the right of C3.
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Thanks, that is great advice. I'm going to read it all and learn how to build the circuit.

 smiley

If its getting hot, it has a lot of current going thru it.
With a speaker in series, that's like having an 8 ohm resistor from +5 to Gnd.
With some voltage drop across the transistor, you could have something like 500mA going from Collector to emitter.

Is 2N706 the correct P/N? Like this?
http://www.abra-electronics.com/products/2N706-Transistor-General-Purpose-NPN-10mA-20V.html
http://www.digchip.com/datasheets/parts/datasheet/417/2N706A-pdf.php

That part is only rated for 10mA.  You're gonna smoke it.

A one transistor amplifier will work much better like this
http://electroschematics.com/6910/single-transistor-amplifier-revisited-part-1/
or here
http://www.fastquake.com/2012/02/simple-transistor-amplifier-circuit-schematic-and-explanation/
taking your output from the right of C3.
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