Simple PC audio amplifier circuit

Thread Starter

tjohnson

Joined Dec 23, 2014
611
I just saw this comment on Stack Exchange regarding the circuit that I used (in post #1):
Just a note that this circuit actually drives an 8 ohm speaker relatively well but the standby C-E current is high (possibly hundreds of mA). For a small transistor this could cause thermal runaway / heat damage. My transistor nearly smoked, I ended up at something like the question asker's original circuit with collector resistor = 100 ohms and big cap in series with speaker. Not quite as loud but no smoke.
so apparently the problem is not a short, and I suppose I should try doing what is mentioned in the comment.

One other question: If I want to adjust the volume of the speaker, should I change the value of resistor R1 or R2? In other words, which one would be best to replace with a potentiometer?
 
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AnalogKid

Joined Aug 1, 2013
10,986
This circuit would probably overheat even with a power transistor. For example, when the output is sitting 1/2 2ay between the power supply and GND, that's 4.5V across 8 ohms, or 563 mA. This current also is flowing through the transistor. While it technically is within the device's ratings, the power dissipated in the transistor is over 1.4 W, which is WAY too much. That amount of current also is almost certainly overdriving the speaker.

I understand what you want to do, but driving an 8 ohm speaker with nothing but a few small NPN transistors is difficult, takes more parts than you think, and absolutely requires heatsinks. Another possible solution is to use an output transformer to reduce the transistor current and keep the DC out of the speaker.

ak
 

Dodgydave

Joined Jun 22, 2012
11,285
OK, thank you for the explanation.

Basically, I wanted to build a circuit to amplify the audio output coming out of my PC's headphone jack to make it as loud as if it was coming out of a speaker. It seems that the best way to do this is to either use an IC in the circuit, or just buy some speakers. It would be neat if there was a program that could amplify sounds being played on a PC by several hundred percent, but I'm not aware of one.
Headphone sockets usually have resistors on the outputs to limit the volume for headphones, so you need to remove them, or bypass them, best to use a Lm386 amp, or use a different output from the pc.
 

Thread Starter

tjohnson

Joined Dec 23, 2014
611
I tried adding a 100Ω resistor and large capacitor as I mentioned, which does prevent the circuit from overheating. However, the sound coming from the speaker is now barely audible, making it in no way superior to headphones. Neither the value of the resistor nor the capacitor seem to make any difference in the volume, and I don't understand how adding a capacitor prevents the transistor from overheating. So I think I will need to add an output transformer, if I can find a schematic online showing how to use one. I may also be able to rip some speakers out of a dead laptop and build a higher quality stereo speaker.
 

Thread Starter

tjohnson

Joined Dec 23, 2014
611
Thanks. The schematic on that page doesn't help me much though. It seems pretty abstract, as it doesn't specify any values for components, and it uses an AC-to-DC transformer, which my circuit doesn't because the audio input it receives is DC. I've been googling, but it seems nearly impossible to find a Class AB amplifier circuit that doesn't require PNP transistors.
 
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Papabravo

Joined Feb 24, 2006
21,159
Do you suppose that might be because the PNP is essnetial to the working of the Class AB device. If you have a design that uses NPN only devices, by all means share it with us.
 

Thread Starter

tjohnson

Joined Dec 23, 2014
611
Another possible solution is to use an output transformer to reduce the transistor current and keep the DC out of the speaker.
Please explain how this would be done. Also:
If I want to adjust the volume of the speaker, should I change the value of resistor R1 or R2? In other words, which one would be best to replace with a potentiometer?
or would something different be done to adjust the volume?
 
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Thread Starter

tjohnson

Joined Dec 23, 2014
611
you said you had connected the audio from the heaphone socket on your computer,is there another audio output?
Oh, I see, you mean that the headphone socket inside the computer may have resistors. Sorry, I had thought that you were referring to the wiring within a set of headphones. However, I'm hesitant to take my computer apart and remove anything.
 

Thread Starter

tjohnson

Joined Dec 23, 2014
611
Believe it or not, I got an amplifier circuit working using only one NPN transistor!:) The sound quality is crude as expected, but I don't think it's too bad for such a simple circuit. I'm happy now because the transistor doesn't overheat, and the volume is about as loud as before. I figured out how do it by reading the Stack Exchange comment that I mentioned earlier more carefully:
Just a note that this circuit actually drives an 8 ohm speaker relatively well but the standby C-E current is high (possibly hundreds of mA). For a small transistor this could cause thermal runaway / heat damage. My transistor nearly smoked, I ended up at something like the question asker's original circuit with collector resistor = 100 ohms and big cap in series with speaker. Not quite as loud but no smoke.
as well as reading this answer on the same page. If I had read this more carefully sooner, I could have saved myself hours of fruitless googling.:eek:
 

Thread Starter

tjohnson

Joined Dec 23, 2014
611
Try this:

Thanks. That circuit is pretty similar to the one that I ended up using, but it has a second transistor which requires a heat sink. Since I stripped some of the insulation off of the headphone connector that I'm using, the sound quality has noticeably improved, so I think I'll just stick with what I have.
 

Thread Starter

tjohnson

Joined Dec 23, 2014
611
I have some questions about how to connect the audio input to my circuit. A cross sectional view of the headphone plug that I'm using looks like this:
wire_cross_section.png
so I assume it's a stereo plug, but my circuit only handles mono. The circuit looks like this:
amplifier.png
What is the best way to connect the plug to my circuit for optimal sound quality? Should I connect wires 1 and 4 on the left and right, or should I do it some other way? Should I join some of the wires together, or should some of them be used for grounding? Also, does it matter what order I connect them to the circuit in?
 
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Thread Starter

tjohnson

Joined Dec 23, 2014
611
Given the schematic in post #38, could someone please answer this question: In order to adjust the volume of the speaker, which resistor should I change the value of?

I thought R3 would be the one to change, but when I tried doing so, it only made the sound quality worse or the volume barely audible.
 
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