Easy to build Headphone amplifier using Mosfets

This instructable is about how to build a simple headphone amplifier using easy to obtain components :)

The amplifier is based around a standard N-Channel Mosfet (IRF740 in my case) and is quick and easy to build with quite acceptable results. I scavenged the heatsink for this off the voltage regulators on an old motherboard. Its rated at about 9C/W (9 Degrees raise in temperature for each watt dissipated). When this cirucit is running, the heatsink is cool enough to touch :)

This circuit uses a simple Class A Common source configured mosfet to drive nearly any set of headphones (8Ohm+). This circuit works through the mosfet being biased into class a operation with a quiescent current of ~150mA. Maximum power output is limited to about 52mW @ 8Ohm through the 33R Resistor in series with the headphones, this power is with onset of some distortion. I test this circuit using my Sennheiser HD201 headphones, they are quite efficient and sound pretty tasty with this circuit! :P

It only uses a handful of components which most electronics builders can find in their component boxes etc. This circuit can work on any voltage from 6v-24v aslong as heatsinking the components is taken into consideration. The lower the voltage, the less power can be delivered to the headphones before distortion occurs.

Any questions on theory or building, please go ahead and ask! If you have any requests for some simple breadboarding circuits, if I have time, i'm happy attempt an instructable on them :)

Enjoy but stay safe!

Warning! I will not be held responsible for any loss/damage of hearing done through use of this circuit, this is for educational purposes only and hearing loss can occur if built improperly!

Step 1 Gather Components

The first and most important step!

Gather all the required components for the circuit:

Resistors:
1x 100K resistor
1x 33R resistor
1x 680R Resistor
1x 47R Resistor (This resistor will need to be rated for 1W as ~0.73w will be dissipated)
1x 4.7R Resistor
1x 4.7k Preset Resistor

Capacitors (All rated for atleast 16v):
1x 0.1uF Capacitor (This capacitor will be used for the input)
1x 220uF (Or higher) Capacitor
1x 1nF Capacitor
1x 22uF Capacitor

Semiconductors:
1x N-Channel Mosfet (I used an IRF740, and mosfet rated for atleast 500mA an 20v will do)

Other:
12v Power Supply
Heatsink for Mosfet

Comments

[blinkyblinky]

How many watts can this deliver?

[pyrohaz]

Hi, as I mentioned in the instructabel, its about 52mW @ 8Ohm Headphones :) Cheers!

[blinkyblinky]

Thanks. You should try ics such as the NJM2073. I built one and it can deliver a few hundred milliwatts but current consumption is high. Lm386s have less but will do. Same thing with LM358.

[pyrohaz]

Completely true, i've used LM386's many a time, just some people find a class A MOSFET amplifier, much more audibly pleasing than a chipamp, personally i'd choose a LM386 for the sake of higher efficiency and ability to go louder but this is just a simple example and can be used cheaply using simple components that will be around most probably for the life of analog electronics :) Cheers for the reply though!

[blinkyblinky]

Do you know how to make an amplifier using 1 AA battery that can act as a good hearing aid. One of my family members needs such a machine. The one he has now is 1 AAA battery (in which he doesn't enjoy) and will last him two months. However, the one I am requesting only needs a one month life. Thanks.

[Jimmy Proton]

Would one amp be too much current for this?

[Jimmy Proton]

Could this be used to amplify a constant current, like say a battery?

[pyrohaz]

I'm afraid not, as you can see the input is capacitor coupled (meaning that only AC can flow instead of DC.

To amplify a constant current, my best idea would be:
Convert to voltage through resistor
Apply voltage gain
Convert output voltage into Current using a voltage controlled current source/sink.

I'm currently reading a book called 'The Art Of Electronics' and it is teaching me more and more on current sources and even shows some example circuits, so if you are really interested in electronics, I say you read up on this :)

Cheers!

[aandrade3]

Instead of a 12v power supply would it be possible to use a USB port to power it?

[pyrohaz]

Hey, chances are you probably could run it off your USB port BUT I wouldn't recommend it unless you are quite sure what you with what you are doing, messing up with USB wires can end up in a blown USB controller! Also, at 5v (USB voltage) you will need to get a mosfet with a lower Vt (Threshold voltage) otherwise, being biased to only 2.5v may not allow it to switch on enough.

Thanks :)

[ironsmith]

we are studying mosfet at the moment in my semiconductors class. I have harvested a few out of my e-waste pile and would like to use them to learn a little more about them. do you have any circuits made up for higher voltages as a learning tool for me?

thanks for posting this one...

[Phil B]

Very nicely done. I tried building a MOSFET project once, and destroyed the MOSFET. I think static electricity was the culprit. I am a little gun shy about MOSFETs now.

[pyrohaz]

Cheers! Yeh, I must confess, at first I was very wary of mosfets (I never got chance to properly use them until the end of my GCSE course, once I bought some myself!). The first time I got to use a mosfet was in a simple 555 timer boost converter and I was using a IRF5xx series mosfet, Not expecting a large output voltage, it shot up to about 500v and I ended up with my first blown mosfet! Not the nicest of smells :P.

I'm happy with using mosfets in audio circuits though where higher voltages aren't included, I still need to learn about driving mosfets correctly in switching circuits so until then, I think its just sticking to audio! If I get chance, I may put up my DIY class D amplifier using these mosfets (One of the first DIY class d amplifiers as far as I know!)