Not thrilled with how a computer soundcard drove my 32ohm headphones so I decided to build myself class-A mosfet headphone amplifier. As with most of my projects, the goal was to keep it simple, keep cost down and try use some salvaged parts. This is a simple do-it-yourself (DIY) headphone amplifier project that is fashioned primarily after the Class A MOSFET Headphone Driver project by Greg Szekeres and to some extent Mark's DIY Class A 2SK1058 MOSFET Amplifier Project. The amplifier concept is simple and follows a typical single-ended class A circuit utilizing an active constant current source (CCS) in place of a passive resistor. A CCS doubles the efficiency of the circuit over that where a passive load resistor is used, bringing it to a maximum of 25%.
There are a couple of items to note. A FET follower circuit will be able to supply high current, but the voltage gain will be less than one. This amplifier will only be suitable in applications where the input signal does not require voltage amplification (such as the output of an mp3 player or computer). Also, a simple single-ended circuit like this will have no power supply ripple rejection and thus any noise in the power supply is going to go right through amplifier. For that reason, you will need to use a regulated power supply. Suitable inexpensive regulated (wall wart) power supplies can be purchased from Radio Shack. 10-20VDC and 750mA should be fine.
The schematic for this project is shown below in Figure 2. An IRF610 MOSFET is used in this example, but a wide variety of FET devices can be used in its place. A LM317 voltage regulator is used for the CCS and the draw is set at 250mA.
DIY Headphone Amplifier Construction
This headphone amplifier will reside primarily on my desk at work, so it needs to fit into an office environment. Fortunately I had a dead Plextor external CD-ROM kicking around that would make for the perfect enclosure and blend in well on my desk. Even better yet, it already had a power switch, power adapter receptacle and RCA inputs on the back as well as a headphone jack on the front. Perfect! The open hole you see on the back is where the USB header resided, but I had previously salvaged that for another project.
Plextor External CD-ROM Enclosure
The amplifier is constructed on ~1.75" square protoboards from Radio Shack (276-148), but any board will work. I only used parts that I had on hand and you can see that I did not use any boutique parts. Plain (but matched) metal film resistors, 1uF mylar input cap and 0.47uF polypropylene bypass cap on the output. The 0.1uF decoupling capacitor is also polypropylene. Some may prefer to use higher quality input and bypass caps and that should improve the sound. You can use carbon resistors, but I suggest you use metal film, particularly for the CCS due to their superior temperature stability over carbon.
DIY Headphone Amplifier on Protoboard
The heat sinks were salvaged from various dead components. The smaller heat sinks are about 1.75" square and only get moderately warm, but keep in mind that the heat sinks are attached to the metal chassis which also helps dissipate some heat. Be sure to isolate the MOSFET and regulator from the heat sinks.
Construction of Headphone Amplifier
The headphone amplifier was first tested (smoke test) using a regulated power supply at very low voltage. The bias is set by varying the 100k variable resistor until the output side of the MOSFET (Source) is at one-half of the supply voltage (Drain). You will want to check and reset the bias a few times in the first few hours as it will drift while everything settles in. The amp worked well between 10 and 20VDC, but seemed to work best at 13V and up. With a regulated supply there was no audible hum. That was not the case with an unregulated supply.
Setting Headphone Amplifier Bias
Next I got a chance to try out my new USB oscilloscope. It is a DSO-2150 which is a dual trace scope with 60MHz bandwidth and a maximum sample rate of 150MS/s. For those interested in such oscilloscopes here is a little more information about my experience with the DSO-2150 USB PC Based Oscilloscope. I checked the sine wave response and as expected, the results were good across 20Hz to 20kHz (the limits of my function generator). Below are two screen shots of the square wave response at 100Hz and 4800Hz.
The top trace (green) is the input waveform and the bottom trace (yellow) is the output. My signal generator is not great and that is reflected in the quality of the input waves. If you compare the input voltage to the output voltage you will see than the gain of the circuit is about 0.8. As you can see in the 100Hz trace, the square wave response is slightly tilted but stable. Tilting gradually decreases as the frequency increases and beyond about 300Hz the square wave response is excellent up to 20kHz which is the limit of my signal generator. Since music is comprised primarily of sine waves this is not a problem as the sine wave response was fine across the audible range.
The final touches were to epoxy the CD-ROM faceplate to an aluminum plate and put the enclosure back together. Since an mp3 player or computer will be used to control the volume, there is no potentiometer on the amplifier. The original volume control knob from the CD-ROM was cut down and glued in place.
Finished Class-A Mosfet Headphone Amplifier
For a simple single-ended amplifier design, the sound is pretty good to my ears. The amp drives my Grado SR80 headphones with ease, while my portable mp3 does not. I even prefer the sound compared to the built-in integrated headphone amp on my NAD C162 preamp.