DIY Class-A Mosfet Headphone Amplifier
DIY Class-A Headphone Amplifier
Not thrilled with how a computer soundcard drove my 32 ohm Grado SR80 headphones, I decided to build myself a desktop headphone amplifier for the office. In this instance I had plenty of voltage gain, but the sound card just runs out of gas with good headphones. This amplifier will only be suitable in setups where the input signal does not require voltage amplification (such as the output of a preamplifier, mp3 player or computer). This amplifier will delivery plenty of current to drive more demainding headphone types.
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%.
Figure 1: Basic Class-A Amplifier Schematic
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 headphone amplifier 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. I've had success with IRF510, IRF610, IRF611, IRF612 and IRF710, all of which worked well. You will want to stay away from IRF530 or IRF540 types (commonly found in power supplies) as there will be terrible roll-off of the highs. Using a simple application of a common LM317 voltage regulator it is configured as a very accurate CCS set to draw 250mA.
Figure 2: IRF610 Class-A Headphone Amplifier Schematic
Construction - DIY Class-A Mosfet Headphone Amplifier
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.
Photograph 1: 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.
Photograph 2: 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.
Photograph 3: 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.
Photograph 4: Construction of Headphone Amplifier
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.
Photograph 5: 100Hz Square Wave Response
Photograph 6: 4800Hz Square Wave Response
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.
Photograph 7: CD-ROM Faceplate
Photograph 8: Finished Class-A MOSFET Headphone Amplifier
Second Build - Class-A Mosfet Headphone Amplifier
UPDATE - Dec 2013. The photos below show the second of these amplifiers that I built. This version of the Class-A Mosfet Headphone Amplifier features a very high quality PCB that was made by a forum member and sent to me as a gift.
Photograph 9: PCB for Class-A MOSFET Headphone Amplifier
For the power supply I am using is a 20VDC power supply from an older laptop. The 20VDC is regulated to 16VDC using a basic LM317 variable power supply circuit. This makes for a ultra quiet power supply which is necessary for this amplifier circuit.
Photograph 10: Class-A Mosfet Headphone Amp with LM317 Regulated Power Supply
The headphone amplifier was built into the enclosure from some sort of old Dolby processor (1990s) that I had to modify slightly. Plain RCA jacks are used for input source. For the headphone output I am using a Neutrik Locking 1/4" phone jack. This is a very good quality 1/4" locking jack and I use these all the time in my builds.
Photograph 11: Second Finished Class-A MOSFET Headphone Amplifier
Sound - 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. See the forum for more photographs and discussion about the DIY Class A MOSFET Headphone Amplifier project.