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. 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%.
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 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 regulator is used for the CCS and the draw is set at 250mA.
Figure 2: IRF610 Class-A Headphone Amplifier Schematic
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.
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 is at half the
supply voltage. 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
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