How to build the Apheared 47 Headphone Amplifier for Grado Headphones

Updated: at 05:00 PM

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Introduction

I should start off by saying that I’m a total newbie to the do-it-yourself electronics scene. I’m interested in this stuff but truth be told I build headphone amps because I want to use them. I doubt I’ll ever become one of those guys who has fifteen different amps of varying designs and applications—but you never know what could happen. I’ve definitely been bitten with the obsession of building the best sounding amp possible for the application and money. I can see that it’s a slippery slope.

The Apheared 47 amplifier in an Altoids Tangerine Sours mint tin.
The Apheared 47 amplifier in an Altoids Tangerine Sours mint tin.

In the summer of 2003, after realizing that I spend all of my time at my desk at work (I was a software developer at Organic at the time), I decided to splurge and get a good pair of headphones. After listening to several, I decided on a pair of Grado headphones. These are terrific phones for anyone interested in perfectly reproduced reference-neutral sound. I bought the SR-225’s—basically they’re middle-of-the-line phones. Apparently even their cheaper lines like the SR-80s are amazing, especially for the price.

I thoroughly enjoyed listening to all my old favorites through the newly acquired detail of the Grados. I liked them so much that I started reading, and lo and behold, everyone was talking about how much better they sound with a headphone amplifier. My first question was “what the hell is a headphone amplifier?”

In short, it’s a device that provides the exact power requirements and electronic environment headphones need to work to their full potential. Think of it this way: the average portable CD player has about 50 cents in parts dedicated to the headphone driver circuit. 50 cents + $300 for a pair of headphones = a bad match. Many manufacturers make commercial units, but since they are geared to the audio enthusiast or audiophile, they cost big bucks. Grado themselves make one for their line of headphones called the RA1, but it costs hundreds of dollars. The kicker is that it contains maybe $10 in parts (plus the wicked-looking mahogany case). Because of this price hike, hundreds of people are building their own amps. Headwize is an excellent place for headphone enthusiasts to group and contains many designs for do-it-yourself headphone amps for all applications and skill levels.

Prior to taking on this project, I had built one other amp—a Cmoy pocket headphone amp. This is an excellent little amp. Thanks to the excellent step-by-step instructions written by Tangent, I managed to get my way through building it. It sounded terrific—a noticeable improvement from what I had been hearing directly out of my headphone jack.

My first amp. A Cmoy in an Altoids Original tin.
My first amp. A Cmoy in an Altoids Original tin.

Sadly, I fried the op-amp in the Cmoy (the little chip inside that does all the work) after listening to it for six months. Refusing to go back to the headphone jack, I took it as an opportunity to build another amp—perhaps one a little more advanced.

Enter the Apheared 47 Headphone Amplifier

When browsing Headwize trying to find an amp that looked fun to build, I found The Apheared 47 amp. It was designed specifically for Grado headphones, which immediately sparked my interest.

Grados are different from most high-end headphones in that they have a relatively low impedance (32 Ohms). Most others are in the hundreds of Ohms. Top-of-the-line Sennheisers run as high as 600 Ohms. This means that Grados require more current than most headphones; therefore, general-purpose headphone amps can have trouble providing power that’s appropriate for Grados. This can change the sound quality rather than the volume—an important distinction. My Cmoy amp could drive the Grados plenty loud, but wasn’t providing optimal high current handling.

The Apheared 47 uses a two-stage op-amp approach that improves current handling. There are four op-amps in the A47: two for the left ear and two for the right. The rest of the circuit is very similar to the Cmoy.

Gathering Info on the A47 Design

Before building anything, I did a lot of reading on Headwize. Apheared had only provided a schematic of the amplifier circuit with no mention of power supply requirements etc. I posted a thread on Headwize that got me started enough to build the new amp on a breadboard. When I couldn’t get this to work properly, I posted another thread. This second thread produced tons of useful information about the power supply requirements and general design of the amp.

The Plans

When I first began to approach the circuit layout on an actual circuit board, I found that I was completely lost. Tangent’s Cmoy instructions had been very clear as to where to place each component and jumper—this time around I had nothing but a power supply schematic and an amp circuit schematic. I broke down and postponed the build (yet another week without tunes at work), and decided to make plans for this amp similar to the plans Tangent made for the Cmoy. The result is below.

The instruction sheet I made for the A47 layout.
The instruction sheet I made for the A47 layout.

You can download it here in PDF format for printing: Download PDF.

The Case

I managed to jam the resulting board into a round Altoids mint tin. Very cool looking if I do say so myself.

The finished amp in its case.
The finished amp in its case.

The guts of the amp. Note that I managed to shove a 1/4" jack into the case.
The guts of the amp. Note that I managed to shove a 1/4” jack into the case.

A closeup of the guts. Notice the OPA2134 op-amp chip.
A closeup of the guts. Notice the OPA2134 op-amp chip.

If you decide to build this amp, I hope I’ve provided some info that will make it easier on you than it was for me. Best of luck! If you own Grado headphones, I think you’ll be happy with the improved performance. Oh, a word of advice: it’s always a good idea to try your amp with a pair of crappy headphones before plugging in your $200+ Grados—I’ve heard stories.

Email me if you have any questions/comments.

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