The Drones’ Hackers

Why does anyone need a headphone amplifier? You can just plug your headphones into the jack on your iPod, can’t you? Yes, of course you can. However there are some high-end headphones that have low impedance — they need more current than the usual high-impedance phones. Often these headphones are low efficiency, meaning for the same loudness they need more input power than more efficient phones. Finally, the output amplifiers in low-power portable devices are often of indifferent quality, particularly at reasonable volume. A headphone amplifier fixes all these problems; and if you make it yourself, you can add extra features like crossover networks.

Pimeta Headphone Amplifier

What’s a “crossover network”? When you listen to your stereo through loudspeakers, your right ear hears both the left and right channels, but the left channel is not as loud and slightly delayed. This is one of the cues that the brain-ear system uses to figure out where a sound is coming from. When you listen to music on headphones, your right ear gets to hear only the right channel. That’s why stereo music over headphones gives that unnaturally wide “inside your head” sound. That’s also why many people (including me) cannot listen to headphones for extended periods. A crossover network feeds a small amount of the left channel — delayed by a short amount — into the right channel, and vice versa. This results in a more natural soundstage, and I can listen to my Denon AHD2000 headphones for eight hours straight without discomfort.

v1.0: The Ramsey SHA-1C

My first attempt was the Ramsey SHA-1C. This is a very easy-to-build kit and even includes a case, but having said that you’ve said everything. It’s very noisy, has indifferent sound quality, and worst of all is completely unshielded. If a GSM mobile phone is in the vicinity, you will be risking your hearing every 55 minutes or when a call arrives. I don’t recommend the SHA-1C as an amplifier. (As an introductory kit to learn soldering etc. it’s just fine.)

v2.0: The Pimeta

After some research on the headphone amp designs on the net, I finally settled on the Pimeta. This is an “internet collaboration” (you can call it “open source”) design, by people who hang out on the head-fi.org forums. It is a three-channel amplifier that uses high-current buffers for output. One of the designers (Tangent) even offers a PCB for it. As a bonus, he also sells a PCB for CMoy’s Modified Linkwitz Crossover as well as a PCB for a small power supply.

What are buffers and why do we need them? A buffer (also called a line driver or just driver) is a 1x amplifier (voltage in is the same as voltage out) that can source and sink high currents. This means the amplifier chip can operate very accurately with high quality since it doesn’t have to worry about driving high currents. And why three channels? In short, a two channel (Left+Right) amplifier has the return currents from the headphones going into signal “ground”, which has some disadvantages that I won’t go into here. Providing a third channel for the headphone’s ground means the return currents are handled by a high-current buffer and not by the signal ground. This also improves the sound — any mischief the headphones get up to is completely isolated from the amplifier chips.

The only disadvantage of the Pimeta is that the buffers it uses — the Intersil BUF634 — are hard to find. Apparently the manufacturer only runs the production line every few years because the demand is not very high — the last time they made any was in the 90s, and now no one has any in stock. Fortunately buffers are a standard component; unfortunately there is no standard pinout for them (as there is for op-amps); fortunately the NatSemi LMH6321 is a drop-in replacement (it only differs in one pin which the Pimeta doesn’t use — no R11); unfortunately the LMH6321 is only available in a surface-mount package, no through-hole; fortunately Tangent’s Pimeta PCB has surface-mount pads (SO-8) as well as through holes.

Amplifier and crossover, with front-panel components

The Pimeta is not a good choice for someone who isn’t already comfortable with soldering and parts selection. It’s not a kit; you get the PCB, and then you have to decide which of the many component choices will work for you, and order them yourself. The traces and pads are smaller than the Ramsey’s, and of course hand-soldering surface-mount chips is not for beginners.

Current drivers are the surface-mount chips

My component choices: L/R opamp: AD823 (gain 4); G opamp: AD843. Buffers: LMH6321. Alps 50K audio taper volume control. Panasonic rail capacitors, Wima ‘reservoir’ caps, no DC-blocking caps. Neutrik NJ3FP6C stereo jack.

Power Supplies

The simplest, safest, and sanest approach is to use an external “wall-wart” that provides the appropriate DC voltage. I’m not sane, I’m more interested in doing something cool and interesting than simple, and Danger is my middle name. I decided my amplifier would be AC powered.

Wall power is not for beginners. It will kill you and your dog, and burn down your house besides. If you have any doubts about your ability to do it safely, don’t.

For the supply voltage, I decided that around ±12V sounded good, which meant an input DC voltage around 24V. I picked a 10 VA 22V (nom.) Amveco toroidal transformer. With Tangent’s TREAD power supply design I get 23.5V out. A fused IEC socket (the kind on PCs) is the power input, a regular PC-style power cord is used.

The Power Supply

Back Panel

Shielding

Since this amplifier would plug into the wall socket, it was important to have a metal case that could be connected to pin 3 of the wall socket. Since the power supply is fully isolated I could have connected signal ground to the case, but I went the “overdesign” route: an additional inner shield that would be connected to signal ground, and insulated from the case. The case is a Hammond 1455N2201, and the cage is made from #16 copper mesh.

Amplifier in copper-mesh faraday cage

In the picture, the front panel is insulated from the cage by a sheet of acrylic glued to the inside of the panel — you can just see it as a darker area just inside the panel’s edges. The shiny bits are the sheet tin that forms the front of the faraday cage. The cage is wrapped in a 0.0.15″ teflon sheet before the amplifier is slid into the case.

The power connector — red plug in the picture — is by Deans, used for radio-controlled (R/C) electric airplanes. I think the signal connector is also a Deans but I forget. It’s a 4-conductor connector often used in R/C gliders for the wing/fuselage joint, to automatically connect the wing servos when the wing is attached.

Also, the three signal wires from the back panel travel inside a grounded brass tube to the front, where the amplifier is. This should reduce any 50 Hz hum picked up from the transformer.

Final assembly: checking fit

How does it sound?

Great! I like the crossover at the “high” setting, and it has a very neutral natural sound. I listen to classical and jazz, so that’s exactly what I wanted. The shielding works great — I can put my phone on the amplifier case and never hear the GSM buzz. However, the headphone cable (and to a lesser extent, the patch cable from the computer) still picks up noise. It’s not very loud but it’s still annoying, so version 3.0 will use fully balanced connections. (That will require modifying my headphone plug for four conductors instead of three.)

Suppliers

For components, I like Digi-Key (based in Minnesota); for things they don’t carry, Mouser (in Texas). Odds and ends from Small Parts Inc.