Portable Headphone Amplifier
you may pose the question, "But why? My MP3 / CD /
Minidisc player can already drive headphones. What is the
point of having an extra box to carry around merely to do
what the player already does?" To answer this, I
must first explain that the onboard amplifier in most
portable music devices is more of an afterthought than a
main design consideration. Quality of the parts is not
paramount; cost savings is. Manufacturers typically chose
the cheapest parts they can get away with for the amp
section just to make the player eke by with the (usually
crappy) earbuds that are included, or the aftermarket
pair from Sony or Koss or Aiwa (just to name a few) that
retail for about $19.95.
These "amplifier" circuits hardly have the current output capability to drive headphones cleanly (that is, without clipping/distortion), and with good reason. Current output costs considerable battery life. What would you rather spend your money on; an MP3 player that boasts a 20 hour battery life, or one that has a mere 6-8 hour battery life?
The external amplifier will extend battery life of the portable music player, because the headphones are not drawing current from it anymore. It is merely supplying a voltage reference signal for the amplifier. Let's do a little bit of math: With a typical set of headphones/earbuds connected that have an impedance of 64 ohms (some are lower, some are higher), at 2.5 volts rms output from the player (aka very loud), the headphones will draw 39 milliamperes. (Ohm's law. 2.5v/64ohms=0.039A). Most onboard amplifier circuits cannot deliver 40 milliamps of current, hence the distortion and clipping. This particular headphone amp has an input impedance of 10 kiloohms. 2.5v/10000ohms=0.00025A, or 0.25 milliamperes. Less than 1/150th the current draw. High current draw causes voltage rails to sag and amplifiers to clip. Without this heavy load, the onboard amplifier in the player can now produce a much cleaner output signal, while the external amplifier supplies the heavy current demand to the headphones.
Ever notice when you're listening to loud music, the cymbal crashes sound muffled and garbled every time there's a big hit of bass? It's a disgusting experience, and that is why people use headphone amplifiers. Plus, they look really cool! I've built amplifiers similar in design to this one in various mint tins for friends, and I still have a couple up my sleeve that I'll be building and selling, including a round tin, and a tin shaped like an NES controller. I've also built a much beefier and robust desktop amplifier with a regulated AC power supply, high-current buffers, and some of the best ICs used in very high-end audiophile grade equipment.
Upon redesign and completion of the desktop HA-1 MkII headphone amplifier, I realized the need for a portable amp. Since the HA-1 now had an internal regulated power supply, it was larger than the MkI version, and not very portable. When I was shopping for parts and an enclosure for the HA-1 at Active, I also picked up the smallest 1455 series enclosure that Hammond makes. I figured I would eventually put a portable amp inside it. :)
No more than three days had passed between completion of the HA-1 than I decided to start work on the PHA. Of course, due to the limited internal dimensions, the design of the amplifier would be considerably simpler than it's big brother, and construction only took a few hours (spread out over the course of a couple days; I had to wait for the ALPS pots I ordered to arrive in the mail. More on this later).
I was hoping I could fit a pair of 9v batteries in the enclosure sideways, in order to provide decent power, but the enclosure is about 4mm too narrow. This limited me to a single 9v battery and a TLE2426 rail splitter for the power supply. I had originally planned to use 470µF capacitors on the voltage rails, then found a pair of matching 1000µF caps in my parts bin. I assembled the amp using these caps, but then remembered I had a box full of dead motherboards. After a brief search, I found a board with several 3300µF capacitors on it that had the same diameter as the 1000µF caps I originally used. They were replaced immediately. Not only would the extra capacity help, but so would the ultra-low ESR.
Assembly went rather smoothly, all things considered. The capacitors, along with the PC board, fit perfectly inside the case diagonally from corner to corner, and the battery fit between the caps like a glove, as well as between the PCB and the back panel of the case. Drilling the front plate for the headphone jacks and the volume pot was a much nicer experience than drilling through a thin, flimsy mint tin. The drill bit actually cut a round hole in the aluminum, versus just plain tearing an opening into the thin sheet metal of the mint tins I used to build my amps into. I don't think I'll be building many more mint-tin amps as long as Active Electronics keeps their stock up of Hammond 1455's.
As for the actual amplifier itself, I did some research on Tangent's website (Thanks for all the information! I learned a lot!) to help me decide which opamp in my inventory to use for the project. I settled on the Burr-Brown OPA227, or more specifically, the dual-channel version, the OPA2227P. This chip will supply up to 45mA of current, can operate down to a supply voltage of 5 volts (about 5.4v in reality), has a quiescent current draw of 3.5 mA, and has a THD+N specification of 0.00005% at 1 kHz. (yes, that is four zeros between the decimal and the 5). It also has a channel seperation of 110 dB at 1 kHz.
Audio-taper dual-ganged pots are absolutely impossible to find locally. It's really really annoying! Active boasts a "large assortment" of potentiometers in their latest flyer, but they have exactly 0 (zero) audio-taper pots in stock. I had to order several ALPS RK097 pots online. They're quite nice actually, and have a built-in SPST switch, which saved precious enclosure space.
Feedback and gain resistors are Vishay-Dale's RN55D metal film 1% parts, hand-matched, and set the overall gain to aproximately 3. This is sufficient for low-impedance headphones, as their volume output is more dependent on current supply rather than voltage supply.
Copyright © 2006 Orcinus Orca