by Chu Moy

"Thank you for your amplifier design. I built it and can't believe how wonderful it makes my AKG K340 headphones sound as well as my Sennheiser 600."
- A DIYer.

While doing research for the article Designing an Opamp Headphone Amplifier, I built a portable headphone amplifier for testing purposes. Each channel uses a single Burr-Brown OPA134 opamp in a non-inverting configuration. It has adequate current capability to drive most headphones without an output stage. I have used it with Sennheiser 465s (94dB SPL) and achieved ear-splitting volume. The amplifier is ideal as a booster for power-conserving stereo sources such as portable CD players and for interfacing with passive EQ networks such as tone controls or a headphone acoustic simulator.

The Amplifier Design

Figure 1

The schematic for one channel of the amplifier is shown in figure 1. All of the parts, except for the opamps, are available from Radio Shack. In several instances though, higher quality parts are available from other sources for about the same price that Radio Shack charges. The parts are commonly available, so look around for good buys. I do recommend Radio Shack's 1/4W Metal Film Resistor Assortment (RS 271-309). It contains 50 resistors in popular values and nearly all of the values needed for this project. The total cost for this project should be no more than $20 - $25 US, assuming you already have general purpose items such as wire (I used solid 22 ga.).

The original opamp for this design, the OPA132, has been discontinued. The OPA134 is the audio-specific version of the OPA132 and will work identically in this circuit. It was selected for its excellent specs: FET inputs for high input impedance and low offset current, 8 MHz bandwidth, 20V/uS slew rate, ultra low noise, ultra low distortion, etc. It has fine PSRR (power supply rejection) numbers, can run on as little as ±2.5V (very important in a portable design) and includes built-in current limiting. The OPA134 costs less than $3.00 per unit from Digi-Key Electronics. It comes in a dual version: the OPA2134, but the single version is easier to wire and avoids thermal crosstalk distortion between the channels. Be sure to get the "DIP" package opamps; SOIC opamps are miniatures that are very difficult to handle.

Other opamps can be substituted, but make sure they will work with battery voltages (as little as ±3V) and are stable without external compensation. Also check the opamp's current capability and current draw. The OPA134 has a quiescent current of about 4mA and will not drain the battery excessively. It can output almost 40mA into a short circuit at room temperature. Modern dynamic headphones need about 10mW to reach full volume. For more information, see Understanding Headphone Power Requirements.

The OPA134 is wired as a non-inverting amp with a gain of 11. At this gain, the output impedance of the amplifier is less than 0.2 ohms throughout the audio range. The high-pass filter C1-R2 at the input blocks DC current and has a corner frequency of about 15Hz. Substituting a 1uF capacitor will lower the corner frequency to 1.5Hz. However, 1uF capacitors tend to be too large for the recommended enclosure. Instead, if a lower corner frequency is mandatory, try increasing R2 to 1M (and scale R1 accordingly). You could omit C1 entirely, if DC input protection is not important. I recommend leaving C1 in the circuit. R5 is an optional load resistor, which reduces residual hiss when the amp is driving low impedance headphones such as the Grados (32 ohms). Because the voltage drop across R5 reduces the maximum output of the amplifier, I recommend trying a 50 ohm resistor first (or as low as 30 ohms). If there is still residual hiss with low impedance phones, then increase the resistor value to 100 ohms

The original pocket amp did not have a volume control, due to insufficient space in the enclosure (but see the next section for information on adding mini-pot volume control). Nor was a volume control necessary since the intended audio sources such as portable CD players and FM stereos already had volume controls. I did want the ability to reduce the input level as required to avoid overloading the amplifier (for example, some portable stereos have very high output voltage levels even when the volume control is set near 0). With R1 = 100K ohms, the LEVEL switch (SW1) drops the input voltage by 50% (6dB). At R1 = 470K ohms (the value I used), the switch attenuates the input by 15dB.

Replacing the Level Switch with a Volume Control

Figure 2

Several DIYers have written me to ask about adding a true volume control to the amplifier. In figure 2, R1 and SW1 are replaced with a dual, audio-taper mini potentiometer. The suggested pot values are 10K to 50K ohms. The recommended enclosure is barely 1" tall, and the front panel is already crowded with and LED, switch and jacks. DIYers have reported good results with dual 10K Clarostat DX585 and Panasonic EVJY10 pots and a dual 50K Noble XVB93 pot.

The Clarostat 585 pots have a maximum value of 10K ohms and can be ordered from Newark Electronics (part no. 585DX4Q25F103ZP). The Panasonic pots come in 10K and 50K values, and are available from Digikey (part nos. P2G1103-ND for 10K, P2G1503-ND for 50K). Carl Hansen, who designed a PC board for a crossfeed version of this amp (see the addendum for details), used to sell the Noble pots for $4.75 each, but does not sell them any longer. A physically larger, more common 100K pot (Radio Shack RS 271-1732) can be used if the value of R2 is increased to between 200K and 1M. (C1 can remain at 0.1uF, and the threshold frequency of the high pass filter will decrease with larger values of R2.)

The diagram above shows how to wire the Clarostat, Panasonic and Noble pots (thanks to Apheared in the forums for the data on Panasonic pots). Use an ohmmeter to confirm the wiring diagram. First, choose one section of a dual pot to check. Connect an ohmmeter to measure the pot resistance from the middle terminal (wiper) to one of the end terminals. Then monitor the meter as the pot shaft is turned clockwise from minimum to maximum. If the resistance increases as the pot shaft is turned clockwise, then the end terminal being measured goes to the amplifier ground. If the resistance decreases as the pot is turned clockwise, then the other end terminal should be grounded.

The Power Supply

Figure 3

The power supply circuit (figure 3) converts the 9V battery into a ±4.5V dual supply. Although the OPA134 could run from a single supply, it (and other opamps) are designed for dual supplies, and a dual supply is required for direct-coupling the output. This virtual ground sits at 4.5V, but works because opamps only care about relative power supply voltages. At idle, the opamp output is still 0V (minus a millivolt or two of offset) without capacitor coupling. However, if the headphone amp will also double as a preamp, add a capacitor to the opamp output to block DC, if the input stage of the power amplifier is direct coupled.

The left and right channels are connected in parallel to the power supply. If the amplifier will be driving high impedance headphones (e.g., greater than 200 ohms) or if the DIYer simply would like more volume than the standard design can provide, I recommend increasing the power supply. Appendix 2 below discusses power supply options in depth: adding dual 9V supply, making a battery pack, recharging 9V NiCad batteries, choosing an AC adapter, etc.

Putting It Together

I assembled the circuit on a printed circuit, 3-hole pad protoboard. I used a Vector Circbord board from Mouser Electronics (Stock No. 574-3677-6). This Circbord has an excellent circuit pattern (featuring numerous bus strips throughout) for this project. Radio Shack sells non-solder-plated boards, which are an acceptable substitute, but the copper will oxidize in time. I cut a small square (about 2" x 1.75") of the protoboard with a utility knife to fit the case (mark a section on the board, score it several times with the utility knife and straight-edge, and then break off the section). When cutting the board, make sure to include at least 3 foil "buses" for the power supply and ground. I socketed the ICs using gold-plated machined-contact sockets which work with low insertion force.

The case is a 2.75" x 4.6" x 1" PacTec enclosure from Radio Shack (RS 270-211) with a built-in 9V battery compartment (Mouser sells similar PacTec plastic cases in the colors bone and black). It comes with both opaque and red plastic front panels. I chose the red plastic panel because the opaque panel was too thick to mount the headphone jacks. The headphone jacks are enclosed units for 1/8" stereo plugs. Radio Shack sells a version of these jacks (RS 274-249). I ordered higher quality units that have spring-loaded contacts from Mouser Electronics (Stock No. 161-3502).

Figure 4

The layout of the switches, jacks and the power LED on the front panel is shown in figure 4. The placements are a little tight, but I think it turned out well. By the way, the LED is a 2mA (max) low current type. It is biased at less than 1mA to conserve battery power and still produces a very bright light. The LED was placed in a LED holder (RS 276-079) before being mounted on the front panel.

Note: If the amplifier is housed in a plastic enclosure, the LEVEL switch must be grounded or the amplifier will hum when the switch is touched. To ground the switch, strip about 1.5" of insulation from a 5" length of 22 ga. solid wire, tin the exposed end if necessary, and tightly wrap the exposed end around the groove at the rear of the metal mounting flange of the switch, twisting the end to form a secure, closed loop. Trim the other end of the wire to a suitable length and solder it to the circuit ground. The same is true if a volume control replaces the level switch. If the pot has a metal shaft and the amplifier will be mounted in a plastic case, the pot housing may have to be grounded to prevent hum. Follow the same directions for grounding the level switch housing.

The project came together very quickly - about two evenings - and without incident. I attribute the quick assembly to the simple design of the circuit and the neat layout provided by the Vectorbord. The circuit was first built on a standard breadboard and then transferred to the Vectorbord. The amp worked immediately when the power was applied. I did tweak the power supply for improved stability. My amplifier does not have a belt clip, but add-on belt clips are available at Radio Shack.

The Results

The sound of the amplifier is excellent, with solid bass and a sizzle-free, detailed high end. It powered my Sennheiser 465 headphones effortlessly. A 9V alkaline battery can power the amp for several days of continuous play (high-capacity NiCad and NiMH rechargeable batteries will also work). When paired with my modified Linkwitz acoustic simulator, which is housed in an identical enclosure, the set make for a truly "dynamic duo". I pack them and a CD player for travel in a Case Logic KSDM-1 case. Since the amp and acoustic simulator are lightweight, they are well-suited for people on the go who like to take with them a complete listening system (of course, you could build both projects into a single enclosure for even greater convenience). Given the low overall cost and the high quality parts used, this project "amply" rewards for the modest expenditure.

Appendix 1: Ideas for Troubleshooting Noise

When built as recommended above, this amplifier is a quiet performer with virtually no background noise. It is more immune to EM and RF interference than some other amplifiers I have heard. The pocket amplifier remained quiet when tested near an old elevator facility that was known for generating loud crackles in another, more susceptible design. Nor did I hear any RF despite that the building had an internal RF communications system.


Figure A1

Nevertheless, there have been a few reports of problems with noise. The first step in troubleshooting noise is to make sure it is coming from the amplifier itself, and not from the audio source. Disconnect the audio source and listen to the pocket amp for any background hiss, static, RF (radio frequency) or EM (electromagnetic) interference. If the headphones are low impedance types (such as the 32 ohm Grados), the amp will function better with the load resistor R5 installed (see figure 1). The load resistor can be added either inside or outside the opamp feedback loop (figure A1). Inside the feedback loop is the preferred method because it keeps the output impedace of the amplifier very low, but some DIYers have reported better performance with the resistor outside the loop.

If the noise is primarily RF or EM interference and is not coming from the audio source, it is probably due to long interconnects and headphone cords, which can act as antennas that channel RF signals into the headphone amplifier. The easiest way to block RF noise is to place one or more clip-on ferrite noise suppressors on the audio cables. They should be located on the end of a cable as close as possible to the input or output of the headphone amplifier. The clip-ons can be removed if the interference is temporary and subsides. See A Quick Guide to Headphone Accessories for more information on ferrite clip-ons.

Another way to deal with RF/EMI interference is to shield the circuit either by putting the it in a steel or mu-metal enclosure (connect the circuit ground to the metal case) or by lining the interior of the plastic enclosure with a shielding foil (such as copper). The bottom of the case where the circuit board rests must be insulated with electrical tape to avoid shorting out the amp. If foil is used, it must be connected to the circuit ground. Copper foil shielding tape could also be used (stain glass supply retailers sell inexpensive copper tape).

DIYers have told me that the high gain of the pocket amplifier can emphasize hiss from noisy portable CD players or other audio sources, especially when driving low impedance, high efficiency headphones. If CD player hiss is a problem, try taking the CD output from the Line Out instead of the Headphone Out - in which case, the amplifier must be constructed with a true volume control instead of the LEVEL switch as discussed above.


Figure A2

Another option is to reduce the gain of the amplifier to minimize hiss. Try a gain between 2 and 6 (R3 = 10K ohms to 4.7K ohms). If the amplifier will also be used with higher impedance headphones that can benefit from higher gain, make the gain adjustable with a switch to select between different value feedback resistors (figure A2). Again, make sure to ground the metal housing of this feedback resistor switch to prevent hum and noise from the switch itself (see instructions for grounding the level switch above).

Appendix 2: Power Supply Options


Figure A3

There are several situations, where the pocket amp could benefit from a higher voltage power supply - when driving high impedance headphones, when the amplifier is being fed from a high gain equalizer or when the listener just wants more volume. With very high impedance headphones (600 ohms or more), the amp may not be able to develop sufficient voltage across the load for maximum power transfer. If the amp is fed from an equalizer or tone control with a high boost, the output of the pocket amp could be driven into clipping.


Figure A4

In such cases, I recommend using a ±9V dual battery supply, which is nothing more than two 9V batteries in series (figure A3) or an external power source such as an AC adapter or battery pack (figure A4). R1 can remain 10K ohms, but any value between 10K and 15K ohms will work fine. Unfortunately, two 9V batteries will not fit in the specified enclosure for this project. The Pac-Tec model K-HML-ET-9VB measures 4.6" x 2.75" x 1.5" and has a compartment for two 9V batteries (Newark Electronics part. no. 93F9946).


Figure A5

Figure A5 shows a simple 15VDC external battery pack consisting of 10 AA batteries in a battery holder. The battery holder is Caltronics model BH107 and has snap terminals which fit standard 9V battery snap clips. Radio Shack sells an 8 cell version (RS 270-387) which will output 12VDC. The cable can be any thin 2-conductor cable. I made my own cable by braiding 3 lengths of 24 ga. stranded hookup wire (2 black and 1 red). Only 1 red and 1 black wire carry voltage; the second black wire functions as a shield.

One end of the cable is terminated with a 9V battery clip (RS 270-324). The red wire from the battery clip will carry the (+) voltage when connected to the battery holder and is connected to the red wire of the cable. Only one of the black wires is connected to the (-) wire of the battery clip; the other black wire is not connected on this side. The other end of the cable is terminated with a submini (2.5mm) 2-conductor phone plug (RS 274-289C). Wire the plug so that the tip carries the (+) voltage. The two black wires connect to the ground of the plug. Insulate any exposed connections with a thin layer of electrical tape.


Figure A6

The power jack is the matching submini (2.5mm) 2-conductor phone jack, closed circuit type (RS 274-292A). The jack is wired so that when the plug is inserted, the internal 9V battery is automatically cut off (figure A5). If the 9V battery were not cut off, the higher external voltage would flow into the battery and possibly cause it to explode. Therefore, the wiring of this jack must be done very carefully. Use a voltmeter to test the jack:

• With the jack unplugged and the 9V internal battery installed, the V+ output terminal should read about 9VDC.
• Insert the plug (do not connect the battery holder) into the jack. The voltage at the V+ terminal should read 0V (meaning that the internal battery has been cut off).
• Remove the internal 9V battery and connect the battery holder (with batteries) to the cable. The voltage at the V+ terminal should be about 15V (or 12V with the 8-cell holder). The voltage across the internal 9V battery clip should be 0V (meaning that there is no backflow of voltage into the battery).



The jack should be mounted in the upper right-hand corner at the rear of the enclosure's cover. Enlarge the mounting hole of the jack, as necessary, so that mounting nut will be installed flush with the top of the insertion tube (see figure A5). Note: the mounting nut MUST be flush with the top of the jack's insertion tube or the power plug will not seat properly - a dangerous situation that could short the battery pack. If either the internal 9V battery or external battery pack gets hot during use, there is short circuit somewhere. Disconnect the battery pack immediately and resolve the problem.



The battery pack also could short if the plug were to come partially loose in the jack. For this reason, I do NOT recommend using this battery pack while traveling, unless the plug can somehow be fastened immobily to the jack. Safer alternatives to the phono plug and jack are coaxial DC connectors, which are found on many portable devices. Coaxial power connectors will not result in a short if the plug is unseated. So far, I have not been able to locate a chassis-mount, mini coaxial jack that is small enough to fit in the enclosure. If the pocket amp is housed in a larger enclosure, Radio Shack sells several sizes of coaxial plugs and jacks (with switches).

The mounting threads of the power jack are in electrical contact with the power jack's ground. If the amplifer is put in a metal enclosure, the virtual ground and the power jack ground must NOT be connected together or the virtual ground will be shorted out. To prevent this occurrence, insulate the power jack's mounting threads from the metal enclosure with nylon washers or electrical tape on both sides of and within the jack's mounting hole. Use an ohmmeter to confirm that the power jack ground is not in electrical contact with the enclosure.



An AC adapter could replace the external battery pack. Most AC adapters are poorly filtered and will introduce noise into the amplifier. The best AC adapter for this project is a wall-wart with a regulated, non-switching supply. The adapter shown above (RS 273-1662) can output up to 12VDC at 300mA regulated. It also comes with a set of interchangeable power plugs, including a 2.5mm phono plug that should be compatible with the power jack in figure A5, so long as the voltage polarity is correct.


Figure A7

The circuits in figure A7 turn the AC adapter into a recharger. Figure A7a is the basic circuit, which comes from Stephen Lafferty's Headbanger amplifier design, except that the value of the current limiting resistor has been reduced from 330 ohms to 220 ohms to give a 20mA charging current with the 12VDC regulated supply. Figure A7b is the same circuit wired with the power jack. Note that the voltage output of these power supplies is always 9V. The 12VDC from the adapter only charges the battery. The recommended battery is a 9V high-capacity Ni-Cad (Radio Shack RS 23-299), which will achieve a full charge in 10 hours in this circuit. A 9V Ni-MH battery will also work. However, Ni-MH batteries are very sensitive to overcharging. The charger must be turned off when the full charge is attained or the Ni-MH battery life could decline.

Appendix 3: Turning the Pocket Amp into a Personal Monitor


Figure A8

Commercial personal monitors for musicians can be expensive, yet are essentially nothing more than headphone amplifiers with a limiter and/or a balanced input option. Figure A8 shows the pocket amplifier with both balanced and unbalanced inputs. The balanced-to-unbalanced converter front-end should be built with very close tolerance metal film resistors to maximize the CMRR. For more information about the converter, see Designing an Opamp Headphone Amplifier.


Figure A9

Figure A9 shows a soft clipper, which can go in front of the gain stage. For P1, try the miniature Clarostat, Noble or Panasonic pots discussed above. The soft limiter operates more like a compressor (figure 8). As the input voltage rises above the forward bias voltage of the diodes, the output voltage varies incrementally by about 1/10 V_in. The limiter's response can be changed by increasing or decreasing R2. For hard clipping which has a harsher sound quality but strictly clamps the audio signal, remove R2 and take the output directly off the diodes.


Figure A10

To preset the limiter threshold, set the input volume control to maximum. Then feed each channel of the limiter an audio signal that is as large as it will ever see (the circuit assumes that the input will exceed 1V). Adjust the Limit control to set the maximum volume in the headphones. Since different models of headphones do not have the same sensitivity ratings, the limiter must be readjusted if the headphones are changed. For tips on setting maximum headphone volume, see Preventing Hearing Damage When Listening With Headphones. For more information on limiters, see Designing a Limiter for Headphone Amplifiers.

A skilled DIYer could build the personal monitor with limiter in the same compact enclosure by using an OPA2132 dual opamp to make some space on the circuit board, though a larger case will make construction much easier. To include the balanced input option, use two OPA2132 opamps. The Clarostat pot listed above should fit on the front panel, but it and the XLR input jack can be mounted wherever is convenient. I also recommend testing the limiter on an experimenter's protoboard first.

11/23/00: Bob Scott put his pocket amp into an Altoids candy box and uses it between his Sony MD player and his Sennheiser HD495 headphones. He writes:

Attached are photos of my amp. I built it into an Altoids tin, partly for shielding, partly for the entertainment value. The only changes I made from your schematic was a slightly larger resistor for the LED to reduce current draw and using a "pigtail" for the input to save some panel space and reduce bulk when "cabled up".

I got the short-handled switches from Digikey. They kept the unit compact and reduced the likelihood of the amplifier turning on accidently. I may build a second copy using "dead bug" construction to see if I can make it REALLY small.



11/23/00: Carl Hansen has designed PC boards for the Levallois version of the pocket amp with Jan Meier's enhanced-bass crossfeed. He writes:

I have been spending the past nine months following your forums and building a number of variations of the pocket amp. I have decided that I have more than a few friends that would like to have one for Christmas in either kit or a variety of assembled form.... Because I have the resources available to me through my work I have gone ahead and laid out a nice little double sided board using Tango PCB, which I have sent to one of the commercial board houses in the Seattle area for a small "prototype" run. My boards arrived last week and I have assembled three of them and they work great!

The board house that fabricated the boards is fully automated meaning that no human hands were involved in the manufacturing process including a complete optical inspection using a robotic vision system.... I would like to sell off some of my excess boards. The price to sideliners in the forum like myself will be $6.50 each (or 3 for $17.00) plus $3.00 S&H which is about the same as the cost for using Vectorboard. To those that have posted contributions to the forum that have furthered the dialogue, particularly regarding the pocket amp, I would like to offer two boards each for free except the cost for S&H.

The specifications for the board are:

Dimensions: 1.80" X 2.45" with routed notches and corners to precision fit Pac-Tec case HML-9VB, leaving a 1.25" space behind the panel for components such as switches and jacks. The amplifier section is designed for dual OPA2132/4s with the crossfeed filters between the amplifier sections. There are provisions for two levels of enhanced-bass crossfeed filters plus flat. A 3 pole, 3 position rotary switch or some equivalent would be required to use all three settings. The filter capacitor component locations have multi-holes each to allow the use of different size capacitors. There is a provision for volume control or high-pass filter resistor. Gain of course is a matter of component selection. Personally I have found a gain of 5 to be the most versatile. There is also a provision for power indicator LED.

Shown below are the Levallois schematic and pictures of the Hansen PC board. For more information about the circuit, see Levallois' entry in the addendum update (p. 1) for May 4, 2000.






Update: C.E. Hansen is no longer selling the PC boards or the Noble XVB93 mini-pots described in the article. Instead, Jon M. Tsukiji (JMT in the forums) is now selling the PC boards for the same price, although he is NOT selling the Noble pots. JMT is also selling completed amps in the Penguin Mints boxes first shown by "Apheared." Contact JMT for pricing on the completed amps and to order the Hansen PC boards.

Jon M. Tsukiji
3142 Spruce Hill Ct.
Antelope, CA 95843
Email: JMT@surewest.net

3/14/2001: Major rewrite of article, including new appendix section on power supply options. Added new high resolution pictures.

3/14/2001: Coffin Lin put this amplifier (with a modified Linkwitz crossfeed front-end) in an old TV remote control case. He used an OPA627 opamp and made R2 and R3 in the Linkwitz filter adjustable instead of R1. The volume control is an Aiko pot in a shunted configuration with a 50K resistor (Dale RN55D), so that the audio signal passes through a single high quality resistor. Regarding his selection of the opamp, he writes:



I found that the OPA637 oscillated, even though the gain was set to greater than 5. The power supply voltage was not symmetric (2V/10V using a 12VDC AC supply). Then I changed the opamp to an OPA627, which was quite good for both my Sennheiser and Pro2 headphones, but the supply voltage was still not symmetric enough (6.4V/6.8V). The OPA134 got best result in stability (6.5V/6.7V), but the sound is too fat for me. So the final version is OPA627 - great detail, sound balance, clear, dynamic.



Lin put the Linkwitz filter at the input to the amplifier. The component values in his version of the filter are:

R1: 30K ohms
R2a, R2b: 15K, 10K
R3a, R3b: 50K, 100K
R4: 33K
R5: 33K
C1: 3,300pF
C2: 10,000pF

The resistors are Dale RN55D. About making R2 and R3 adjustable, he says:

I mistook R2 for R1, but on the Excel worksheet simulator, R2 can still alter some balance. I think that adjusting R3 is more effective than adjusting R2 (I forget which switch is for what resistor.) One has more stereo (good for dance and rock) and the other is more natural (good for jazz).

12/26/2001: Revised value of the current limiting resistor in figure A7. I reviewed Stephen Lafferty's circuit for charging a single 9V NiMH battery. The value of current-limiting resistor in Lafferty's circuit assumes that the specified unregulated 12VDC adapter will output 14VDC, because the amp is a very light load for the adapter. The recommended adapter in my project has a regulated output, so the output should be 12V exactly (or fairly close). Therefore, I changed the value of the resistor from 330 ohms to 220 ohms to get a charging current of about 20mA.

12/26/2001: Here are three candy box amps from forum members Doh, Droche and LivingPlasma. Doh put his Hansen-board amp in a Penguin Mints box (first shown by Michael Shelton - a.k.a. "Apheared"). He writes:



It looks like Apheared beat me to posting a Penguin Mints amp, but I swear I didn't steal the idea! Penguins rock! I'm afraid my amp isn't nearly as DIY cool as Apheared's creation, but it's only my second amp and I just learned how to solder a few weeks ago!

As you can see, the Hansen board is mounted upside-down in the tin with the power and crossfeed switches sitting right underneath the op-amps. It has dual pigtails and one position of crossfeed plus flat. There's no LED and volume is adjusted via an inline volume control from radio shack (soon to be replaced by a DIY version that uses the panasonic pot once those parts get in). I don't have any of that fancy tape, so I actually just stick a metrocard under the lid before I close it. (Haven't gotten around to glueing it in with some artist's spray mount quite yet).

I think that there is still enough space in the box to wire the pot inside if anyone feels like giving it a try. Personally, I like the flexibility that a modular volume control gives me. On the other hand, I'm still trying to think up a way to get rid of the pigtails to improve portability.

Just a note on drilling the holes in an altoids tin or other metal candy container. What I found to work really well are the black and decker "bullet" tip drill bits. They have a small extension at the point that bites into whatever you're drilling into so that the drill bit doesn't slip. The tip works its way through the metal fairly quickly, so after it's through you have a pilot hole that holds the bit steady while the rest of the bit does the work. Using these bits, I found drilling holes up to 1/4-inch to be no problem. The bits are available through amazon.com, but should be widely available.

Droche put his amp in the popular Altoids tin. He writes:



For any of you who think that building an amp is too difficult for a beginner- I am proof that it isn't. I started browsing the forums a month ago with no electronics experience whatsoever. After browsing for a while, I put in a few orders and before I knew it, I had a headphone amp. It took a few tries to get it into the box, but I finally got it in after removing the headphone jacks and adding pigtails and removing the pot. It was well worth the effort. I was amazed at how much better the sound out of my portable MD player got. Thanks to everyone here for all the helpful info.

Livingplasma put his amp in a round candy tin. He writes:



Not to take the attention away from Apheared, but I just couldn't help it after seeing all thse proud people post their version of the CMoy pocket amp. Those who have been here a while will know I had a string of bad luck making my first cmoy, this is what I came up with the leftover parts. It's the basic CMoy amp made with an OPA2134 and modified for the Meier crossfeed (changed some values so I could use a 50k pot and smaller input capacitor; yes, it's unbuffered). Input is through the pigtail, has a LED power indicator and uses one submini toggle for power and one for the crossfeed (on or off). Measures about 3 inches in diameter (not counting the controls), and just under an inch in height. Schematically, I think it's very similar to Tomo's version.

Drilling the holes on the side of the tin is annoying, to say the least. After a certain hole size, it's really hard to drill a hole, the bit catches on the metal and goes ripping the case apart (lesson learned trying it with a Altoids tin). I just made the hole as big as possible with the bit, then reamed it with either a screwdriver or a knife. The opening for the volume pot (it's those panasonic ones) is a square, I think I used some old diagonal cutters I didn't mind messing up and some pliers to bend and break the tin.

12/28/2001: Kenji Rikitake (a.k.a. "bdx" in the forums) built two versions of the pocket amp with an opamp-based virtual ground. He says:



The amp on the left in the picture is the OPA2134 version; the one in the middle is a single-amp version (OPA134). The breadboard on the right is for the OPA2134 version. The basic amplifier circuitry is same, but I changed the value of the feedback resistors to 1k/4.7k ohms pair. The 1k ohm resistor at the + input of the opamp protects it from accidental overcurrent or overvoltage (though the probability is very low). This is generally recommended when you make a non-inverted amplifier.



I tested with several different opamps. OPA2134 showed its excellence (only 3mV maximum output offset). NJM4580DD worked OK though it had 70mV maximum offset. The NJM082D (TL082 compatible) also worked but it could not fully drive my Sehnheiser HD414 Classic. Note that the NJM2043DD didn't work (caused self oscillation).



I also added an opamp voltage follower for providing the virtual ground, to stabilize the voltage exactly to the 1/2 of the unipolar power supply (namely a 9V dry battery). I tried the OPA134 as a unity gain buffer for the virtual ground driver and then the BUF634 (as ppl suggested). The quiscent current of the chip lowered from 4mA to 1.5mA or so, and the amplifier sounded the same. Note that OPA134 and BUF634 are virtually pin-compatible if you use the OPA134 for a unity-gain buffer. The 1N4002 diode protects the circuit from accidental inverse voltage connection.

The circuit is put into an aluminum case (T-SIN Denki TM-1) which can hold a 9V dry battery inside and can mount a 47mm x 72mm breadboard widely available here (Sanhayato ICB-88 compatible) in Japan. The FatBrain.com stickers are some of which I've got from the bookstore. The size of each amp is 87mm(W) * 31mm(H) * 103mm (D). I built the circuit on a glass-epoxy DIP breadboard. Since I proved the amp works fine with my Sony D-E880, Diamond Rio500, and Sehnheiser HD580 as well as with HD414Classic, I think I've got to build another one for my wife sooner or later.

More details about Bdx's amp and power supply can be found here.

2/21/2002: Added note about insulating the power jack ground when the power jack is mounted in a metal enclosure.

2/22/2002: Forum member Tangent has created a tutorial for electronics newbies who are interested in building the pocket amp. Many DIYers have found the tutorial very helpful. Please note that Tangent's opinions are not necessarily the same as this author's.