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 popular dual version: the OPA2134, which contains two opamps in a single package. 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.

Amplifier Frequency Response

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.

If the amp will be driving low impedance headphones (32 ohms or less) such as the Grados, see appendix 1 for ways to optimize the amp for low impedance loads. R5 is an optional load resistor, which is explained in appendix 1. It can help reduce residual hiss and keep the power supply balanced.

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 enclosure in the prototype is barely 1″ tall, and the front panel is already crowded with and LED, switch and jacks. Mini dual pots are hard to find. Currently, Tangent’s Parts Shop is selling the ALPS RK097, a dual 10K audio mini pot, for a reasonable $3.25. Digikey sells the Panasonic EVJY10 series pots in 10K and 50K versions (part nos. P2G1103-ND for 10K, P2G1503-ND for 50K) for less than $3 each. The excellent dual 10K Clarostat 585 conductive plastic pots can be ordered from Newark Electronics (part no. 585DX4Q25F103ZP) for less than $3 each. Radio Shack sells a physically larger, dual 100K pot (RS 271-1732), which will work 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 and Panasonic pots. The ALPS pot has the same wiring as the Clarostat. 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. Choose the largest filter caps (C1 and C2) that will fit in the enclosure. I used 220uF caps, but would gladly have replaced with 330uF or higher caps if my enclosure had been bigger. Appendix 3 below discusses power supply options in depth: adding dual 9V supply, making a battery pack, recharging 9V NiCad/NiMH 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 PacTec HML-9VB (Mouser 616-62582-510-039 or 616-62578-510-000). It measures 2.75″ x 4.6″ x 1″ with a built-in 9V battery compartment and both opaque and transparent red plastic front panels. (Note: PacTec may discontinued the red panels). I chose the red plastic panel because it’s thinner and easier to mount the headphone jacks. Many DIYers have been using colorful candy mint tins as enclosures. If the tin’s interior is conductive, it must be insulated with electrical tape or it could cause short circuits. 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 can be either a low current type or an ultra-bright type. It is biased at less than 1mA to conserve battery power and still produces a very strong light. I used a 5mm LED placed in a LED bezel (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: Tweaking the Amp for Low Impedance Headphones

The OPA134 opamp produces a small DC offset voltage, which does not affect the amp’s performance when driving medium to high impedance headphones (over 100 ohms). Low impedance headphones (32 ohms or less) can cause the power supply to become unbalanced, because a small current flows though the load, even when the amp is at idle. This table compares the power supply voltages with the Sennheiser HD600 (300 ohms) and Sony MDR-G52LP (24 ohms) headphones connected to the amp.

Amplifier Load	V+	V-
No headphone	3.9V	-3.9V
HD600 (300 ohms)	3.9V	-3.9V
MDR-G52LP (24 ohms)	4.2V	-3.7V


Note: the battery by itself measured 8VDC.

There is disagreement about whether this almost negligible offset is worth the trouble to fix. With opamps other than the OPA134 series, the offset might be higher and the power supply imbalance could be greater. The offset has not damaged any of my headphones, but it might impact performance slightly by reducing the amp’s power output, injecting noise and/or draining the battery. To determine whether a certain headphone unbalances the power supply, measure the V+ and V- values with and without the headphones plugged in (and no music playing).

For those who want to reduce or block the offset current, figure A1 shows two ways to modify the amp for optimal performance with low impedance headphones: a) add a load resistor or b) AC-couple the amp’s output. A third way is to rebuild the power supply with an active virtual ground device like the TLE2426 or an opamp-based equivalent. Active virtual ground circuits are described in the addendum.


Figure A1

Solution A is the simplest and allows the output to remain DC coupled. The load resistor (figure A1a) will help stabilize the virtual ground and reduce any hiss or noise in the system. The load resistor does create a voltage divider effect with low impedance headphones, and so may lower the amp’s gain and maximum output power and possibly alter the frequency response. Some say that the pocket amp’s gain of 11 is too high for low impedance headphones, so the small drop in gain due to R5 might be desirable anyway. Choose a R5 value just large enough to stabilize the power supply without too much volume loss. I recommend a 1/4 watt, metal film resistor in the 20-50 ohm range.

Solution B avoids a voltage divider effect because although the capacitor blocks DC current, it is largely invisible to audio frequencies. The circuit in figure A1b shows how to switch between AC-coupled and DC-coupled outputs for the highest fidelity with medium and high impedance headphones (the load resistor in solution A could be switched too). Choose the largest value electrolytic capacitor that will fit in the enclosure. A 220uF capacitor will give a flat response down to about 22Hz in 32-ohm headphones.

Use a high quality, low impedance electrolytic capacitor to minimize any sonic coloration. High quality electrolytic caps don’t have to be expensive. The Nichicon Muse KZ series 470uF, 25V sells for less than $1.00 at the time of this writing. The Panasonic FC and FM series caps are also less than $1.00 each. The exotic Elna Silmic II series (which feature a silk fiber dielectric instead of paper) has a 470uF, 25V unit for less than $2.00 each. By comparison, an ultra high-end type like the Black Gate 470uF, 16V typically sells for around $12.00 each and is not recommended for this amp.

Appendix 2: 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.

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 amp is driving low impedance headphones (32 ohms or less), try installing R5 (see figure 1) and/or AC coupling the amp’s output as described in appendix 1.

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 3: 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, such as the Switchcraft 850X (Mouser 502-850X). 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, such as the Switchcraft TR2A (Mouser 502-TR-2A). 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. Safer alternatives to the phono plug and jack are coaxial DC connectors, which will not short if the plug is unseated. When the amp was being constructed, I could not find DC coaxial jacks small enough to fit on the side of the case. The Switchcraft 712A (Mouser 502-712A, Jameco 281842) fits in a 0.313 inch hole. The mating plug must accept a 2.5mm (0.1″) pin, such as Switchcraft 760 (Mouser 502-760, Jameco 281877).

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 NiCad/NiMH trickle charger with a 20mA charging current. Trickle charging takes longer but is gentler on the battery. The circuits are identical except for the value of the resistor that sets the charging current. Figure A7a is for the specified NiCad battery, and A7b is for the specified NiMH battery. The 9V NiCad from Radio Shack (RS 23-448) has a capacity of 120mAh and should achieve a full charge (8.2V) in about 5 hours. The 9V NiMH (PowerEx MH-96V230 by Maha) has a higher voltage and almost double the capacity of the NiCad. It will take almost 10 hours to fully charge (9.6V). NiMH batteries are very sensitive to overcharging. The charger must be turned off when the battery is fully charged to avoid shortening the battery’s lifespan. The 1N4001 diode prevents the battery from discharging backwards if the 12V adapter is not turned on but is still plugged into the amp.

Appendix 4: 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. This simple wiring trick for converting balanced signals to single-ended signals isn’t free: the signal amplitude is cut in half, but the loss can be compensated by turning up the volume. A true balanced converter that preserves the signal amplitude and noise rejection can be found in Designing an Opamp Headphone Amplifier.


Figure A9

Figure A9 shows an adjustable clipper, which can limit headphone volumes to safe levels. The maximum voltage that the headphone can see is 0.7Vp (the forward bias voltage of the diodes), so the clipper is most effective with high efficiency headphones of low to medium impedance (less than 200 ohms). High impedance headphones may not achieve enough volume even at the maximum setting. In that case, try replacing each diode with two diodes in series to raise the clipping voltage to 1.4Vp. The clipping effect is a little harsh because of the hard cutoff by the diodes. P1 is a trimmer pot or an inline stereo volume control, such as those made by Koss or Radio Shack.


Figure A10

The limiter can be set for only one headphone at a time. Different models of headphones have different sensitivity ratings, so the limiter must be readjusted if the headphones are changed. The more accurate and safest way to set the limiter is with an audio level meter and a headphone coupler (or artificial ear) sold by audiometric suppliers. If such equipment is not available, the limiter can be set by ear, but with less reliable results.

For initial testing, it is a good idea to use a pair of disposable headphones with the same impedance and the same or higher sensitivity as the intended headphones. Begin by turning the amp’s volume control to minimum. Do not connect the headphones yet. Feed an audio signal into the amp and turn up the volume until the diodes are forward-biased and clipping the signal. Use a voltmeter set on AC to confirm that there is about 0.7V across each of the diodes. The voltage should stay at about 0.7V even if the volume is turned up higher, indicating that the diodes are clamping the signal.

Set P1 in both channels for maximum resistance or set the inline volume control to minimum volume. With the trimmer pots, only one channel can be set at a time. With the inline control, both channels are set simultaneously, but if the channels don’t track precisely, always set the limiter based on the channel that is louder.

Connect the disposable headphones. Adjust P1 or the inline volume control slowly until the headphone volume reaches the desired level. Confirm that the limiter is working by turning up the amp’s volume control. If the volume increases, reduce the volume by readjusting P1 or the inline control. Repeat until the circuit clips at a consistent volume level. Then turn the amp’s volume control down to minimum and plug in the intended headphones. Slowly increase the volume and confirm that the clipping level is set correctly.

Once the pots are set, the settings must be protected against accidental change. While trimmer pots on a circuit board would be protected by the amp’s enclosure, it’s best to fix the thumbwheels in place with a dab of white glue. If an inline volume control is used, wrap the thumbwheel with electrical tape. 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.

c. 1998, 1999, 2000, 2001, 2008 Chu Moy.