Nr. Parts description Basic kit
1 PACTEC enclosure top plate +
1 PACTEC enclosure bottom plate +
4 Enclosure screws +
2 Battery contacts +
1 PACTEC enclosure beltclip +
1 Enclosure frontplate  
1 Cable to connect battery contacts with the PCB  
1 Two-sided PCB +
1 9 mm ALPS potentiometer 2x100 kOhm with ON/OFF switch  
1 Dip-switch horizontal 2 x ON-ON (S1)  
1 Volume knob +
2 1/8" stereojack CLIFF JYS35 (or similar)  
3 Scottky Diode  
2 LM6171 opamp DIP-package  
1 LM2662M  
1 PCM2704DB  
1 12 MHz Quartz HC49/4H enclosure  
1 USB-mini-B receptacle (MOLEX 54819)  
1 LED low power  
1 Power adapter  
3 22 microH inductor  
1 470 microH inductor  
2 3-fold pin-header (S5 & S6)  
1 TLE2426CLP +
5 22 Ohm SMD-resistor 1206 size  
6 1,5 kOhm SMD-resistor 1206 size  
4 3,3 kOhm SMD-resistor 1206 size  
1 6,8 kOhm SMD-resistor 1206 size  
4 10 kOhm SMD-resistor 1206 size  
2 12 kOhm SMD-resistor 1206 size  
2 22 kOhm SMD-resistor 1206 size  
2 56 kOhm SMD-resistor 1206 size  
1 1 MOhm SMD-resistor 1206 size  
2 33 pF SMD-capacitor 1206 size  
2 47 pF polystyrol  
2 330 pF polystyrol  
2 1,5 nF polystyrol  
4 220 nF polyester RM 5  
3 68 nF polypropylen  
2 180 uF RM 2,5  
9 560 uF / 6,3V RM 3,5 (12,5mm height)  
8 470 uF / 16V RM 3,5 (12,5 mm height)  
This article presents the design of a relatively simple but nice sounding portable headphone amplifier.

To build this amp requires some basic skills in soldering as well as in electronics design. The description of the various aspects of the circuitry is kept rather short. For more information you have to consult textbooks, data-sheets, and/or other sources of knowledge (e.g. HEADWIZE). Sorry, but apart from the info given below I will not help you with your quest for more knowledge!

Optionally the amp has a build-in USB-DAC and can be connected directly to a computer. The quality of sound easily surpasses that of an average soundcard as found on most computers.

Apart from the enclosure and the PCB only standard parts are used that can be easily found in most electronics shops. To facilitate your job I do offer two kind of kits that provide you with some of the more difficult to get parts. You only have to pay for material costs and for shipping fees. Just drop me a mail if you're interested.

!!! Note that the use of this design is for personal applications only. Commercial applications are not allowed !!!

Happy building!
                                Jan Meier
A DIY PORTABLE HEADPHONE AMPLIFIER WITH USB-DAC
The amplification circuitry of this amp is build around an opamp. However, instead of the standard solution that uses a non-inverting opamp stage with a fixed amplification factor that is preceded by a potentiometer to attenuate the incoming signal, this design uses an inverting opamp stage with a variable gain factor. The gain factor is set by the potentiometer. There are some major advantages to this somewhat unconventional approach.

Very importantly background noise at low volume settings is strongly reduced because of the very low gain factor of the output stage. In a more conventional approach with an output stage with a fixed gain factor the noise of this output stage can become very dominant, especially when using high sensitive headphones.

Also feedback is stronger at lower..medium sound levels and thereby distortion is further reduced when compared to conventional approaches.

The voltages of the input stages in an inverting opamp setup are kept at ground potential and do not fluctuate. Thus influences of non-linear input capacities are strongly minimized. Inverting opamp stages normally sound better than non-inverting stages.

Of course there is also a drawback. The phase of the original input signal is inverted. However, since most people are unable to hear any difference between an inverted and a non-inverted signals this aspect is mainly of academic interest only.
The schematics of the design are shown in the picture above. The blue components are the one needed for the amplification circuitry and the power circuitry. The black components are needed if people are also interested in implementing the USB-DAC. The schematics show the LM6171 opamp but there are many other options possible, each having its own sonic signature.

Basically the left amplification channel only requires one single opamp IC1 and two resistors, R11 and the variable resistor represented by the 100 kOhm potentiometer Rpot. The amplification factor is equal to Rpot/R11.

C1 was added to improve stabilisation of the opamp. In principle it is not needed when using low-speed opamps but in combination with the LM6171 its use is mandatory to prevent oscillation of this extremely fast (100 MHz, 3600 V/usec slewrate) opamp. Even with slow opamps it never hurts to leave this capacitor in place, provided you use one of high quality (polystyrene or polypropylene are recommended!).

R9 helps to decouple the non-inverting input of the opamp from RF-noise on the signal ground and prevents fast opamps to work as a radio-receiver. Again, this is not necessary when using slower opamps but it never hurts to keep this resistor in place.

R13 and the switch S5 were added to be able to increase the gain factor when driving high-impedance headphones which need higher output voltages. When S5 is closed R13 is placed in parallel with R11, thus lowering the gain factor as set by Rpot/R11. The maximum amplification factor of the amplifier is 17 dB. This allows to drive inefficient, high impedance headphones even if a source with a low output voltage (portable) is used. However, if people only use high or medium efficient headphones and/or sources with a high output voltage (stationary CD-player) then it is recommended to use a potentiometer with a lower resistor value. This will simply improve volume control and prevent channel imbalances at very low sound levels (which are an inherent "property" of mechanical potentiometers).

R15 provides a low resistance DC-path for the input bias currents of the LM6171 and helps to reduce any offset at the output of the opamp. The input bias currents of the LM6171 are relatively high (bipolar inputs) when compared to most other opamps (having fet-inputs) and the only DC-path provided is through the potentiometer. At high gain factors, when the resistance value of the potentiometer is high, the product of high bias current and high resistor value results in a rather strong voltage offset. R15 is not required when using fet-opamps.

R17 connects the output of the opamp to the negative voltage rail and drives the output stage of the opamp into class-A operation. With some opamps this greatly improves sound quality but it also increases current consumption. The optimal resistor value strongly depends on the opamp model used and the supply voltage (as well as personal taste!). Trying is the only way to know.

R19 is only required when using fast opamps that don't like to drive high capacitive loads (headphone cables) and improves stability.

R1 and C3 form a low-pass filter that keeps RF-signals away from the inverting input of the opamp.

R5..R8 and C5..C9 (in combination with R1 and R2) form a crossfeed filter that can be activated with S1. If crossfeed is not required these components can be left out.

Power is supplied by a 9V battery or a 9 .. 28 Volts external powersupply. The power voltage is splitted by a TLE2426 rail-splitter to create positive and negative powerlines of 4,5 .. 14 Volts. It is not allowed to connect an external supply and a battery simultaneously. The amp is protected against reversed voltages by D3. The amp is switched ON/OFF by S2..

For people who also like to implement the USB-DAC please check the website of Texas-Instruments to download the data-sheet of the PCM2704. The function of the black components in the upper left side of the schematics will thus become apparent.

It is possible to connect the analog outputs of the PCM2704 to ground using 680 Ohm resistors. These drive the output stages of the PCM2704 into class-A and help to improve sound quality. You will not find an explicite place for them on the PCB but it is relatively easy to add them by hand.

Be aware, soldering the PCM2704 is extremely complicated as this chip comes in a 28-pin SSOP package! It requires a very experienced hand and proper soldering tools to do the job! Implementing the USB-DAC is not a job for beginners!!!

The circuitry around the LM2662 was added to create a negative 5V powerline from the positive 5V USB-powerline to power the amplification section so the amp can be used without battery or external powersupply when connected to USB.

Below please find a list of all the parts needed when building the USB-version of the amp.

All resistors, ceramic capacitors, and diodes are SMD components in 1206 size.

All capacitors in the signal path are specified as polypropylen or polystyrol. Alternative choices are possible but be aware that the quality of these capacitors has a strong influence on sound quality.

The potentiometer made by ALPS has a built-in ON/OFF switch. Of course alternative choices as well as the use of a separate switch are possible.

The JYS35 stereo input jack has built-in switches. If a plug is inserted it automatically connects to the analog signal. If the plug is removed it automatically connects to the signal of the USB-DAC. This jack with the same pin layout is also offered by some other brands. Check the CLIFF website for more detailed information.

Some parts, especially the enclosure and the PCB, are rather difficult to organize. Since I do have some parts left from previous production runs two kind of kits are offered, one basic kit that offers enclosure and PCB and a more extended kit that also offers difficult to get parts for the USB-DAC. People only have to pay for material costs and shipping. If interested simply drop a mail at:

meier-audio@t-online.de

Note 05 Jan. 2010: Parts for the extended kit are no longer available. Therefore only the basic kit is offered now.