OPA 2134 Stereo Headphone Amplifier
The purpose of this compact headphone amplifier is to boost the output of an MP3 player, mobile phone or PC that otherwise would not have enough power to drive certain headphones to an acceptable level. It's based around the Burr-Brown OPA2134PA dual opamp which lends itself perfectly to this project. The goal was to achieve the best possible sound quality with the minimum number of components using a single rail supply, and although it doesn't claim to be top end Hi-Fi, it does sound pretty good and has a nice detailed presentation
Because of the single rail power supply the circuit is AC coupled. It can run on 12V as shown here or if preferred, it will run quite happily on a 9V PP3 battery. Although it's accepted that AC coupling isn't as good sonically as DC coupling due to capacitors in the signal path adding distortion and phase errors, I dont think it's a show stopper here since most music sources have DC blocking capacitors in the output anyway and I've never heard anyone say "This music would sound so much better without series capacitors!" It's still best to keep the number of capacitors to a minimum though. Another advantage of AC coupling over DC coupling (apart from not requiring a split + and - power supply) is that your headphones can't be damaged by DC voltage at the output if a fault occurs. Note that there are no current limiting resistors in the output because being an opamp and not a power amp it only produces power in the low milliwatts so it's not going to cook anything!
The opamp is configured as an inverting amplifier. The gain of an inverting opamp is easily calculated by dividing the feedback resistor (Rf) by the input series resistor (Ra). In this case, 680K divided by 100K gives a gain of 6.8 which is just about right for this application. 100K was chosen for the input resistor to give the amplifier a high input impedance, allowing a smaller value input capacitor to be used. An input coupling capacitor for audio might typically be a 1uF electrolytic, but for this project I wanted to use polyester capacitors which unfortunately would be physically too large compared to an equivalent value electrolytic. Increasing the input impedance enables a capacitor of smaller value and size to be used, in this case 0.22uF. It's all to do with RC time constants and high pass filtering but that's another subject! Suffice it to say, a 100K resistor does the job. The two 47K resistors connected across the power supply form a potential divider to bias the non inverting inputs. This will set the outputs to exactly half the supply voltage, allowing them to swing equally up to supply and down to ground
The circuit is built on stripboard as is my thing, but I specifically wanted to fit it all onto a standard 25 x 64mm board. I usually like to use screw terminal blocks to connect my circuits, but as there is simply no room on this board, I opted instead for Molex KK connectors. The amount of pins is kept to a minimum too, by having a common ground with the inputs, outputs and power supply all being earthed through their respective sockets which are mounted together on the rear panel of a small extruded aluminium enclosure (Hammond 1455C801). A metal case also helps to shield the electronics from external electrical noise. If you're wondering why there's no volume control, I decided that because most things already have one built in, another one wasn't really needed
Socket wiring (earthed through rear panel)
Warning: This amplifier can go loud and could damage your hearing if used at high volume for long periods. Please be safe!
A good quality amplifier needs good quality interconnects to really give of its best. You don't have to spend a fortune though, as there are products available from Rean (a brand of Neutrik) who make connectors that are consistently well made and good value. There is also a low cost cable made by Van Damme with a foil screen that conveys remarkable musical detail for its price
Another opamp that's worthy of a listen is the LM4562. To me it sounds a bit more laid back than the OPA2134 but it's all really a matter of taste
If running the amp from a battery, it's a good idea to add protection against damage from accidental polarity reversal by placing a 1N5817 schottky diode in series with the supply. Schottky diodes have a much lower forward voltage drop across them than standard diodes so less power is wasted