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Headphone / Pre Amplifier





I'm not sure whether to call this a headphone amplifier or an audio preamplifier as it can be either. There are similar projects on my website but here I went for the simplest circuit with the minimum component count that could still provide the quality of sound I wanted. It's based around the OPA2134 opamp which I like due to its detailed presentation and effortless ability to drive low impedance headphones. The circuit is configured as an inverting amplifier which conveniently uses 4 components less than the non-inverting version without compromising quality. The circuit is AC rather than DC coupled as I prefer to use standard +12V DC as a power source. DC coupled circuits require a split + and - supply which would add to the complexity









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 spot on. 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 inverting inputs. This will set the outputs to exactly half supply voltage


The circuit is built on stripboard as is my thing, but this time I specificaly wanted to fit it all onto the smallest standard 25 x 64mm board. If possible I like to separate the components so there is always at least one hole between them which is achieved here. Keeping the amount of cuts and links to a minimum is also important. 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




Socket wiring (earthed through rear panel)


Why? To me it seems that there just isn't enough output level from portable music sources and PCs to drive a power amplifier or larger hi-fi headphones to a reasonable volume. Amplifiers and headphones should be capable of going loud, but won't if they don't get enough drive, which is increasingly the case. The circuit described here supplies that extra drive and provides an improvement in sound output level and quality that is quite amazing!


The Little Extras


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) whos connectors are consistently well made and great value. There is also a low cost cable made by Van Damme with a foil screen that conveys remarkable musical detail for its price. The parts are available in the UK from Maplin (details below)


3.5mm plug:  Rean NYS231BG (Maplin Pt.No. A94QT 1.39)
Phono plug Red:  Rean NYS373-2
(Maplin Pt.No. A30QT 1.19)
Phono plug White:  Rean NYS373-9 (Maplin Pt.No. A32QT 1.19)
Van Damme Black Pro Grade Classic XKE cable (Maplin Pt.No. N96JT 0.89 per metre)