OPA 2134 Stereo Audio Booster

The purpose of this audio booster is to increase the output of an MP3 player, mobile phone or PC that otherwise would not have enough power to drive an external amplifier 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 and although it doesn't claim to be top end Hi-Fi, it does sound pretty good and has a nice detailed presentation


The circuit runs quite happily on a 9V PP3 battery and in order to achieve this, it has to be AC coupled to aviod having to use a split + and - power supply. 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

Veroboard layout

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. 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. I decided not to add a volume control because most things already have one built in so another isn't really necessary

Socket wiring (earthed through rear panel)


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

3.5mm plug: Rean pt.no. NYS231BG
Phono plug Red: Rean pt.no. NYS373-2

Phono plug White: Rean pt.no. NYS373-9
Screened cable: Van Damme Black Pro Grade Classic XKE