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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
 Schematic
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)
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) 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
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