Desktop Audio Switch box

Posted on Jul 26, 2012

One of the more frequently requested projects on HeadWize has been a switchbox for selecting multiple audio inputs. I built this passive preamp/switchbox several years ago to switch between two stereo sources (a portable FM tuner and portable CD player) AND two stereo outputs (a computer speaker system and a headphone amp). It's very compact (perfect for the desktop audiophile), easy to build and uses commonly available components - including a shunt-wired linear taper pot for a volume control. The simple, low-cost design puts it within reach of beginning electronics DIYers.

Desktop Audio Switch box
Click here to download the full size of the above Circuit.

A passive preamp is basically a preamp that has switching functions and a volume control, but does not amplify the incoming signal. Because there is no amplification, the input signal must already be at line level, which is loosely defined in the range of 0.5V to 1.0V. An active preamp can increase the signal strength several times before feeding it to a power amp; however, modern power amps (and headphone amps) have enough gain that they can be driven directly from the line-level source. With the exception of certain phonograph cartridges and microphones, just about every other audio component I have ever encountered has a line-level output. Portable players may have a separate line-level output, but if not, the headphone output can be used instead - with the possible penalty of slightly more noise. Figure 1 shows the schematic for the passive preamp/switchbox (hereafter called the "switchbox"). It uses DPDT switches to toggle between the two sets of inputs and two sets of outputs. The input and output grounds are all connected to the common circuit ground. Most audio components use a common ground between the channels. The circuit must be modified to work with components that require separate grounds for each channel (see the next section for details). The volume control is probably the most interesting feature. Since the enclosure I wanted to use was very small, I needed a mini dual pot with audio taper for a...

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