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A Passive Preamp Switchbox for the Desktop

A Passive Preamp Switchbox for the Desktop

One of the more frequently requested projects on HeadWize has been a switchbox for selecting multiple audio inputs.
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.

Construction

A passive preamp/switchbox is one of the simplest electronic projects that there is. I’ve shown one of many possible configurations here. Despite the cramped space of the enclosure, it went together very quickly. I didn’t have to use a circuit board. The only “loose” parts were the shunt resistors, which I soldered “free-floating” to the hookup wire. The tension of the wires was adequate to hold them in place. The resistors could have been soldered directly to the pot as well. Speaking of hookup wire, I used 22 ga. solid wire throughout. Be sure to keep all wiring short and neat.

I chose standard 3.5mm (1/8″) stereo mini jacks for the input/output jacks, because this type of jack is common in portable players. Radio Shack sells a version of these jacks (RS 274-249). I ordered higher quality units that have spring-loaded contacts from Mouser Electronics (stock no. 161-3502). I strongly recommend the jacks with spring-loaded contacts, as they are not expensive (less than $1.00 US each) and less prone to metal fatigue, which can result in intermittent connections. A good alternative to mini jacks would be RCA-type jacks, which are found on most non-portable audio gear.

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