Line Amplifier

line_ampIn my daily activities I do a lot of hacking around with audio circuits in my lab. When experimenting with ideas, it’s often a problem matching signal levels between devices. Some internal parts of an audio device might need to work at a higher or lower level than the surrounding sections. A finished product obviously has all the right matching figured out. But on the bench sometimes it’s nice to turn things up or down,  generate large signals to test overload characteristics, or deal with weak signals from guitar pickups or other high impedance sources without having to build buffers and other interfacing circuits over and over. So one day I sat down and built the device you see in the photo. It’s a two channel audio line amplifier designed for use in the lab. It was designed to be quick and easy to build, and has a number of interesting features which make it suitable for working on many kinds of audio circuit design and testing.

Features

  • two channel design with internal isolated power supply (each channel is electrically isolated from the other)
  • each channel has a floating ground which can be referenced to the circuit under test to reduce ground loops
  • AC coupled input and output
  • adjustable gain – from 1 to n, where n is selected by the value of the gain pot
  • mute switch – useful when things are humming or a cable is loose
  • high Z switch – selects between approx. 470K or 47K input impedance
  • output attenuator – divides the output of the amplifier to feed sensitive inputs
  • +/-15V internal power supply for large signal swing
  • mains powered

One drawback is that it only handles unbalanced signals, but other than input/output circuits in professional gear, balanced lines are rarely used inside equipment, which is what this is designed for. If you work on any manner of analog audio circuit design, a line amp like this will save you a lot of time!

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