Instruments and soundcards can put out too much signal for the inputs of your mixer or multitrack. Instruments often use headphone outputs instead of line outputs. An attenuator is needed for such instrument outputs. Soundcard outputs come in two types. A soundcard with a line output has two output jacks, one for speaker outputs, and the other for line out. Other soundcards have speaker outputs, but no line outputs. An attenuator is needed to obtain a line signal from the second type.

Most powered computer speakers expect to be powered by a headphone or speaker output, and load the signal lines accordingly. So the speaker output of the soundcard is expecting to be loaded down with a low impedance. A soundcard speaker output is nominally designed for 1 or 2 volts output max at 8 ohms.


If the soundcard output is fed unloaded (no 8-ohm load connected) into a line input, things are different. Depending on the design of the soundcard, the output can go as high as 20 volts without the 8-ohm load attached. So you need an attenuator to cut the power of this output down to a level safe for your equipment. Distortion can also result from improper loading.


You can build an attenuator cheaply if you can't buy one. The following attenuator is for obtaining a line-level signal from speaker outputs on soundcards and from headphone jacks on music keyboards:

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Footnote: If neither insulated wire in the 1/8" plug cable is red, use a continuity tester or ohmmeter to find out which wire goes to the ring (the contact in between the other two contacts) of the plug. Think of the color of that wire as "red".

Footnote: If the left and right channels are reversed, you have a nonstandard sound card. Trade the wire on upper terminal 2 with the one on upper terminal 5.

Footnote: If the signal is still too powerful for your trim control, add another pair of 1K resistors, one across screws 1 and 3, and one across screws 4 and 6. If the signal is too weak instead, change the resistors across screws 1 and 3, and across 4 and 6, to 5 K instead of 1 K.


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