Building A Guitar Amp
Line Out Box
Last Updated 02/22/04
By Paul J. Marossy
There have been
a few ocassions when I wished an amplifier had a line out jack, but it didn't
have one. It's not too hard to add one to an amp, but it's not always practical
or advisable to do, especially if it's something like a vintage blackface amp
or something. The next best thing then, is an external box that is in series
between the amplifier speaker jack and the speaker cab which has a line out
jack. This is something that is very simple to make and is relatively inexpensive,
too. I created this thing mainly for use in a studio environment, and one could
use any number of speaker cabinet simulators in conjunction with this for some
interesting studio effects and unique tones.
This is basically the line
out jack section on the Super Twin Reverb. It takes the signal coming
from the output transformer to the speaker jack and sends out some of that
signal to the line out jack via a voltage divider arrangment consisting
of a 2.2K resistor followed by a 270 ohm resistor connected to ground. This is just an example. The line out arrangement should
be optimized for your particular amplifier. Generally, the attenuation is a 10:1 ratio. This page will guide you through the process of designing a line out for your amp. The speaker must still be used in conjunction with the line out jack,
or damage to the output transformer can occur. You may find that a treble cut cap is necessary if you connect this device directly to a mixing console.
The box I used is an aluminum project enclosure, one of several that were donated to me,
and measures 2-3/4"x2-1/8"x1-5/8". I cut it down so that
it stands 1" high. I also used a bypass switch for the line out,
just because I like to be able to switch things off and on. The round
black thing is just a plastic plug to fill in a hole previously drilled - the
result of an ill-fated project I attempted with this enclosure before. The circuit worked, but it wouldn't fit in the enclosure. Oops...
Here is a picture of the interior. The cap on the switch doesn't do
anything, I was just lazy and didn't want to remove it, the switch was
salvaged from another project. I decided to use an RCA jack for the
line out for various reasons. I used 20AWG wire rated for 300 volts.
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