There are many ways to build a line out device. One of the simplest is just a voltage (potential) divider. I recommend you read the full write-up on the Elliott Sound site above to get a good understanding of how this works. The basic concept is to drop most of the output signal (normally sent to a speaker) across a power resistor, then pass a little bit of signal to the input of another amplifier. The photo on the right illustrates the analysis and basic equations (click it to see the full size version).

The first step was to calculate the ratio of voltage out to voltage in. I wanted a drop of about 40:1. I had some 2.6 KOhm and 125 Ohm resistors (30 cents each), so I used them for this project. To get approximately 40:1, I used a 2.6K Ohm resistor for R1 and two 125 Ohm resistors in parallel for R2. This gave me a voltage drop of about 43:1, close enough! I did not want to have to have the amp plugged into another amp AND a speaker, which is why we used the Power Resistor. I used a Radio Shack 10 ohm/10 Watt resistor (about $1.50).

Note that in the above simulation I used 100 volts in at 1000 Hz. This is just to make the numbers easy to see. My amp actually puts out about 6.2 VRMS, which gives me about 6 watts of sound.

Here is the basic assembly of resistors in the voltage divider. The output from my amp connects to the left and right ends of this assembly. The input to the power amp connects to the right end and center tap.

IMPORTANT NOTE:

The input to the line out device comes from your small amp, but it is still high power (6 watts for the Thick Mary). You need to use a speaker wire equivalent for the input into the line out device.

The output of the line out device needs to use "line in" or guitar cable style wire. It goes to the input of your next amp via a guitar cable you plug into the line out device. I recommend you shield this wire inside the line out device to minimize noise going into the power amp.

Here is a layout of everything I thought I would use for this device. Ultimately it wouldn't fit in the film canister, so I used a little Radio Shack box.

Remember that we want to use shielded wire for the output side of the line out device, because this is a small signal that will feed the preamp in your power amplifier.

So, you have the voltage divider, an input plug to connect to your small amp, an output jack to plug a guitar cable into (connects to power amp), and some sort of box to put it all in.

After trying the film canister, I tried a Bayer aspirin bottle to house the line out. This proved just a bit cramped, so I purchased a Radio Shack hobby box for about $3.

Click the photo for a large format view of how everything fit into this box. I drilled a few cooling holes in this box to help dissipate the potential 6 watts of power consumption.

Notice that the input side uses thick wire, while the output side uses shielded signal wire. Actually I shielded all the wires and connected the shields to a common point. The output is very quiet, with virtually no noise added to the signal.

Here is the completed line out. The box is 3 x 2 x 1.2 inches. The nice thing about this device is that I can crank Thick Mary all the way up and use that overdriven signal in a power amplifier.

You could actually use this device for much larger amps than 10 watts, as long as you can adjust the output of the amp. First plug the amp into a set of speakers and set the volume to about conversation level. This should be far less than 10 watts. Then feed the signal into another amp. If you have a multimeter you should probably measure the output of the line out to make sure it not so high as to damage your power amp.

IMPORTANT NOTE:

You CANNOT use this device for amps that put out more than 10 watts if you turn the amps all the way up. This device will only dissipate 10 watts before overheating. As long as you keep the amp output turned down, you can use the device, since the amp will be putting out less than 10 watts. But if you crank a Super Reverb through this thing, it will literally melt.

If you really want to hear the sound of your power amp cranked to distortion without splitting your eardrums, build a line out with an appropriately large power resistor.