how do i add a lineout to my amp?

[Bmxbarspin724]

I saw once that this guy put a line out inbetween his amp and his cab. so his speaker was connected but he was still able to go to a separate amp. are there any products out there that can do that?

thanks, noah.

[boyer]

This is a pretty straight forward mod that anyone who can solder can do.

Have a look at the schematic of a Fender PA-100, or something else with a line out.

What I usually do is remove the ground switch (should be the first thing you do to an old amp anyway along with of course replacing the 2 prong power cord with a 3!).

Then, you can put a jack in place of the ground switch. Run a lead from the speaker out to the jack through a 1k resistor. Then you can use a 5 or 10k pot for volume. Solder something like 200 ohm, 1/2w resistor across the jack.

While you're at it, get a 150 ohm @ 15w (for a 50w amp) resistor and put it between ground and the hot tab on the speaker jack. This will help prevent destroying a lot of components in your amp if the speaker for some reason becomes unplugged while in operation.

Hope that helps,
Sean

[boyer]

Sorry, here I am assuming that you have a vintage amp... Silly me.

You can do the above mentioned to pretty well any amp (tube or transistor). Take a lead from the speaker output, pad it with the appropriate resistor/pot, and hang a 100-300 ohm resistor across the output jack. If you like you could build this into a little project box or something, with three jacks:

Speaker in
Speaker out
Line out

Wire the two speaker jacks in parallel, and then do the above mentioned to the line out jack. You'd then have somewhere to mount the pot as well, and wouldn't have to touch a thing inside the amp. Also, you could use it on different amps just by un-plugging it and putting it in your pocket.

[Chuck H]

There's actually more to it than that. Read here:

http://music-electronics-forum.com/s...ighlight=chuck

[boyer]

Hi Chuck,

Good point. I'm figuring on a 50-100 watt amp. It goes without saying that the series R will change a fair amount with the power output, but the load R should usually stay around the same, yes?

[Chuck H]

Yes, the load R is usually good at 500 ohms or less. I use 500 because it allows me to use a standard pot value without an extra resistor across the lugs. The dropping R value is also impedance dependant since different impedances of the same wattage generate different voltages. The power amp is current dependant, but the line out is a voltage dependant circuit. The chart in the attached thread makes it all clear (thanks Enzo).

Chuck