Adding a line out jack - resistor values?

[Apexelectric]

I’m modding an amp for my son and he asked me to add a line out jack so he can go direct to a mixing board for low volume/headphone band practices in his apartment. I don’t think that accurate reproduction of the amp sound is critical. He just wants to be able to cut the speaker out of the equation and send signal to a mixing board.

The amp is a 18W PP EL84 home brew with an 8 ohm output. I don’t really have room for a pot for an adjustable output level but I do have room for a mini 3 position switch to give some range to the output level. The amp does have a dummy load resistor in it already so if the speaker is unplugged the 8 ohm resistive load remains, albeit without the inductive load.

So the simple voltage divider option was the one I’m hoping to do the job but am open to other options if they don’t require too much room. Just wondering what values to use. Was thinking of a 2.2K from the output jack hot to the line out hot and varying the leg to ground from 100R to 400R. Possibly add a cap to tame the treble?

Thoughts?

[labb]

https://schematicheaven.net/fenderamps/deluxereverb_II.pdf


2.2k and 270

[Apexelectric]

Thanks Labb.

Anyone see any reason for varying the resistance on the ground leg? If so by how much?

[sluckey]

If you have room for a mini switch then you have room for a mini pot. But surely the mixing board has an input pot. If so, just use labb's recommendation.

[Apexelectric]

Ok. Will just keep it simple. 1/2 W resistors will do the job?

[2deaf]

Anyone see any reason for varying the resistance on the ground leg? If so by how much?

Maybe, if you wanted a -10dBV / +4dBu switch for the two different line levels.

18W at 8 Ohms is 12V (all values will be rms values in this post).  But I don't think you would want to design a line out that requires full power to get a nominal line level.  Half power seems like a good value to me given that line level devices usually have plenty of headroom above their nominal level.  Dividing 12V by the square root of two gives me 8.5V at the half power level of 9W.

+4dBu is 1.228V, so we need a voltage divider with a 2.2K that will give 1.228V out with 8.5V in.
8.5V x R/(2200 + R) = 1.228V
R = 372    Moving up to the nearest E24 value is 390.

-10dBV is 0.3162, so 8.5V x R/(2200 + R) = 0.3162
R = 85 and the nearest E24 is 91.

91 and 390 are strikingly close to your original idea with 100 and 400.  The recommended value of 270 seems like a good catch-all one to me if you don't want to have a switch. 

[jjasilli]

You need to consider the board's input impedance.  All boards expect low z.  Some boards accept hi z but it doesn't work as well.


The fx out mentioned is hi z to drive guitar pedals which expect hi z.  If you go with that you should buffer with a DI box.  To avoid that use a low z circuit out from the amp.

[2deaf]

Z_o looks like 240 Ohms to me with labb's recommendation.

[pdf64]

An output impedance of 100 - 400 ohms would seem to fulfil many definitions of ‘low impedance’, in the context of line or mic level signals?
I think most mic or line level inputs have an input impedance in the range 2k-100k.

[labb]

Some info here:  http://music-electronics-forum.com/showthread.php?t=25684&highlight=slave+line

[jjasilli]

Zo looks like 240 Ohms to me with labb's recommendation.
OK That's fine for direct to a board.  (Tough not for guitar pedals!!). 

[Apexelectric]

Thanks for all the info. I decided to use a 3 position switch with multiple values to give some options, anywhere from 470R to 240R to 120R on the leg to ground. That should satisfy multiple situations.