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Headphone Box
Written by Administrator   
Monday, 13 November 2006

We all know this situation. We build a brand spanking new awesome amp. We start playing it, we start cranking up the gain, we're in tone nirvana. Then, it hits us. "Honey, can you play something else and turn it down?". Yup, we start to annoy people around us.The solution? Why not use headphones or a line out! This is what this article is about. This is a simple headphone / line out box for tube amps. It has 3 parts:

- resistive load to simulate the load of a speaker
- variable output section to bring down and control the volume going to the headphones / line out
- a very simple tone shaping section to grossly simulate a speaker frequency range. This is by no means a complete cab/speaker simulation but a simple way to cut the extreme lows and the high frequency to make it more pleasing to the ears.

Lets look at the schematic:




For the lineout


R1 is the resistive load. In my implementation, this is 4 x 2ohm resistors rated at 25W
each. This should be good for about 50 watts. I would not try a lineout or headphone on an amp that is more than 50 watts with that setup. Adjust accordingly, i.e. a 4 ohm resistor to replace a 4 ohm speaker or a 16 ohm resistor to replace a 16 ohm speaker.

R2 and R3 is the voltage divider that brings the output to a manageable level for line out level. Actually, this output can be used as in input for a second amp or into a microphone input of a recording console. The signal is too weak for a line input into a recording console.

C2 and R5 is the bass cut filter. This cuts any frequency below a certain level. To
calculate these values, I have used the following site: http://www.sengpielaudio.com/calculator-RCpad.htm . In this example, we are cutting everything below 160hz.

R4 and C1 are the treble cut filter. This cuts any frequency above a certain level. In this example, we are cutting everything above ~5khz.

For the headphone out

R6 and R8/R7 form the voltage divider to bring the output the headphone levels. The reason that a 22 ohm resistor is paralleled to a 1k pot is to make the pot act like a 22 ohm pot rather than a 1K pot.

R6 and C3 form the bass cut filter. This cuts any frequency above 1.6khz. This works
surprisingly well with headphones!

Picture of my build

How does it sound?

Well, here is a short sound clip of the lineout. This was recorded straight into a zoom
PS-04 palmtop studio. Completely dry (i.e. NO processing / effects on the PS-04)

http://www.sopht.ca/sounds/headphonebox/lineout.mp3

Last Updated ( Friday, 23 February 2007 )