Phil .. yes one for each of L and R channel.
Thanks Arjan .. actually that Keen's box doesn't house that headphone noise-reduction adapter. Was used for another guitar/amp thing I built a few years ago and thought those mustard metal boxes looked neat so lol.
Keep in mind that the info above is suitable for phones with about 50 ohm impedance. You would need to adjust the resistor value if you use with phones with substantially higher or lower impedance. Pretty easy voltage divider math.
Also, I tried a 15 Ohm resistor which gives you about -9dB (about 1/3 voltage) drop in headphone output and there is still plenty of gain. Of course you are wasting a lot of playback signal power in the resistors (one for each channel) but 1/4W resistors are still fine for power dissipation.
My test gig with 15 ohm resistors is shown above.
p.s. .. to check for this "hiss" background noise, try this simple listening test:
In a quiet room, put your phones on and plug in. Without playing any tracks and with Stereo level at max and Monitor level at 12:00, listen to the difference between phones plugged and quickly removed. You should hear a faint hiss. On my system that level doesn't change even if the monitor level is turned up full (because that gain control is in the digital part of circuit).
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