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Old 18th November 2012, 07:23 PM   #1
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Default Headphone distribution block (passive)

I'm looking for some help designing a simple headphone distribution block. Here's the problems I'm trying to solve:
1) Distribute stereo signal from headphone amp to up to 4 different headphones
a. Headphone impedance varies from 30 ohms (earbuds) to 300 ohms (HD650’s)
2) Independently control volume on each output
3) Desire (not absolute): Passive circuit – simplicity has high value
Here’s a schematic (sorry for the cruddy image) I found that is probably close but obviously missing pots and any insight on resistor value in respect to above impedances.
Click the image to open in full size.
Questions:
1) What resistor values should I use?
2) What size pots should I use?
3) Should I design different signal paths for each impedance?
4) What is the net effect when output 1 volume is changed on output 2,3,& 4?
5) What am I missing?
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Old 18th November 2012, 09:38 PM   #2
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Short answer: I think I'd replace the 100 ohm parallel resistors with 20-50 ohm series resistors, and follow that with 100 ohm rheostats on each branch.
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Old 18th November 2012, 09:54 PM   #3
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I've read somewhere that there's a rule where your impedance should be 1/4 your resistance. That would make me think 1k pot. Is that even a "real rule"? Also why rehostat and not pot?
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Old 19th November 2012, 07:10 AM   #4
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Quote:
I've read somewhere that there's a rule where your impedance should be 1/4 your resistance.
Okay. I don't know how that relates to your project though.
Quote:
That would make me think 1k pot.
Okay. Where is that 1k pot? And what is it that is Z=250 ohms?
Quote:
Is that even a "real rule"?
I'm not familiar with it, so I have no comment.
Quote:
Also why rehostat and not pot?
Basically, the main purpose of your project as I see it is to nullify, to some degree, the result of connecting parallel loads to a headphone output. Adding more resistance(s) in parallel is obviously not the most effective way to do that. Being 100% passive, one can expect only so much isolation among branches using practical resistance values. In any case. With the different possible headphone impedances, rheostats seem to me to be the better option. You may need to experiment a bit to find the optimum values; the ones I gave were just suggestions.
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Old 19th November 2012, 04:21 PM   #5
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Thanks for the info! I'm new to circuit design so I'm still figuring all this out. Hence starting with a simple circuit.

My Z=250ohms was the approximate impedance of the 300ohm Senn HD650's.

Should I use switching or non-switching jacks?
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Old 19th November 2012, 10:34 PM   #6
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Non-switching jacks are alright. The dual pots you are able to find can have some bearing on the final product also. The wide range of headphone impedances to deal with passively puts this a little above simple; four op amp voltage followers would probably be simpler IMO.
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Old 19th November 2012, 11:55 PM   #7
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I'm not one to get my head stuck up my own behind. Simplicity is the goal here so should I be looking at distribution amplifier circuits instead?
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Old 20th November 2012, 12:34 AM   #8
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Nothing wrong with your original concept, but if there is no need for a passive solution, active devices can make things easier and the result more satisfactory.
So yes, a distribution amp. You need buffers, to maintain a load on the single headphone output and isolate the multiple headphone jacks with their potential wide range of load impedances.
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