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Old 19th January 2013, 06:06 AM   #1
Qfactor is offline Qfactor  Singapore
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Default Headphone distribution schematics

Hi,

I'd like to DIY a simple device that uses one 9v battery to mix 4 iPads (4 inputs) that goes to 4 outputs that can drive 4 earbud-type earphones.
This is so 4 people could plug into this device and listen to one another.

Been looking at the headphone distribution amp schematics but they're usually either many inputs to single output (mixer) to one input to many outputs (distribution).

I'm a relative noob in electronics but can solder and have done audio kits before
Can someone please help?
Thanks!
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Old 19th January 2013, 06:35 AM   #2
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Quote:
mix 4 iPads (4 inputs) that goes to 4 outputs that can drive 4 earbud-type earphones.
Quote:
Been looking at the headphone distribution amp schematics but they're usually either many inputs to single output (mixer) to one input to many outputs (distribution).
There's your answer. You need to build both the mixer and distribution amps, then connect them together. The single output from the mixer becomes the distribution input that is sent to the 4 headphones.
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Old 19th January 2013, 09:27 AM   #3
Qfactor is offline Qfactor  Singapore
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Thanks for the reply!
In that case, how long would an average 9v battery last to power these 2 circuits?
I teach on the iPad and would like to group 4 students to a group, in class, for an ensemble!
Also, any particular links you could point me to for schematics of these 2 circuits?
Thanks again!
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Old 19th January 2013, 12:56 PM   #4
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It's hard to say just how long a 9V battery would last. 10 hours or thereabouts? Rechargeables would have less run time, but be cheaper than buying disposable primary cells.
Is the tether of a wall wart supply too inconvenient? I like to avoid 9V batteries whenever I can. What about two small 6V SLA rechargeables? Those would last all day long and could be recharged overnight, and be easy to design for a bipolar supply.
I don't have any links to post right now. These are typical application circuits that can be found in datasheets and app notes. Using op amps such as NE5532 or OPA2134 should work great.
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Old 19th January 2013, 02:30 PM   #5
Qfactor is offline Qfactor  Singapore
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Hi,
If a 9v battery can last about 10hrs, it'll do just fine!
In fact if a 9v rechargeable is a little shorter, it'll still do fine. I just don't want a 1hr battery life that I'd have to charge in between class periods.in any case, music lessons are only about twice a day or so.
I'm not planning on SLA-types as I wanted the circuit to be inside a 4"x4" casing so its easily collected and stored after lessons
Will search out for any ready circuits for the chips you've suggested. Are LM386 chips workable too?
Thanks again!
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Old 19th January 2013, 02:48 PM   #6
Qfactor is offline Qfactor  Singapore
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Another thought...
Since I'm not too particular on the quality of the audio, what if its a passive 4-channel mixer which feeds into a headphone distributor?
This way, I could save one part of the circuitry having to be powered.
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Old 19th January 2013, 10:26 PM   #7
paulb is offline paulb  Canada
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Look at Figure 18a and 18b here:
Designing An Opamp Headphone Amplifier | HeadWize
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Old 21st January 2013, 09:43 AM   #8
Qfactor is offline Qfactor  Singapore
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Hi,

I found a 2W+2W stereo amp kit that I was wondering, could be easily modified to do my 4in-4out headphone "distributor"
Question is, for the input and output, what value capacitor and/or resistor should I change these to, in order for:
1) 4 passive inputs hooked up to the input and
2) 4 outputs branched out from the current spkr out to drive 4 "typical" iPad-type ear pods?

Would it work?
Thank you for any advice!
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Old 21st January 2013, 09:48 AM   #9
Qfactor is offline Qfactor  Singapore
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Oops!! Here's the image attachment!
image.jpg
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Old 21st January 2013, 10:37 AM   #10
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Yes, you could do that to simplify the build. At each input connection, place a 10uF capacitor and 10k resistor in series. The other ends of the resistors then all connect to the 10k potentiometer shown in the schematic. Replace the 220uF output capacitor with a 1000uF capacitor, and connect each of the output jacks to it.
And I would power it from four 1.5V cells instead of a 9V battery.
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Last edited by sofaspud; 21st January 2013 at 10:41 AM.
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