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Old 21st May 2008, 04:26 PM   #1
Dan2 is offline Dan2  South Africa
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Default low voltage headphone amp

hey guys, a friend of mine has an xbox360 and wants me to build him a small headphone amplifier for it, because the xbox only has a line out (which is just not loud enough)

the thing is most preamp circiuts run on 12 volts, which is a hell of a lot of batteries (my friend wants it to be portable for LAN'ing) and i know it can be done with a much lower voltage - a portable mp3 player uses just 1 aaa battery and that is loud enough.

what i really want to do is build it so it can get its power from one of the usb plugs on the xbox, so no batteries will be required.

i also want it to sound quite decent - as i am sure the usb has enough power to do (i think usb has 5v, 200mA output???)

i was thinking of using an opamp, but if you have a better idea please share.

thanks in advance
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Old 21st May 2008, 05:40 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

You need a small power amplifier not an op-amp. Basic difference
is the load impedance they are designed to drive, op-amps do not
do 8 to 32 ohm loads (typical earbud / headphone range).

http://www.national.com/mpf/LM/LM4863.html ?

/sreten.
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Old 22nd May 2008, 07:49 AM   #3
Dan2 is offline Dan2  South Africa
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thanks sreten, thats perfect for my project!

just one question about usb. if i just hook this chip to usb 5v and gnd, it wont cause any damage, right? (2.2W/5 = 440mA)

and thanks for the info on the opamp this is why i always ask the questions before i start building or buying!
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Old 22nd May 2008, 09:56 AM   #4
sreten is offline sreten  United Kingdom
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Hi,

TBH I not exactly sure but I doubt there will be any problem for
headphones. I have a USB powered external hard drive that
has two USB connectors for the case one port is not enough.
Indicates that the current capability of a USB port is not fixed.

/sreten.
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Old 22nd May 2008, 10:04 AM   #5
AndrewT is offline AndrewT  Scotland
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Find out something on the specs of his headphones. Impedance and sensitivity as a minimum.

If they are 32ohm and 90dB/mW then 500mW into 8ohms will give 2.8Vpk into his phones producing 111dB peak but delivering just 125mW.

He doesn't need a lot of power. He does need current ability to suit his load.
Find out about the load.!!!!!!

BTW,
if the maximum average power output is 1.25mW (-20dBV ref 500mW into 8r0) then the phones are producing 91dB average, that is still too loud.
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regards Andrew T.
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Old 22nd May 2008, 10:48 AM   #6
Mooly is offline Mooly  United Kingdom
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Quote:
Originally posted by AndrewT

BTW,
if the maximum average power output is 1.25mW (-20dBV ref 500mW into 8r0) then the phones are producing 91dB average, that is still too loud.
Pardon ! What. Quite right to
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Old 22nd May 2008, 02:02 PM   #7
Dan2 is offline Dan2  South Africa
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Quote:
Originally posted by AndrewT
BTW,
if the maximum average power output is 1.25mW (-20dBV ref 500mW into 8r0) then the phones are producing 91dB average, that is still too loud.

sorry thats a little over my head!! i don't know what you mean by too loud - cos you can set the volume down. i'm just wandering if the usb port has current limiting so it wont blow.
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Old 22nd May 2008, 02:14 PM   #8
AndrewT is offline AndrewT  Scotland
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we're talking about 1.25mW average power and 125mW maximum power on short lived transients (if the load is as the example).

That should be enough to work out the amplifier quiescent requirements from the datasheet.
Then decide if there is spare capacity in the USB supply.
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Old 22nd May 2008, 04:46 PM   #9
Mooly is offline Mooly  United Kingdom
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A USB port has enough "power" to blow your ears off. I see it was mentioned earlier that MP3's provide enough volume from a AAA cell. These almost certainly ( and I have not actually seen a circuit for one ) use a switching type output stage, and they may well also step up the 1.5 volts with a DC DC convertor.
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Old 22nd May 2008, 11:39 PM   #10
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A pair of LM386's should also do the trick. They run fine on 5V, and need minimal parts.

Another way is to use a dual op-amp and use each to drive a pair of descrete output transistors and drivers - gives better sound.
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