Looking for an inline 3.5mm amp

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FnordMan

[H]ard|Gawd
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Apr 22, 2011
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something that's wall powered or can be made such would be ideal.
less than $50 would also be good.

The short of it is: i've got an odd audio setup that involves 3.5mm audio being pulled off an hdmi swtch that's a bit quiet. Need to bring it up the volume a bit to match the desktop. (audio from the switch is fed into an aux input)
 
Nobody has any ideas? Let me re-phrase it a bit (after a bit of searching)
Anyone know of a decent headphone amp? (i'm hoping a headphone amp will work here) Preferably one easily modded to be able to take external power. (i'm open to a DIY solution, have soldering iron and solder)
 
Look for something for lower impedance headphones, they offer more gain.

Alternately, the simplest way to build something like this would be to get a pair of isolated DC-DC converters (maybe 5v to 9v or so) to make a +/- 9v supply. Use a 5v wall wart or USB power. Add a couple of Os-con caps to filter/buffer the supply and a ceramic cap across the power pins. Use an op-amp like an OPA2132 or LM4562 (http://www.ti.com.cn/cn/lit/ds/symlink/lm4562.pdf) in the non-inverting configuration (http://en.wikipedia.org/wiki/Operational_amplifier_applications), I'd recommend a gain of about 4 using a 3K and a 1K resistor and an Rin of between 10K and 100K. See fig 2 and 3 here: http://sound.westhost.com/dwopa.htm
 
A headphone amp will work as long as your load impedance is at least as high as headphones you might otherwise use with it, which I imagine it is, as your "aux input" should have a much higher impedance than any headphones. Something simple/common like the CMoy is fine. Op amps aren't generally THAT important but if you want a high-end one, AD8599, AD8620, or LM4562, OPA627/637 (depending on gain) are some good ones.
 
Nice, thanks for the input from both of you.
Going to talk to a friend about modifying a CMoy design to run off of 5vDC. Wonder if 5v, 500mA is enough, i'll have to dig around to try and find some power usage numbers.
 
FnordMan said:
Nice, thanks for the input from both of you.
Going to talk to a friend about modifying a CMoy design to run off of 5vDC. Wonder if 5v, 500mA is enough, i'll have to dig around to try and find some power usage numbers.
Click to expand...

Yeah, that's actually overkill. Even 250mA should work fine. 100mA would likely work. Though depending on the quality of the power you may want a pi filter (parallel cap to ground then a series inductor (or resistor for slightly less good results) then another cap to ground) to filter the power which would also require some extra current from the supply.

Op amp currents are generally on the order of microamps, not milliamps. In fact, ideal op amps are considered to draw no current from their inputs, but as we know such an ideal doesn't really exist.
 
If you're going to run off of 5v, look for a rail to rail 5v op amp like the OPA317 or OPA2320, a "normal" op amp just won't work right off of only 5v. You'll need to create a virtual ground using something like a TLE2426 or an op-amp reference to half of V+. You'll probably need to limit yourself to a gain of 3 or 4 to keep from clipping.

500 ma is more than sufficient, this circuit should only take a few ma.
 
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