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Old 1st December 2011, 01:11 AM   #1
Skree is offline Skree  United Kingdom
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Default help needed with micro amp project for 16ohm earphones

I'm trying to build a portable device to enhance my on-the-move audio. Just got a new nokia 101 phone as source and i'm pretty impressed with it(great mp3 player in them and the X-01), particularly through my Sennheiser MX880 earphones(16ohm). The trouble is, they're not a headset so i can't use them for calls. I also find the sound on them lacking in weight a little after my cheap old sony mdr-ex35 iems. They just aren't as forceful, as present, as... forward? and if you really crank the volume on the phone, the AVLS clips treble which drives me nuts. I also would like to experiment with a crossfeed circuit as this rig really shows up when sounds are only coming from one ear - a lot of the fine detail i missed on my EX-35s for example. In addition to this i'd like to be able to get full sound with only one earphone in. To this aim i have been researching the Meier bass enhanced crossfeed.

So the box i've designed has a 4 pole 3.5mm headset input socket, a mono mic output, and a stereo 3.5mm output. but that's about it. I'm thinking of trying to make it in a little plastic case made to protect a 9v battery, but need an amp circuit small enough to also fit in there - power supply wont be much of an issue - it's only to run a couple of hours at a time at the most, and i can get ultra tiny li-ion cells. Heard CMOYs have issues with low imp. earphones like mine? I don't need much gain - enough to account for the crossfeed and to add a little force to the senns.. but the amp HAS to be really really small like one of sijosae's micro CMOYs

can it be done? anyone got any circuits for me?
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Old 6th December 2011, 08:50 PM   #2
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Interesting set of requests. I've been thinking about some of this and I've been reading on the problems that face low impedance loads.

Here's some light reading on the topic of headphone DC-reducing caps vs. crosstalk and their highpass filter characteristics: http://www.ti.com/lit/an/sloa043/sloa043.pdf

Here's a good circuit on page 2: http://www.national.com/ds/LM/LM48820.pdf
It's fixed gain so you can manage the volume from your PMP/phone. There's an eval board available for it as well. If you want seperate volume control, you'll probably have to go with something like the LM48821 as the center of your circuit if it really needs to be super small.

You have such a large list of wants that you will likely have to cobble it together yourself.
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