Mobile Headphones Sound Enhancement Device

Hello there!

I don't know if I'm alone with the problem of having a cheap smartphone with a below-standard headphone output volume and a mid-range-pronounced tone, paired with a rather high-impedance favorite set of headphones, but if thats your problem too, here's your relief, provided you dont mind carrying an extra device with an extra battery in your pockets :)

The principle by which these problems will be solved is the combination of active low and high pass filters that leave a frequency gap in the mid range to compensate for the excess given by the sound source and at the same time boost the whole signal.

The big advantage of this thing above software solutions is that the output volume is independent of the type of your phone, but only depends on the combination of voltage you provide by the extra battery and the headphones' impedance.

The downsides of this project are:

• the mentioned extra pack to carry (maybe you will find a design solution that makes you look less suspiciously terrorism associated ;)
• and - if you're really picky - a little noise. It really is just a very very little hissing that will not be heard unless your surrounding is really quiet and your listening at low volumes, however this will not be a problem as the device is there for raising the volume in a rather busy surrounding.

ATTENTION: DONT USE 9V-BLOCKS!

I am hereby providing a guide to making a device that should increase the joy of your listening experience, however be aware that irresponsible use, in particular the use of high voltages (such as 9V Batteries) for power supply can easily DAMAGE YOUR HEARING!! (it can even kill your headphones!) Keep the voltage down and the input volume as low as possible at first, then increase to where it feels comfortable. I am not liable for any damage that follows from ignoring these lines! (not even if youre stopped at the airport when they think its a bomb ;)

• Soldering iron
• (best be a) continuity tester
• adhesive such as hot glue including the gun, however I wont keep you from screwing things to gether if that's more your thing

4x LM386 low cost, good quality audio OpAmp

2x 100 k resistors

2x 330 Ohm resistors

2x 68 nF Capacitors*

2x 100 nF Capacitors*

2x 470 nF Capacitors*

2x 390 µF Capacitors*

1x Capacitor of a few hundred µF between the battery poles

1x 3.5mm (or what your sound source provides) stereo plug (you can design the device to have a cable between it and the phone, in this case you will need the cable and the housing of the plug; not the case with my design)

1x 3.5mm (or what your favorite headphones need) stereo jack

1x board to solder to; I used the separate-holes kind of, but feel free to design and etch your own layout!

- some connection wire for the board

1x Housing (larger heat shrink did it for me)

1x Power supply

• 3.7V mobile phone battery (including a mounting frame) OR
• 3x rechargable 1.2V AA/AAA batteries
• 2-3x fresh non-rechargable 1.5V AA/AAA batteries

ONCE AGAIN: DONT USE 9V-BLOCKS!
Using supply voltages higher than 3..5V can easily DAMAGE YOUR HEARING!! (it can even kill your headphones!) Depending on the headphones' impedance:

• 4Ohm - HIGHLY DANGEROUS, dont use with that device at all!
• 8Ohm - TRY WITH GREAT CARE even with 3..5V you can have very loud results and hissing noise
• 16Ohm - Thats mine, stay with the statements in this instructable, still - be mindful!
• higher ratings - youre more or less safe to proceed and maybe even raise supply voltage - still - do so with care!

Whatever your specs are - keep the voltage down and the input volume as low as possible at first, then increase to where it feels comfortable.

Also - whenever you are testing with headphones: dont wear them! If you hear nothing just approach your ear to the phones!

Of course you can also add output resistors to increase the impedance, but that is wasteful of battery power.

Well, you're right if you throw in that the LM386 data sheet says it needs at least 4V, but that is to achieve the distortion specs at given amplification rate. it will start to distort sooner the lower the supply voltage is, so see for yourself what suits your headphones and ears! Mine works quite well even at voltages where the phone this battery belonged to is completely dead (<3V)

* Try to get small built capacitors - most of the time it is a good start to look for low voltage ratings!

Well... Sorry if you expected some 1-component-at-a-time-instructions or a comprehensive layout at all.. I have to say I am neither a big fan of my own layouting skills, nor do i have time for those details. I also have to appologize i use a freeware schematic drawing software that has a maximum parts counter and keeps me from drawing the full schematic.

I can imagine this is a little disillusioning and questions whether this should actually be called an instructable, but the way i see it is: i am providing the principles of solving this problem; even if i leave the details to you (it is even highly probable that you will find a better layout and way of arranging the components so the overall thing suits your needs).

i can assure you the information i am giving you is correct and accurate, as I spent some time picking and changing the components hunting for the best overall tone.

Now - whether you want to arrange your device more slim but lengthly, more square-flat-planarly, dense-cube-like or specially hook-like shaped to complement the shape of your cell phone, battery pack and pocket...

• ...try to plan the arrangement of the big parts - mainly the plug, jack and the 4 LM386s and the larger capacitors. Maybe you even want 2 boards, 1 for either each channel or high/low pass groups.
• Be aware of the jack and plug polarities and if in doubt check with a continuity tester
  
  • shaft/sleeve...ground
  • ring................ch 1 (left/right, no matter as long as plug and jack are connected to the same channel)
  • tip..................ch 2
• the plug is the input, so make sure to ground the sleeve and all LM386s to the same net, and use the channel pins for input of each channel depicted in the schematic
• make sure your layout does what the schematic shows
• Each channel separates to low and high pass, so keep track of who is who and connect the outputs correctly to the output jack (better checked one time too much than too little)

When the board is soldered with all components and the thing is working (check left/right-correctness with a sound source that features a pan knob or with music where you know what's left and what's right), then make sure the jack (and plug if you decide to mount it directly to the board as i did) are mounted really soundly to the board, hot glue worked to my satisfaction, just make sure it is hot enough and binds sufficiently to the surfaces

Choose a battery holder arrangement (directly to the board? where exactly? or separate?)...

If you choose to use a phone battery, you will need to destroy a phone using that type of battery to get a well suited battery holder - unless you want to build or 3D-print such a container and equip it with the contact pins. Make sure you connect the power supply wires to the negative and non-regulated positive pole of the battery (most probably it will work with just one of the positive poles at all)

And choose a kind of Housing. For me, a larger heat shrink worked best, consuming the least space possible and having exactly the length i cut it to (well, also consider its shrinking in length - proportionally!).

Aaaand DONE :)

Walk the streets with a smile where you used to frown about the volume bar being maxed in your felt mid-way!

Or about the sound that wont fit even with extreme EQ settings, or in some cases EQs even choke down the overall volume... Even though I actually am into software-solving things, some of the artificial "intelligence" just needs to be hardware-hacked.

Experiences & Future Prospect:

• I was astonished how good the sound of that device got and how low the noise levels were.
• Also one battery charge was enough for a damn loooohohohot of listening hours! i dont know how to quantificate it, but it outruns any phone (even the old, non-smart ones), ipods, minidisc, walkmen, discmen... actually i never had any mobile device that worked with a single battery charge unit for so damn long!

• I tried to use the battery of the phone i am actually using as sound source - however i did not manage to get that working. Still there must be a way since you can buy some mobile-mounted face fans that use the power from the phone-jack for their motors on ebay
• probably it would be better to do all this in SMD, since pocket space really is an issue.

Thanks for reading & feel welcome to share your thoughts!