This is a new page dedicated to my not-so-new interest in Analog audio projects.  Audio amplifiers, preamps, etc. are exceedingly easy to build & making your own is not only fun but you can customize your setup for whatever you do (especially if you are a musician) to create Audiophile quality circuits from scrap parts.




Portable Hi-Fi Professional Mic Preamp
Here is a painfully simple portable hifi mic preamp with many advantages over store-bought ones. 
For one, it's entirely portable with a very long battery life and, using salvaged bluetooth headsets or building your own, you can charge the cell phone battery via USB.  It also uses the Texas Instruments INA217, a professional-grade audio preamp seldom found in regular consumer-end products.  It integrates three precision opamps and all the necessary parts saving time, space, and quality.


To download project notes, schematics and datasheets for this project, click here





Here's the completed preamp I built.  I used a salvaged cell phone battery and for a charger I used one of those "Pocket Picture Frames"  (possibly the most pointless invention ever, lol).  That's what was laying around free but you can use any bluetooth headset as a charger or salvage one elsewhere.  The Pocket Picture Frame board is that PCB on the left with the "blob" chip- a jungle IC incorporating the li-ion charger among other things.  I soldered in a new USB plug and used the pin 5 (ground) to shield connection when a USB cable is plugged in to activate the charger.  This way the board doesn't drain unnecessary current.  It has a nice tri-state LED for the status too.
The only parts I actually had to buy were the enclosure, volume knob and the XLR jack ($6 at RadioShack).  Everything else was sampled & salvaged.  I added copper shielding over the entire thing in the end too.  Not necessary but it helps reduce noise the circuit will pick up when the gain is high.

Application?  Many obvious uses, but I'm using it to record high quality music and also for a mic input to add to a boombox/portable soundsystem.  A nice way to have good sound on the go for street or cafe performances.  Currently being used for a weekly poetry reading too :)

(Sort of) Hifi Bluetooth Headset

Bluetooth is great technology if you know how to modify it correctly.  Most headsets today have terrible mic sound because they place the microphone up by your cheek and most sets use very poor quality mics as well.  The other issues are 2) bad signal reception and 3) poor headphone speaker.

Using any half-decent old pair of headphones and and scrap computer headset you can make a very nice Bluetooth setup:



Obviously a large pair of DJ-style headphones work the best since you can cram the bluetooth inside.  I used a horrible <$5 no-name headset and it sounds great now.  Since the Bluetooth is mono I connected the two speakers in parallel to decrease resistance and give a louder output.  The largers speakers improve the quality and sound output a lot too.  Then I wired up the 3 necessary buttons for functionality and a USB port to charge instead of the annoying proprietary jacks they use.  Wire in a quality extended mic (this was from on old Xbox headset), a little antenna, a bigger battery and you're set!  I ran cable into the other earphone to fit a battery from broken ipod nano.  Any 3.7v Li-ion or Li-polymer will work great.

Many people have told me that I sound like I'm on a landline when calling from this headset.  In fact, not a single person can tell it's Bluetooth until I tell them!  These are very comfortable too. I prefer that any day to some pinky-sized thing in my ear.

HACKING BLUETOOTH FOR USB CHARGING
OR FOR YOUR OWN CHEAP 3.7v Li-Ion CHARGER


Found a cheap bluetooth headset but it's missing the charger?  Wan't to salvage cell phone batteries or other Li-ion or Li-Poly cells but don't want to pay $20+ for a charger?  Read on...

Lithium Ion batteries have been king of portable electronics since the early 2000 era.  With good reason--they hold the most power per size/weight of any battery chemistry there is.  But they are needly little buggers, sensitive to even 1v changes in charging or discharging.  Personally, I've grown tired of spending $3 on protection boards only to break five of them building a new project. 
And having to share a charger for my gazillion battery packs is annoying.  But USB charging is ideal and an easy way of charging single cell li-ion.

What has both of these (circuit protection and charging circuitry)? and is cheap?  Old Bluetooth headsets!
Here's the Motorala HS850 guts:


The important part here is the circuit circled in red.  I've found this in multiple Bluetooth headsets and was at least standard for some time.  It's marked "AZN" usually and through some miracle of Googling I found out it's a BQ24010 by Texas Instruments!

Very simple circuit that you can plug any 5v source into to charge li-ion batteries.  Here's the schem info from the above PDF:

USB 5v in to pin 1....yep, it's that easy.  Now you can use this cheap $5 headset as a 3.7v li-ion charger.  :)  or connect a USB plug to any Bluetooth you have with this chip. 
and just in case your Bluetooth doesn't turn on and you want to fix it, look out for those protection chips circled in the pic above.  Also standard packages used in multiple headsets.  Usually the protection/monitor circuitry is a separate board on the battery itself, which makes sense since it is designed to break in case of a battery fault.  You can trace the power output from the MOSFET and tie a new battery to there if yours is damaged.

Applications?  Other than the obvious, it's great to throw this board in any portable enclosure with a cell phone battery for a great portable power setup.  See the mic preamp project above for an example.  This could also be useful for portable mixers, distribution amplifiers, headphone amplifiers, LED projects, and anything else you can think of!

Mod your "Speaker Bomb"

While working on music the other day a friend of mine pulled out this new toy:




and of course I had to notice the little bomb with the cute face.  But more importantly, this is an IMPRESSIVE super-mini boombox, one of the very best I've heard actually.  The reason I guessed for this was a) a high-ass filter for audio (c'mon apple/altec lansing/etc. clue into this!) for low-distortion and a nice sealed enclosure with a class D 2 watt amp.

So what's the catch? 
Well for $10 (shipped!) you're going to have some problems.  After picking one up I cracked it open, and found my guess about the charging circuit was right:


Instead of a decent (or any) charging circuit for the li-ion battery there is simple a diode (!)  I have seen this a lot in the super-cheap-knock-off sound systems on the market now.  It saves a lot of cost I suppose, but it also makes sure your battery will die in <6 months (if you're lucky it will last that long!).  The reason?  These tiny batteries are delicate and need precise voltage and current charging, especially in the 90%+ charge cycle where they should be "trickle charged"- meaning only a very small amount of current should be used to charge them.  Not to mention that this thing charges via USB, so if you have a port rated at 5.2v (for example) the diode can output up to 4.5v to the battery directly (!)  This will really damage a li-ion cell.  They're never meant to take more than 4.3v (absoulte max).   A great site for understanding how this works (and all battery physics) is Battery University (click here to readup on it).

So, If you noticed the bluetooth section above, I put 2 and 2 together and cut out the nice charging circuit in an old headset, pulled out that diode, and attached it inside:


That is actually the same exact bluetooth headset used on this page years ago!  Been sitting in my stuff all this time :P  And the small SOIC in the main circuit is the amp by the way (in case you didn't guess).  It's rubbed off but most likely it's one of those new national 2w amps on the market, like the LM380.  They are in all these things.  It's pretty well built in there considering the price though, the blue LED is even surface mount and the caps are all surface mount ceramics.  Impressive with the $10 price tag. 
The charging circuit was super simple to hook up.  I just cut out the board making sure all the components were there, wired the +5v in to pin 1 (there's a on/off transistor there which is easier to solder to) and the battery + to pin 10 (the mini resistors close by are connected to pin 10 and were again easier to solder to).  Battery - is ground.  Ideally, I would like a charge state LED connected too, but it's a little extra work and isn't completely necessary.  So voila!  This little toy should last at least a couple years, maybe ever 5 or 6 with proper charging!