TDA1308 Headphone Amp...

[sean]

I recently built a nice simple headphone amp to use around the house with my Rhodes pianos and electric guitars.  I used a module built around the TDA1308 headphone driver chip - www.nxp.com/documents/data_sheet/TDA1308.pdf.

The module I bought is the red one on Amazon (search for TDA1308).  I bought mine from seller Qianson, because it is fulfilled by Amazon.  I bought a few of them last year, when it was a bit cheaper (about six bucks).  The identical module is available from a bunch of different sellers, so you can get them for under two bucks if you buy five or ten and wait for snail mail. 

The module comes with zero documentation, so I got a magnifying glass and reverse-engineered it.  The Amazon sellers have very detailed photos, and it is a really simple circuit, so it was an easy job.  I have one multimeter that can measure capacitance, so I have made estimates of the capacitor values used. 

This circuit board is complete with input and output capacitors, so you don't need any external components except the connectors.  It provides plenty of gain (I have to turn my Rhodes stage piano volume knob down to six or seven), and it works well with electric guitars.  The finished project sounds great (very clean and noiseless), and is plenty loud.

This is what the tiny little circuit looks like:




I had a few construction design goals:
- make it run mono when plugged into the Rhodes, but have the option for stereo input when desired
- have a 1/4" stereo headphone output, and also 1/8" headphone output (so I don't have to hunt for an adapter)
- run off of three AAA batteries (4.5 Volts)
- turn power off when the input plug is removed
- stuff it into a plastic box that would not scratch the piano


This is what the circuit looks like all wired up:   




This project cannot be constructed economically, because you can buy something pre-made for cheap.  This project can't compete for price unless you get the TDA1308 board for cheap, and use an old plastic container for the chassis.  Behringer sells the MicroAMP HA400 and the MicroMON MA400 for twenty-five bucks.  I haven't tested either, but I expect they aren't too hateful.  Pyle PHA40 is even cheaper.  However, all of these products have two problems:  they expect line-level input, and they don't run on batteries.  Then it gets really ugly if you search for a "guitar headphone amp."  There are a number of tiny little devices that might be acceptable and run on batteries.  The Monoprice 611500 costs twelve bucks; and it must be awesome, because they clearly state "Pro Audio" on the label.  So I can't tell you that this project will save you money, but it will provide a high-quality headphone amp that will make you happy.


If you choose to build one yourself, here is the shopping list of components you will need:

1 each  Tiny red circuit board with TDA1308 headphone amp, found on Amazon
2 each  1/4" stereo jacks, Switchcraft 12B or 112BX
1 each  1/4" mono jack with NC switch on tip, Switchcraft 12A or 112AX
1 each  1/8" stereo jack, like CUI SJ1-3513, Amphenol ACJS-MV35-3, Switchcraft 35PRM3, or similar
1 each  3-cell AAA battery holder, like Keystone 2484 or MPD SBH431-1AS
1 each  Plastic box
Assorted wire, shrink tubing, solder, and tools from your basement or garage.


If you pay top dollar ($7.89) for the little red board, you should be able to gather the materials above for less than twenty bucks.  Except for shipping, that will add eight bucks.  This also assumes that the plastic box is found for free, and you already have wire, solder, and tools.

I used a plastic tub with a screw-on top that was formerly filled with Body Shop Body Butter (the 6.75oz size).  It is about two inches tall, and four inches in diameter.  It is sturdy enough to support all the connectors, and the lid screws on very securely, but is easy to open when I need to replace the batteries.  It worked out great.  My second choice was going to be a Tupperware or Rubbermaid food container.

This is how it turned out:




I can't find a SPICE model for the TDA1308 anywhere, so I didn't do any circuit modelling.  However, the little red board would have a circuit schematic something like this:




There is also a purple-colored board with the TDA1308 available on Amazon.  I have not tried that unit.  It looks like it implements the "typical application" circuit from figure 4 of the TDA1308 spec sheet (linked above).  This purple unit is set for unity gain, which might provide more bandwidth (but we don't need it).  The purple unit also has much larger capacitors at the output, which might provide better low-end response (which would be useful on a Rhodes).  Maybe someday I will spend the seven bucks to buy one for testing.

Sean