Sound Amplifier for CE Device
(Contributed by Allan Teo - an OziExplorerCE user)

(We have not used or manufactured one of these amplifiers, so we cannot make any recommendations or answer any questions you may have about this article.)

With the ability of the OzCE software being able to announce proximity points and steering instructions, I needed a 'Plug and Play' amplifier to allow me to hear the announcements without disturbing everyone in the car and not breaking the bank.


This project took 5 days to debug , even though it only took 15 mins to build the kit.

I will make this as short a description as possible to get you going. I am not going into building the kits, but how to solve car audio related problems and to integrate it into the AMP.

In my particular example , I use a HP Jornada 560 , HP 12v lighter adaptor for the jornada and a Kit based, TBA 820 IC amplifier.. If you are owning a vehicle that uses 24 volts, you need to find a kit that allows for 24-25 volt input or build a 7812 IC based voltage regulator circuit to protect the amplifier.

I recommend IC based Amp kits use TBA820 and LM380 ICs, I do not recommend LM386 based ICs as the voice quality is not too good.
Try http://kitsrus.com/kits.html, kit 97, My TBA820 is from Future Kit FK 602, 2 w power amp.

All these cost around AUS $13 each.

If you don't want to build a Kit, you can buy the Radio Shack 277-100BC mini Amplifier Speaker. This is based on the LM386 IC but it will have to do if you cannot solder.. This Kit DOES not require a voltage regulator even though it says 9 volts DC only, it can take up to 15 volts, so you can plug it in immediately. Computer mini powered speakers are also an option.


For what ever amplifier kit you build or ready made one you buy, you will probably encounter two main problems

1) Oscillation noise from the power supply of the PDA
2) Alternator Whine from stepping up your car's RPM.


The wiring schematic of this whole setup is to buy a 3 to 1 lighter adaptor and use this to power the jornada, GPS and Amp.

 

Solution to Problem 1 = Buzzing sound heard when no input, caused by Jornada Power Adaptor
(Note: Other adaptors that did 12v to 5 v, buzzed badly too)
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To Solve Problem 1, I bought a AUDIO Ground loop isolator from a car audio shop.

A ground loop isolator contains an isolation transformer for each channel.
The transformers generally have a 1:1 ratio which neither boosts nor cuts the audio level.
The audio is magnetically coupled through the transformer's core.
Since DC can not flow the transformer, the DC path is cut and the noise is eliminated.

These are also available from radio shack. part # ,Ground LP Isolator (270-0054)

As shown in fig 1, the HEADPHONE OUTPUT from the Jornada is plugged into the INPUT of the transformer and the output simply goes into the audio amp circuit.

 

 

Figure 1

A,B,C = sound path, Jornada headphone, to GLI to Amp board
D,E = Alternator Whine Filter, Toroid with 2000 UF E Cap.

 

The Ground Loop isolator I purchased cost about AUS $10.00

The Inside of the GLI.

 

You now need to dismantle the GLI and use a Marker pen to mark the Ground inputs and Ground Outputs, same for the signal input.. You need to replicate this soldering of wires to your Kit amp. After marking, desolder the original wires Use a SAW to separate the two transformers as we only wanted mono sound.

Buy a coiled stereo shielded twin audio adaptor and snip off one end exposing the wires. Identify the ground wire and solder it into one side of the GLI, choose 1 signal line and solder that to the GLI, Snip off the other unused signal line. Use 2 short wires and solder the output of the GLI to the input of the AMP.

You have finished curing this problem. Plug the Jornada into your DC 12 V to 5 volt adaptor and the buzzing noise will be gone.

Solution to problem #2 = Alternator Whine
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Once you have the whole amp ready, test it by driving around.. if you hear a whining sound that seems to go up and down with your car RPM, you have alternator whine.

If you have some cash to spare, you can pick up a EDC-36, Filtered power adaptor from Alinco .. They make amateur radio equipment and 1 EDC-36 can power the Jornada DC adaptor, Your Garmin GPs adaptor and this AMP. This will cost you almost AUS $50.00

However if you can wind or find a Torodial Transformer in an electronics Junk Store, you can get away with spending only AUS$ 5.00

My Electronics Junk Store coil looks like the above. However you can buy one from a proper store at

http://www.davidnavone.com/filters.html

The EDC-36 is a PERFECT solution, absolutely no noise.. The DIY solution will cut off 97% of noise, You probably won't notice it. However with the EDC-36, all you need to do is to desolder the wire from the 3 in one Cig lighter adaptor and solder the output of the EDC-36 into the adaptor.

Other alternatives are available but are bulky, in our case we want to have it all in one box.

 

Integrating the solution
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The toroid transformer uses 2 wires and will be used to power the amp circuit inside the amp enclosure. Shorten the output wires as much as possible.

Choose 1 output wire and solder it to the 12 Volts from the 3 in 1 car adaptor.. solder the other end to the +VE side of the amp circuit board.

You are DONE!

If you wish to IMPROVE it furthur, buy a 2000UF 25 volt Electrolytic capacitor and solder the + lead to the 12volt input point and the - side to the amp circuit
board, yes across the same leads as the Toroid transformer.

All the components
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