DOUBLETALK AUDIO OUTPUT ADAPTER =============================== Frederick Stam Intelligent Access Microware (IAM) IAM@braille.uwo.ca June 1997 This article describes an adapter which can be built to connect the output of the DoubleTalk-PC synthesizer to the auxiliary input of a computer sound card or other grounded amplifier. The audio output signal from the DoubleTalk synthesizer originates in a bridge type amplifier chip. (TLC2471) The output is DC coupled and the chip really complains if one of its leads is connected to chassis ground. Simply connecting an audio patch cord from the output of the DoubleTalk to a grounded external device or your sound card line input will eventually destroy the amplifier chip. Of course this makes Texas Instruments happy since they get to sell you a new amplifier chip. In short don't plug anything into the output of your synthesizer but the speaker ... or the adapter shown here. Since the amplifier chip has DC voltage on the output leads an audio output transformer can be used to isolate the output and safely extract the audio signal. The isolated (primary) side of the transformer can then be directly connected into the Line-in connector of a system sound card. The adapter shown here uses parts which are readily available from Radio Shack. It can be plugged into the output of the DoubleTalk or the external speaker. The adapter provides audio to both the Left and Right channels of your sound card (though not in stereo). Once connected you will need to set your sound card mixer settings and the DoubleTalk volume control to achieve the best sound level. If you plug the input of the adapter into the DoubleTalk external speaker you can also control the level using the speaker volume control. There is also some reduction in the audio level because the output transformer primary impedance doesn't match the input impedance of the sound card. PARTS REQUIRED: 1 mono mini phone plug (RS #274-287) 1 stereo mini phone plug (RS #274-284) 1 audio output transformer (RS #273-1380) 2 feet of audio cable 3 conductor heat-shrink tubing or electrical tape TOOLS REQUIRED: soldering iron and solder wire cutters/strippers Teflon fingers (if your new to soldering) ASSEMBLY: - Cut a 1 foot section of the audio cable and attach one of the center leads to the tip connector of the mono plug and attach the shield lead to the ring connector. - Attach the remaining piece of cable to the stereo plug. One center lead to the tip and the other to the center ring of the stereo plug. Connect the shield wire to the ring connector and connect the other end to the center tap wire of the transformer primary (don't forget to slip on some heat-shrink tubing before attaching the wires). - Connect the two center wires to the two outside wires of the transformer primary. - Connect the two wires from the mono plug to the two wires from the other side of the transformer (secondary). - Wrap any open connections with electrical tape and continue wrapping the entire transformer and connections with tape to strengthen it. - When you are all done, simply plug the mono plug into the DoubleTalk output or speaker and plug the other end into your sound card. Remember to set your sound card line-in mixer setting for best volume. __________________________ || __________________________ | )||( | | )||( Primary | ~= ===| Secondary )||(__________________|=== == =~ ~= ===| 8 Ohms )||( |=== == =~ | )||( 1K Ohm CT | |_____________________)||(_______________________| || Mono mini plug Output Transformer Stereo mini plug (To DoubleTalk (To sound card output or ext. aux. input jack) speaker jack)