Speaker Driver  

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Flat Speaker Driver

Figure 1 shows the schematic for the flat speaker driver. Note that the LM675 is not unity gain stable. Hence the somewhat unusual scheme where we divide the signal down just to amplify it back up again.

Figure 1: Schematic for flat speaker driver.

Conventional Speaker Driver

To convert the flat speaker driver to work with the new conventional speaker, first remove the 3.1 ohm matching resistor. Then, add a 1.1 ohm resistor and 0.22 µF cap from the output to ground. (Note: you probably actually want a 1 ohm resistor, but we're out as of this writing). Lastly, you need to add a 100 pF cap across the feedback resistor to prevent oscillations. See Figure 2 below for more details.

Figure 2: Schematic for conventional speaker driver.

Oscillations

Beware oscillations. Figure 3 below shows the output of the conventional driver circuit without the 100 pF cap. A signal generator was used to input a square wave. The results are not pretty.

Figure 3: Driver output without cap.

Figure 4 shows the output with the 100 pF capacitor. Much better.

Figure 4: Driver output with cap.