Line Level > Input Preamp > DSP Core > Class D Amplifier > Speakers

-Line Level

Typically upto 3V peak to peak, although can be more.

-Input Preamp

Inverting opamp of gain -0.3. The normally connected to ground terminal is actually connected to a trimmer potentiometer to add a DC offset to the output, to bias it into the range of the ADC. Calibrated to 1V. Isn’t calibrated to 2.5V as the radix format used in the DSP algorithms work best at the lower end of ADC range.

-DSP Core

Uses dsPic30F3012 with C compiler. Implements 3 band EQ with overall volume control. Parameters controllable over I2C.

-Class D Amplifier

Uses quad NPN darlington array IC. One terminal of the speakers is connected to +5V, the other to the output of the array. Very dirty and cheap Class D(?), but the problem is quiescent current as its not using a dual rail power supply, but this can be managed with duty cycle reduction when no audio is connected.

-Speakers

Nasty and cheap 0.3W 8 ohm speakers just for testing.

-Enclosure

Mounted LCD screen and rotary encoder with Pic16F, to allow I2C control of DSP IC. In progress picture: