Here is the speech input module for my Vocoder project. It consists of a microphone preamp, a line input, tone control and a level indicators for the microphone input. The amplification of the microphone preamp is adjustable with one resistor (R8), to fit your needs. A level indicator is added to prevent overdrive and achieve a good SNR for your input signal. This stages are followed by the tone control. You can adjust bass, middle and treble over a wide range. The output buffer drives the signal for the analyzer module.
Specs and features:
- Microphone and line input
- Line input 10Vpp (adaptable)
- Level indicator for Mic input
- Sound control Bass, Middle, Treble
- +/- 15V PSU. Runs on +/- 12V as well (With minor changes in some part values)
- Schematic Microphone input
- Schematic line input
On page 01 you can see the microphone preamp and the level indicator for the microphone signal. The microphone preamp is build around the SSM2019. You can adjust the amplification range for the SSM2019 to your needs with the voltage divider R9, R10, P2 at the pins 1 and 8. Refer to the datasheet for values. C27 blocks the DC offset of the SSM2019. You can adjust the amplification of the next stage with changing R3 if needed. With P2 you can silence the microphone input completely. With the level indicator you can see if your microphone input level is in the desired range for best SNR
On page 02 is the line input and the tone control. The line in input is laid out for 10Vpp signal. Change R22 to your needs if necessary for other input levels. The tone control is nothing special. Just a standard application. You can adjust bass, middle and treble over a wide range.
- Feed a 10Vpp/1kHz square signal to the line input.
- Set the tone control to neutral (Input and output signal should look nearly the same on the scope).
- Adjust TR1 so that the output level equals the input level when the tone control is set to neutral.