PC Sound Card with Stereo Microphone Line Mixer

The circuit is aimed to build a computer sound card with input line in order to use a two mono stereo microphone.

TLC272 – a precision dual operational amplifier with low offset voltage drift, low noise, high input impedance, and speed approaching that of general purpose BiFET devices and combined with a wide range of input offset voltage grades TLC274 – a precision quad operational amplifier with low offset voltage drift, low noise, high input impedance, and speed approaching that of general purpose BiFET devices and combined with a wide range of input offset voltage grades 7805 – a 3-terminal 1A positive voltage regulator with output transistor safe area compensation, internal short circuit current limiter, output voltage of 5V, 12V, and 15V, internal thermal overload protection, no external components, and current output in excess of 0.5A

Aside from having a stereo microphone in its input, the circuit is also capable of regulating the gain of the microphone stage and be modified to the final level of external audio that is not dependent with each channel of microphone. This method has been implemented in order to create a mixing of the two mono microphones. Included in the circuit is a classic supply for Electret microphones which is a type of condenser microphone that utilizes a permanently charged object to eliminate the use of a power supply. In the design, an input line can accept two types of plugs with the level being regulated at the input by the variable resistor RV4 which makes the sound mixing possible together with the input line.

Two RCA plugs are being used at the input A via connector JF3 and JF4 while a classic stereo plug with 3.5 mm jack is used at input B. the mixture of signals is being done by the IC2A and IC2 B, which drives the stereo output that comes from the jack JF6. The supply of the circuit is fed into the jack JF7, which can be slightly more than 9 Vdc from an external single power source. The function of IC3 7805 is to regulate and stabilize the incoming voltage to 5 V. This voltage value has been proposed to prevent overdriving the sound card which can cause damage on the circuit itself. The values of resistors R14 to R19 and R21 to R24 can be modified in order to achieve the desired percentage of gain level of the proportional stage.

Most sound card microphone inputs are requiring a minimum signal level. This could be in the range of 10 mV but older cards like the 8-bits would need as much as 100 mV. The impedance of the soundcard microphone input is typically 1 to 2K ohms depending from card to card. It is better to keep in mind that the type of microphone that works best with computer soundcard is the electrets microphone. There are two ways by which the signal level can be strong enough to hear. One way is to increase the sensitivity of the sound card input can be increased, to easily detect the signal from the microphone. This is done by using the software supplied with some sound cards to allow the user to increase the gain or sensitivity of the input, either by click and drag or a set of check boxes. The other way is to amplify the microphone signal before it goes to the sound card input, if the input sensitivity cannot be increased. This is done by using a microphone preamplifier or mic-to-line amplifier.

Soundcards can be manufactured in different versions like from Creative Labs where they produced Sound Blaster soundcards such as SB16, SB32, AWE32, AWE64, or Live. Some brands, like the Shure microphones can be interfaced with computer sound cards. Since soundcards are mostly used in computers, it is better to know its main components such as the digital signaling processor (DSP) which does all the audio processing like echo, tremolo, 3D effects, vibrato chorus, reverb, etc.; the digital to analog converter which converts the computer’s audio into an analog signal to be sent to a sound system like amplifier or speaker; the analog to digital converter which converts an input analog signal into digital data that a computer can process; the internal input/output connectors including telephone answering device having a green connector, auxiliary inputs having white sockets, and CDROM connector having a black socket; and the external input/output connectors with a MIDI connector having gold-colored feature, an SPDIF digital output using a coaxial cable with RCA connectors at the end, a microphone input, a line-in jack, and two normally light green standard 3.5 mm line-out jacks.

Source:users.otenet.gr/~athsam/audio_mixer_pc.htm