After finishing the microphone amplifier and the AF amplifier modules, it is time to test them. For the experienced builder with scope and signal generator available it is no problem. If you are new to building, and don't have these available, there is a very inexpensive option that may even work better.
I spent more than 25 years working as a field service technician. During this I spent more time than I want to think of dragging scopes, and other pieces of test euipment around. During the last years before I retired, I found several useful pieces of software that allowed you to use PC sound cards as scopes and or signal generators. Although they were not suitable for calibrating the equipment I worked on. They were more than adequate for trouble shooting, and verifying basic system operation
Although these worked better with actual sound cards, they also work quite well with the very inexpensive USB sound cards that are available simular to this one.Other USB sound cards will give more options such as dual channel input, but are more expensive.
Only prblem I have found with the USB card is that sometimes there are higher frequency signals found because of traffic on the USB bus. This has normally been higher than the 5 KHz. I think of as the upper limit for frequency response in comunication equipment.
They only requite a simple interface to provide usable results. I have tried several, from basically just DC blocking capacitors,to ones with internal amplification.
The one I use most of the time uses a simple X1, X10 voltage divider along with DC blcking capacitors on the output to the MIC input on the sound card. If you use an internal sound card with a Line Input, you can build a two channel version of the interface.
This is also built with perf board, and use right angle header pins as output connectors. The header pins let me use JST jumpers to connect to modules being tested or to build simple fixtures for making testing easier. Instead of a switch I use a 3 pin header and jumper to selecdt voltage level. You could also replace the voltage divider with a 1 Meg. Pot. to make construction even easier. I used some 3.5 mm. jacks for the signals going to the sound card. You could elminate them and use a 3.5 mm stereo cable cut in half and wired directly to the circuit.
They have similar basic functions
2 channel scope with XY display
FFT based frequency spectrum display
Functions are selected from a set of tabs above the scope display. First thing you need to do is select your input and output device, and set the scope paramateurs. Amplitude calibration can be done later. For ease of learning the interface select scope loopback for input and output device
First function to check out is the Oscilloscope function selected by clicking on the first tab. To the right you have controls for setting the voltage range per channel and the timebase for the sweep. There are also options for setting trigger type , slope, and threshold.
One common way to check frequency response, is to set the signal generator start frequency, turn on sweep and set the end frequency and sweep time. I usually set the end frequency to about twice the highest frequency response I am trying to measure. Then on the frequency display set peak hold and set to dB and auto scale. Let it run a few passes and you have a good indication of the frequency response of the amp or filter. Do this with two different signal levels and you can find the gain or loss of the device being tested.