Using a computer as a scope

I have a MacBook. On the Mac there are a number of programs which can display audio signals as would an oscilloscope. One I like is called Signal Inspector which as well as displaying as an oscilloscope can also display spectra and generate signals. There is another very useful program called PTHVolume which gives you an audio routing selection and gain controls in the menu bar.

The range of the spectral display depends on the settings of the Audio MIDI program where you can chose the A/D sample rate for the audio inputs (up to 96kHz giving a 48kHz bandwidth).

But the audio input to the computer is fairly low level, around 100mV at maximum gain. If you want to check, for example the frequency response of an audio power amplifier then it can have an output of 100V Peak-to-Peak which will severely overload the computers input. On the other hand if you want to monitor low level signal down to just a few milli-volts them the computer is not sensitive enough.

This calls for a preamplifier. And here is one with switchable gain of on the input of 0/-40dB (approximately) and a gain of 0/+20dB, giving a total of 0-60dB range.



Looking at the circuit, the input attenuator needs to be aggressive, for if an input of 100V P-P is to be tolerated and an output of 1V P-P (max) is needed then it should switch 0/-40dB so the lower resistor is therefore just 100R. This would give gain settings of -40, -20, 0, +20dB.