The input amplifier is now designed to roughly cover a range from -20dB to +20dB, which translates to an amplitude amplification of x0.1 to x10
Today the frequency response of the ampilifier using a standard PC-Soundcard and the Shareware Version of the audioTester Software. This is far from an accurate measurement but it gives me quite a good hint that we are on the right way. I already noticed before that the output of my PC-Soundcard is slightly unbalanced and this can be seen again in this measurement. I also want to note that I measured the frequency response of the Soundcard only as well and used the result as calibration data for the actual measurement.
With +20dB gain the frequency response looks like this (with groundlift activated)
Without groundlift (here we short the input Capacitors on the Ground line C24 and C28) The spike on the left is unclear why but maybe due to an error in calibration data.
Results: In +20dB gain Mode we see a drop in gain at around 16kHz. Groundlift does not really effect the frequency response in the audible spectrum.
With -20dB gain the frequency response looks like this (with groundlift activated):
And without groundlift:
Results: with -20dB gain we see an unbalanced Situation with groundlift activated. And we further see an steady increase in gain at around 6 to 7 kHz.
In overall the Results look acceptable – I also ran an online radio station through the input amplifier today and listened to music and the Music sounded nice – it sounded a tick more crispier when ran through the input amp.
Together with Michi we will use a professional audio analyzer to do the measurements in the next weeks