SA: PC sound card oscope
don't make this SA, see last section.
8/26 2012
11/25/2012
For audio experimental, the most cheap spectrum analyzer is PC based[except the PC], a simple buffer circuit do all the work you need. then you can use a free version trueRTA or the SoundCard oscope do all the audio related analyze work: the O-Scope, spectrum analyzer, THD meter , all in one.
there is 2 mazing adapter circuit F.Y.I.
My version Probe
I use xoscope's input protect and oscilloscope probe compensation circuit and use Rodney Byne's buffer: the MPF102. that does work, for 1:1 probe, it works well, but for 10:1, the 60Hz power line interference can not be eliminate, so i use 1x only, that's enough for me now.
the beautiful box make it a cute instrument of my Lab.
Using is test a simple sine oscillator, Audio, I'm Coming
THD 3%
protection
1. input: 0.6V :cause if you power down/or off , and connect a power siganl to it...., mpf burned
2. output 100R+0.6Voutput + addtinal capcitor protection: some pc sound card had a 2.5V dc offset !!! damit !
LOOK HERE!
the buffer i built derived from Rodney Byne's one . but i add two pair of protection diode. that's would introduce the distortion while input level reach 300mVpeak. make it um-convenient to meter distortion. thanks Rodney Byne remind me this.
so D3,D4, D2,D5 should be remove. and power on before you start to use it ! (thanks Rodney again).
Simple dummy "buffer"
i use a very rough "dummy protection" circuit: 0.47uF serial with a 560R resistor as buffer circuit, and it works very well than this buffer. use this very simple circuit sound card can handle <1.5Vpeak signal and noise floor drop to -110dB with a decent audio cable.
use this "buffer" the signal level you get is a little bit small than the accrual level, cause the sound card input impedance is not "infinity".