On Tue, May 12, 2009 at 6:43 PM, Joerg <joerg...@analogconsultants.com> wrote: > Michael B Allen wrote: >> Anyway it looks like their AC generator is using 2mV. So the 20mV >> value I used to get a good output SIN looks closer to reality which >> means my model is probably ok. >> >> I wish I had a real oscilloscope to find out what my guitar is really >> putting out but xoscope doesn't seem to produce voltages (but I'm not >> surprised since the sound card is probably oblivious to such things). >> Maybe I'll have to get one of those PC oscilloscopes. >> > > I don't know much about E-guitars, other than having enjoyed a live > rockband here this weekend. But AFAIK the magnetic pickups are high > impedance, above 100kohms. Much higher if piezo. Sound cards in contrast > are in the kiloohm range so most likely the signal from your guitar > collapsed the instant you plugged it into the sound card. Also, keep in > mind that sound card inputs often carry a little DC voltage to feed > electret microphones. You can measure it with a voltmeter. That can > cause a DC current through the pickup coil and saturate the core. > > If you want to display the waveforms or an FFT spectrum on the PC > cheaply you could use an emitter follower as a buffer so the voltage > gets transferred 1:1. Then a high input impedance is provided to the > guitar even if you connect a sound card.
I think I would rather buy a PC oscilloscope only because building the buffer circuit requires time and knowledge that I don't really have. Again, this is not work - it's just a hobby. I'm still trying to wrap my head around the concept of impedance in general and especially it's relation to frequency and how capacitors work and so on. So I think it would be better to get something that I know is going to just work. Has anyone tried the "Parallax USB Oscilloscope"? It's only $139. It can certainly handle audio and I assume it will tell me voltages, do FFT etc. The only problem I can see is that only has 8 bits of resolution. > Probably the input impedance must be set to a certain value (by means of > an extra resistors) so the guitar "likes it" and "sounds alright". This is very interesting. I had no idea the difference between the impedance of the sound card and a guitar amp was so significant. I built the circuit last night (although I haven't tried it yet). So now I have to wonder about it's impedance wrt to the sound card. Another thing I was thinking about doing was using siggen to feed the preamp circuit a SIN and then feed the result back into the sound card. So what is the impedance of the sound card output jack? Now I need a high impedance SIN generator. I get the feeling the whole sound card solution is not going to be ok. I either need a high impedance buffer circuit or an oscilloscope. This is a lot harder than I thought. I think I have a lot of reading to do ... Mike -- Michael B Allen Java Active Directory Integration http://www.ioplex.com/ _______________________________________________ geda-user mailing list geda-user@moria.seul.org http://www.seul.org/cgi-bin/mailman/listinfo/geda-user