Michael B Allen wrote: > 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. >
No idea. All the PC-scopes I've looked at so far only produced yawning. The Picoscopes seem pretty good but expensive. >> 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. > If you connect it smack to the collector of a transistor (via a capacitor, of course) then the impedance will be loading it down. The gain of a single transistor stage like yours is roughly equal the combined resistance hanging on the collector node divided by the emitter resistor. So your gain would drop. > 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. > That depends on the card you have. Some can drive small speakers directly and are in the 10-20ohm range. Others only offer line level output and that would be in the kiloohm region. > 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. > Read up on them. I have used the sound card of laptops at clients and found noise problem that >$10k audio spectrum analyzers were unable to see. Sometimes to the utter amazement of engineers witnessing this. Seen some jaws drop ... > This is a lot harder than I thought. I think I have a lot of reading to do ... > Yep. And the good news is that thanks to the Internet such information is essentially free of charge. Or to stay with the tune of this group, open source :-) -- Regards, Joerg http://www.analogconsultants.com/ _______________________________________________ geda-user mailing list geda-user@moria.seul.org http://www.seul.org/cgi-bin/mailman/listinfo/geda-user