Re: gEDA-user: Signal Source Setup for Electric Guitar?

[Michael B Allen <ioplex@xxxxxxxxx>]

On Tue, May 12, 2009 at 6:43 PM, Joerg <joergsch@xxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx
http://www.seul.org/cgi-bin/mailman/listinfo/geda-user