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

[Gareth Edwards <gareth@xxxxxxxxxxxxxxxxxxxx>]

2009/5/12 Joerg <joergsch@xxxxxxxxxxxxxxxxxxxxx>
> >> For guitar pickup modelling, you might like to do a bit of Googling on
> >> "spice model guitar pickup" or similar.
> >
> > Yes, that is a very interesting page. This is precisely the kind of
> > thing I'm interested in. I had no idea the capacitance of the cable
> > was so significant.
> >
> > 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.
> >

Unless you are building a pickup, don't get too hung up on modelling
it - it's a complex and changing impedance across frequency (stray and
interwinding capacitance, leakage inductance, DC resistance etc) and
the unit-to-unit variation in impedance and output voltage is large.
You've already spotted that cable capacitance plays a part - the tone
and volume controls loading the pickup are probably just as important.

The trick is to use a high impedance amp front end that isolates the
pickup from the rest of the circuit (JFET or, my personal
preference/prejudice, a valve triode stage); then you can simplify the
pickup model to a simple generator without worrying too much. In my
amp work, I use a 100mV pk nominal signal - this is recommended by
Kevin O'Conner in his excellent Ultimate Tone series of books:

http://www.londonpower.com/catalog/index.php?cPath=3

However, modelling can only get you in the ballpark of gain staging
and frequency response - eventually you have to evaluate with your
ears, not with SPICE!

>
> 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.

Agreed, guitar into soundcard is sonic disappointment but JFET source
follower would be much better. Here's a good one (not mine, but
derived from a Win Hill circuit he posted on Usenet):

http://www.ciphersbyritter.com/RADELECT/PREJFET/JFETPRE.HTM

Cheers
Gareth


_______________________________________________
geda-user mailing list
geda-user@xxxxxxxxxxxxxx
http://www.seul.org/cgi-bin/mailman/listinfo/geda-user