Jay, if you have a good, high-impedance voltmeter and the input section is coupled only with resistors, it may work OK. I read very close to 1 MegOhm on my old Fender tube amps, which are resistively coupled. But my Peavey solid-state amps are capacitively coupled. As Ed says, you need to make AC impedance measurements. Here's a simple explanation of a reasonable way to measure input impedance in this case:
http://www.sunynassau.edu/users/schoenf/refshts/measzin.htm
A bit more detail is given here:
http://ist-socrates.berkeley.edu/~phylabs/bsc/PDFFiles/bsc1&2.pdf
The upshot is that the input impedance may be a function of frequency. In addition, if the input signal is large, it may also be a function of the input signal level. One generally makes these measurements with sufficiently low input voltage levels that the input circuit is not overloaded, i.e., runs in its linear region.
Brad, the load may change the overall frequency response of the combined pickup-impedance, Zp(jw), and load input-impedance, Zi(jw), system. One can view the pickup as a Thevenin equivalent circuit, with the voltage induced in the pickup coils, Vp(jw), as the voltage source and Zp(jw) as the pickup Thevenin equivalent impedance. The load is directly connected to these, and we're concerned with the voltage dropped across the load input, Vi(jw). This is a simple voltage divider, and Vi(jw) may be computed for any angular frequency w as:
Vi(jw) = {Zi(jw)/[Zp(jw)+Zi(jw)]}*Vp(jw)
or = {1/{1+[Zp(jw)/Zi(jw)]}}*Vp(jw)
This is the voltage transfer function form. You can see that the pickup drops significant voltage signal at frequencies where |Zp(jw)| is not very small compared to |Zi(jw)|. If the load impedance varies with frequency, it distorts the effect of the pickup frequency response.
Ed, of course pickup makers need to understand how the various design parameters affect the pickup voltage generated and the frequency response of the coil. I think that guys like Bill Lawrence have this well in hand. I agree with you that serious measurements across lines of pickups would be useful to establish a baseline for players and designers of equipment.
I know that it is often difficult to do this, but it would be useful to translate such technical ideas into descriptions that are meaningful for guitar or steel players. I don't think most players are going to ever think in terms of frequency responses or transfer functions. They need something simple and intuitive to help them decide if a pickup, pedal, effects unit, or amp will sound good with what they have.
As far as input impedance goes, I believe it is well understood that everything in the signal chain should be low-output-impedance into high-input-impedance as much as possible, unless one wants to muddy things up. Some people do - e.g., jazz and blues guitarists often do that intentionally.
Donny, I've used plenty of EQs. As you say, EQ doesn't add attack or overtones that aren't there, nor do they equalize the many nonlinear things involved in the sound. They can also mess with the linear phase response and add a lot of noise of their own. Used judiciously, they can be very useful, but I don't like using them to make large changes in guitar sound. It may make a Les Paul or SG with HB credible in a single-coil context, but I've never heard anybody really make one sound exactly like a trad Tele to my ears.
Billy, from what you're saying, it sounds like you should be in good shape. You don't say what steel/pickup you use, but the LDR volume pedal specs say the input impedance is 410 KOhms in full-frequency input, and 100 Ohms output impedance. The RV-3 and Steel King both have input impedances of 1 MegOhm. I assume the RV-3 also has a very low output impedance. If it were me, I would probably try running into the RV-3 first, to take advantage of its 1 MegOhm input impedance to buffer the pickup, then into the volume pedal, then into the Steel King. But it may not make that much difference - all these devices have fairly high input impedances. In addition, noise generation may be a bigger issue than input impedance. It's important to have things that raise the noise floor, like compressors and overdrive, early in the chain so they can act on the cleanest possible signal. When in doubt, try different orders and see what sounds better. That is always the acid test for me, regardless of frequency responses, input and output impedances, transfer functions, or any amount of mathematics.
<font size="1" color="#8e236b"><p align="center">[This message was edited by Jim Phelps on 06 October 2005 at 05:00 PM.]</p></FONT>
