I need a basic theory lesson....

[dave251]

Two questions:

Will an inductor cause a phase shift in the signal?

Will a capacitor cause a phase shift in the signal?

[David Schwab]

I'd say yes to both, but it depends on how they are used and at what frequency.

[dave251]

In series, in a passive pickup circuit. I'm making the assumption that the capacitor will retard the lower frequencies more than the inductor will retard the high freqs.

IN otherwords, a magnetic pickup will shift(delay) the higher frequencies' phase away from the attack transient, while a capacitive device( such as a piezo) will delay the onset of the lower frequencies...

So the two vastly different voltage generating schemes would have an offsetting effect....

Just a thought, but I don't KNOW for sure....

[SkinnyWire]

http://www.play-hookey.com/ac_theory/filter_basics.html

[Joe Gwinn]

Two questions:

Will an inductor cause a phase shift in the signal?

Will a capacitor cause a phase shift in the signal?

Yes to both. Why do you ask?

[dave251]

I'm interested in guitar pickup design....and what influence the mag pickup has as an inductor, and the influence of the capacitance of a piezo device. I've built more than 150 of these circuits into my guitar designs, but I'm not clear why they do work.

My question is really just a confirmation of what I "hear" as the instrument is being played, that is....I'm using these two disparate devices, not only for their signal generation attributes, but also, respectively, as an inductor AND as a capacitor.

The circuit is PASSIVE, and seems to work best into a Fender style tube preamp.

[David Schwab]

I'm interested in guitar pickup design....and what influence the mag pickup has as an inductor, and the influence of the capacitance of a piezo device. I've built more than 150 of these circuits into my guitar designs, but I'm not clear why they do work.

Hey Dave, I've seen your work... nice looking instruments. I remember reading about your pickup setup a while ago. I thought that was very unusual.

My question is really just a confirmation of what I "hear" as the instrument is being played, that is....I'm using these two disparate devices, not only for their signal generation attributes, but also, respectively, as an inductor AND as a capacitor.

It's probably a very complicated thing going on, but the magnetic pickup is surly loading down the piezo, since they require a very high input impedance. Un buffered piezos can be very clacky sounding, so the loading from the pickup is probably smoothing it out.

I'm not sure what the piezo would be doing to the magnetic pickup.

The circuit is PASSIVE, and seems to work best into a Fender style tube preamp.

This is probably because of the input impedance of those amps. If you want it to be consistent you could consider using an active buffer set for the same input impedance as the Fender amp.

I have to wonder since you make very acoustic based instruments how they would sound with pickups that aren't loaded down. You are getting a good composite signal, but not really getting the full frequency range of either pickup.

But this reminds me of an experience I had once... I was playing around with an old Mosrite bass I had. I had the tone control turned all the way off, so no highs were being heard. I had the bass plugged into a Univox Unitron, which was a copy of a Mutron envelope filter. I had it set for high pass. I had modded this pedal to have a built in mixer, so I could blend the high pass setting with the straight bass signal, and it also had a post filter boost.

With the dry signal off, and the filter set for a very high cutoff frequency, you obviously didn't hear much of anything, since the tone was down on the bass. When I cranked the boost all the way up, I got the best bass tone I ever heard.. very smooth, with a nice crisp top and and warm round bottom. Almost an acoustic tone. But also a lot of noise from the boost. The tone was very different from the stock Mosrite tone.

Something odd was going on there, but I've never figured out what.

[dave251]

Well you wouldn't believe how many iterations I went through trying to find something that would work with my "solid body" design. BUT, I'd like to figure out how to predict the outcome. While the piezo devices I use are all very similar, the magnetic pickups are all over the map....there are just so many designs to choose from and there's no way I can test them all.

THe piezo influences the mag....through getting rid of some of the upper midband "electric" tone, typical of a magnetic pickup. It's right in that frequency response range where the piezo really starts getting efficient. So it sort of takes over in those frequencies. I can hear MUCH more bottom end AND top end when the piezo is in the circuit. Sounds to me like mag pups are really a mid band device....not much below about 250hz, not much above about 2500hz.

I'm not really after "acoustic tone" BTW....but more of the "acoustic sensibility", that is, the dynamic input from the player and how it relates to the dynamics at the output. In otherwords, the "harmonic blossom" that occurs during the attack transient. For so many years we've all been so focused on sustain, that the attack transient has been ignored to a large degree regarding pickup system architecture.

So this "MagPi" passive system is the result. I'm also trying to find the least amount of gain stages to get a reasonable "clean" volume. Something useful say, with a five piece jazz group in a noisy nightclub. I believe the more gain stages you have, the more you "squash" the dynamics, the harmonic blossom. So I joined this board in the hopes of finding some new avenues, a new way of looking at what I've been doing...a bit less "seat of the pants" and a bit more engineering.