I am terribly confused ... I tried myself at building a low impedance guitar pickup some time ago.
Quick LowZ Pickup Intro
Basically just two loops of aluminium with two magnets, a humbucking guitar pickup with a single loop per magnet.
The three tongues are the terminals, but the outer ones are connected so that there is only one output plus ground. The magnets poles are on the top and bottom, one of them is rotated 180 degrees. This results in the desired humbucking effect.
I hooked up the whole shebang to my audio interface (connected to the Gnd and Hot Pins of an XLR with the Cold shorted to ground) and lo and behold there actually is audio coming from this thing.
Problem is that the output is super quiet and I have to crank the gain to +30db before I can hear something. My plan was to design a simple opamp preamplifier to boost the signal as close to the source as possible.
Now I definitely am no electrical engineer, but this is not my first rodeo. So I looked up the basic inverting opamp circuit and hooked it up on a breadboard. More specifically I used the schematics from here, but modified it to give me about 35x gain (about +30db). Hooked the output up like before, signal to hot, gnd to gnd and cold. This is a TL072 for testing, I meant to switch that out for a ultra low noise opamp once this is working.
(If I remember correctly this is Figure 4 from the Nuts'n'Volts article, just to give you an impression of my breadboarding conditions, I just noticed that it doesn't even show the output connection)
Hum! Instead of sweet guitar sounds I get hum. Definitely no signal in there.
What I Tried
For testing purposes I hooked up a poti to control the gain and found that the hum disappears at unity gain. The signal is audible again then.
Did a two stage design with a unity gain and a 35x gain stage. Still humming, no signal.
Checked the unity gain bandwidth thingy, but at 3 MHz for the TL072 35x gain shouldn't be a problem.
Tried a 1:35 transformer instead... But transformers are magic to me and I didn't manage to get anything from it.
I suspect that it's the super low impedance of the pickup itself. Basically it's a short against ground. But this is where my electrical knowledge fails me.
TL;DR: What am I missing in designing my LowZ guitar pickup preamp?