I'm trying to follow the schematic on this page to create a preamp/impedance matcher for a balanced piezo contact mic. I've reproduced the schematic below.
I've built it on a breadboard and I'm finding that the output is at least 20dB below that of the contact mic on its own, and has considerable noise. I'm finding it difficult diagnosing this, not having very in depth electrical engineering knowledge myself. I wonder if someone could suggest the likeliest causes?
My JFETs are 2N3819, matched by hand using the technique suggested on that page. My R3 is 1K, and my R1,R2,R5,R6 are 3M3 (easier to source). My capacitors are polypropylene, rated for 100V, and all resistors are carbon.
- My JFET matching went wrong. I gather that unmatched JFETs would result in noise, but would they also result in a signal drop where there should be gain?
- The assumptions the circuit design makes about the internal architecture of my audio interface are wrong, ie. "It takes advantage of the fact that phantom power is fed to the balanced XLR inputs via pair of closely matched 6.81k resistors. These resistors are used directly as the drain resitors in the differential amplifier rather than the more usual method of connecting an additional pair of matched resistors to each line to create a +48V supply rail and then feeding the signal back through DC blocking capacitors. Since the mixing desk already blocks DC at it’s inputs it’s not a problem that the signal is floating at +45V or so."
- Some of my component selections were wrong: an exact 3M value is critical, or the resistors should be metal film.
Many thanks for all help and suggestions.