Rap 'n Tap
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From: Bill Steinhour
Date: 2/11/01
Time: 11:12:57 PM
Remote Name: 205.188.200.36
I noticed a couple of things about the opamp configuration that would make it very diffucult to reproduce an operable circuit. First, the opamp's noninverting (+) input is tied to circuit ground and so is it's negative supply pin. That's OK for a family of opamps like the LM324 with a commom mode input range that includes the negative supply but the 741 isn't one of them. I'm suspicious that the guy who built it may possibly have used a 324 or similar device and then specified a "741 type" in the parts list. You could add a couple of 10 K, or so, resistors, one from the + input to battery Positive. and the other from + in to batt Neg. That would at least get one of the inputs biased right.
But then, the inverting input configuration seems flaky too. With the capacitor where it is and a huge 10Meg feedback resistor, a "hobby" type opamp with a sloppy input offset could have the output biased to a supply rail.
If the designer happened to luck out and pick a leaky (very leaky) capacitor he may have accidentally biased it and set the gain in a workable range. Who knows? Try and duplicate that, though.
Probably the easiest way to get a working circuit from what you've got is to reverse the inputs to get a noninverting amplifier configuration and add a second 9v battery from the (-) supply pin on the opamp to ground (batt. positive to ground). Connect the capacitor to the amp's (+) input. Add two 100k resistors, one from each input to ground. Connect the 10 Meg resistor between the amps output it's (-) input (I'd probably try a 1 Meg first for a gain of 10 - make sure it works and then go higher on the value if needed).
After writing this it seems like it is probably a very confusing way to modify a circuit. I just hope you're not scratching your head more now than when you started. And I'll bet there's probably an identical or better diagram on someone's website. Does anyone in the core XtalSS group know of one to suggest? Also can anyone tell me of a way to post a schematic for general viewing? >Bill S.<