Update to microphone circuit...

After less than ideal performance using the unit below (hissing on high gain, whining from engine noise, hollow sound, etc), I did some more digging on the net to look for possible fix... I find this article/document to be very informative and deserve a mention here on my blog for others to checkout...

• Working with Cranky Op-Amps
• Op Amps for Everyone (PDF-2MB-464 pages)
  

I'll be changing my opamp tonight with one that has very low 'input bias current'... I have a few LMC6081 on hand and I'll use those... the opamp I use now is CLC449 (obsolete) with about 50uA input bias current... the LMC 6081 is spec'd for 100pA! Orders of magnitude lower than the CLC449...

I'll add the output and grounding resistors as suggested by 9W2DTR... I'll set the output resistor to 100R and the ground resistor to 100k... I'll add some ferrite beads on the power lines as well to tackle the engine noice...

I will update on the performance later... stay tuned... :-)

Update: March 25, 2008
The fixes above worked... audio is cleaner, clearer, louder with no whining...