I am carrying out a few acoustics experiments with electret mics. I am currently using the attached preamp circuit and measuring magnitude and phase using a 1Mohm Zin impedance analyzer, which works fine. I need to start carrying out the same experiments on a vector network analyzer with a 50ohm input impedance. My current preamp circuit is getting loaded down by the VNA.

The VNA has a +30 dBm +-7 V MAX input @ 50 ohm, so ideally this would be taken into account with voltage limits ect.

Any suggestions on what type of amp circuit I could use? Low noise is especially important across as a wide frequency band if possible (20 Hz - 30 kHz +).

I found these circuits posted by Audioguru and SgtWookie:

Thanks a lot!

 

Look at the data sheet for a OPA1641 - http://focus.ti.com/docs/prod/folde...=hpa_audio_opa1641&HQS=Sample+PR+opa1641-preu

 

How well would that chip handle the 50 ohm load and be able to supply close the the +30dBm to get full scale measurements?

I came across this high current op-amp that seems suitable: TS921?

 

Gain and freq response are no problem, but you might have to use something like an audio amp to drive that 50 ohm input - http://www.national.com/mpf/LM/LM1876.html#Overview