KLUDGE@AGCB1.LARC.NASA.GOV (08/27/90)
Have any of you instrumentation amp gurus out there ever built an amp with an extremely low-impedance (sub-ohm) input? I say this because I routinely use ribbon mikes which run around 2 ohms output at the most, and usually contain a step-up transformer to match it to a more reasonable impedance. Trouble is, the distortion in the transformer is usually pretty high, and magnetic pickup becomes a problem. I'd like to eliminate the transformer and put a line-level amp in the mike head itself, but I don't know just how feasible this is to do. --scott Stay tuned next week when Kludge builds an FM IF strip with video-grade op-amps and gets abused by the Motorola "Manna from Heaven" distributor.
jgk@ames.arc.nasa.gov (Joe Keane) (08/31/90)
Getting low-noise performance with such a low impedance is tricky. A good low-noise op-amp has an input voltage noise density of around 3 nV/Hz^.5. With a very-low-noise matched transistor pair you can cut that in half. This is still about an order of magnitude above the thermal noise of a 2 ohm resistance. Plus your feedback resistors generate a lot of noise unless they have very low resistance, which causes other problems. So you can see why people want to use a transformer.