Sometimes the old stuff is still the best. At room temperature, a mechanical chopper combined with a high-ratio step up transformer and high impedance follower amplifier is unbeatable for low noise, low impedance signal amplification. The solid state devices that replaced mechanical choppers in most applications have much higher resistance, so more intrinsic noise. You should save the chopper for possible future use - and if there is an input transformer that goes with it, especially save that. Some circuits used a transformer (typically 1:100 ratio) to reach high gains for tiny signals into the nV DC region - without it, the noise level swamps the signal. The transformer is the key to the noise performance, but it wouldn't do much good without the low on-resistance of the chopper. The main problem with mechanical choppers is that they wear out - I think 10,000 hours is a typical life specification, and there's no way to know the history of a salvaged part. Transformers should last virtually forever, unless abused. As an example, I built a transformer-based preamp for my lock-in analyzers, that provides 1000X gain with 300 pV/root Hz noise, when the source Z is low enough - around a few ohms. I could make this unit measure DC by putting a chopper in front of it, but it must have very low resistance to preserve the noise performance. There is no conventional IC or discrete amplifier that I know of that can directly amplify with this noise performance, without using a transformer. The analyzer front-ends typically use "regular" low noise, broadband IC amplifiers that have around 7 nV/root Hz - about twenty times as much. The lock-in analyzer makers used to offer special transformers for this purpose. There are tradeoffs, of course - the transformer circuits only work well over a limited frequency range like 1 Hz to maybe a few kHz, unlike the ~0 to 100 kHz that an analyzer can run. Also, they only make sense in low-Z circuits - higher Z adds lots of noise, and dramatically affects the transformer frequency response. As I recall, 300 pV/root Hz is about the intrinsic noise of 50 ohms at room temperature, so in the more normal high impedance realm, conventional amplifiers are good enough. Ed