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Low Noise Microphone Preamplifier

2015-01-04 14:52  
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Low Noise Microphone Preamplifier
This is a design for a low noise microphone preamplifier, which is ideally suited to low impedance (600 Ohm nominal) microphones. One limitation is that it is not balanced, which is not a problem in a home recording environment, but will allow the mic lead (and case) to pick up noise with long cable runs or in a hostile environment. As shown, it is not really suitable for professional work (although it has been used on stage in its unbalanced form with good results), but the addition of a 1:1 microphone transformer on the input will convert it into a balanced preamp with very high performance. In many cases, a transformer will actually outperform active balancing circuits, because there is (or should be) no ground reference. The shield of the balanced cable must be earthed of course, but in my experience with live music and studio work, less noise is picked up if the internal wiring is floating. It is most regrettable that good mic transformers are rather hard to come by, and are expensive. If you happen to have a suitable one in your junk box, give it a try with this circuit - I doubt that you will be disappointed with the result. I have used this circuit in Front-of-House, foldback (monitor) and studio mixers, and managed to obtain excellent results - I still have a little 6 channel mixer (which I use only occasionally now) using this circuit, and have never been even slightly tempted to replace it with even the best of opamps (or Figure 3 for that matter).