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I'm glad you were able to make progress!

The passive preamp has a high output impedance. The amp has a high input impedance. This creates a problem where the interconnect cable can have a lot of hum and noise coupled to it from nearby wiring, because there is no low impedance anywhere in that circuit to keep the induced voltage low. Think of this as a simple voltage divider problem. There is a high impedance in the stray capacitance that couples the noise to the interconnect cable. Normal source components would have a low output impedance, and the voltage that this divider could develop on the interconnect cable wire would be very small.

However, with the passive preamp, the impedance to audio ground is much higher. This allows the divider to develop a much higher hum voltage on the interconnect cable.

Your observation about plug polarity recalls one published many years ago by Enid Lumley in The Absolute Sound. It was summarized with detailed instructions by Greg Weaver in Soundstage (see link). Your ability to reverse the plug polarity permits you to optimize your system. However, I still feel that you should investigate a proper grounding scheme for any equipment that has a three-prong IEC connector, for safety. The hot and neutral wires could be reversed internally if necessary to deal with a hum problem.


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  • I'm glad you were able to make progress! - Al Sekela 17:35:32 04/03/07 (0)


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