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KM 56 power supply and other concerns
Author: David Satz
Date: 11/19/2006
Hello, Richard. An NKM can power a KM 56, but if your NKM hasn't been used in a long time, perhaps someone should check and adjust its output voltages under load (i.e. with the microphone connected).

The KM 56 has a balanced, transformer-isolated output, so it really doesn't matter whether or not phantom powering is present on the input that you connect it to.

The tube for this type of microphone is the Telefunken AC 701k, which is a selected version of the basic AC 701. The responsible microphone manufacturers (especially Neumann and Schoeps) selected further from among the batches of tubes which Telefunken delivered, so "just any" AC 701k isn't necessarily suitable (nor are all of the "k" markings genuine on the tubes which one sees on eBay, etc.). But reliability was part of the fundamental design for these tubes, and 40 years of service isn't an unreasonable expectation with them. If your microphone shows no symptoms now, replacement may not be an issue for you even 10 or 20 years in the future.

When the time does come, tubes are still "somewhat available" though good ones sell for a couple hundred dollars or thereabouts. Selecting and replacing an AC 701k is beyond the skill and experience of most audio technicians today, though--so in practice, you'd probably want to choose a repair facility or repair person that specializes in this type of work.

The other major part of a KM 56 which is delicate and, at this point, becoming nearly irreplaceable (more than the tube), is the capsule. The KM 5x microphones had capsules with ultra-thin metal diaphragms. Neumann doesn't manufacture this type of capsule at all any more, not even for replacement parts. So if some bozo tries to test your microphone by blowing into it while the powering is on, or uses it to close-mike a kick drum without a really good wind screen, your alternatives might not be pleasant.

--best regards  
 
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