EEVblog #616 – How Microphone Phantom Powering Works

Doug Ford, former head designer at Rode Microphones explains 48V phantom powering of microphones and how it works.
There is a ton of stuff here:
Electrostatic and electromagnetic noise, differential amplifiers, common mode noise, transformer magnetisation current, JFET differential driver circuits, balanced impedances, phase splitters, miller capacitance and how it matters, and he explains how the Schoeps amplifier topology works.
Forum HERE

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  1. this deep review of microphone techniques is absolutely gorgous and instructive. thank you so much Doug and Dave for your fantastic work!

  2. a good, deep and sincere review of all the “wank technologies” out there on the pro audio market is so much needed !

    you wont burn in hell at all Doug, you’ll deserve a nobel prize for it!

    • If you liked that part of Doug’s talk, you might like the YouTube video “Jörg Wuttke, Understanding Microphones”. He was technical director of Shoeps, and in some ways, I think that video makes a good companion to Doug’s perspective.

      I used to have almost exactly the same attitude that Jörg Wuttke expresses in that talk, and it took me many years to get over it. 😉

  3. I loved watching Doug’s quick responses to all of Dave’s odd questions!

    I have one rather longish soapbox comment on phantom power:

    Phantom power is covered in the IEC 61938 standard, and anyone implementing it should abide by the official specs (so that all products work with all others). For example, the two 6.8k resistors for P48 (48v phantom power) need to be matched within 0.4%, so common 1% resistors don’t cut it. You need to match them by hand, or use resistors with tighter tolerance.

    Years ago I purchased an inexpensive mic preamp made by a well-known manufacturer, and when I measured the “48v phantom power”, it was only 27 volts! When I complained, I was told that although it wasn’t a full 48 volts, it should be sufficient for “most” microphones. It was returned very quickly.

    There are many other examples of products that failed to meet the specs, and could result in various troubles when used with certain models of mics or preamps. The amazing part is that it is pretty easy to do things right.

    • Actually, they don’t have to be high tolerance resistors. They simply have to match.

      Remember, a 1% resistor just means you get 6.8k +/- 1%. But if the absolute resistance doesn’t matter, you can probably get two 1% that are without 0.4% of each other, even if both are 1% down or 1% up.

      Maybe you can get away with 5% resistors.

      This is a good thing as it means you can use ICs to do phantom power – in an IC, absolute resistance is really hard to achieve. But matching resistors in silicon is *really* easy. So easy that most designs are done such that they are based on ratios of resistance, and not absolute resistance. So if you need a resistance twice that of another (e.g. R and 2R), it’s really easy to do, and really accurate as well.

      But if you need it to be 1000 ohms, you’re only getting 25% tolerance or worse.

      Most phantom power devices that are battery powered barely meet the spec because well, they’re battery powered and at best, the charge pump can’t supply all that much.

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