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Old 15th August 2019
  #7
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David Rick's Avatar
It's not an oversight

Quote:
Originally Posted by 9xSound View Post
I know that I'm just grousing about what is either design laziness on the part of manufacturers ("That's just how they're made, man"), or a failure to see that a problem even exists.
It's not laziness; it's best practice. The vast majority of tube mics, being older designs, use linear power supplies. (Neumann built a newer one with a switch-mode supply, and there was no end of people grousing about it.) That means they all have a big-assed power transformer, and several secondary rectifier stages, all radiating magnetic fields that are very, very difficult to shield against.

Here's a picture showing the insides of one of my favorites, a Josephson C725. On the left edge you see an AC input board that holds the IEC power connector, line voltage selector switch, and power switch. Mic manufacturers buy this as a complete assembly from a company like Schurter, who certify it for safety in dozens of countries. Next to it is a large board containing the actual power supply. This board is a potential source of noise. It can have hundreds of volts on it, compared to millivolt or lower signals in the audio path.



The right-most side of this board has the DC output connections, and that's the only part of the board it's safe to be close to. No surprise, then, that that where the third board containing the audio connectors and pattern switch is located. Anywhere else would be noisy, and also risk running afoul of regulations on minimum safety spacing. One thing about this particular mic will warm your heart, however: The audio board also contains a green power-present LED. David Josephson could get away with putting it there because LED's run from low-voltage DC. But most tube mics use an AC-powered incandescent lamp, which needs to be on the left side for reasons already stated. Why did they do that? Because, when most of them were designed, LED's did not exist.

Tube mic supplies are arranged in this way because nobody wants to drop $6,000 to $12,000 on a mic with audible 120 Hz hum.


David L. Rick
Seventh String Recording