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Old 15th August 2019
Gear Maniac
9xSound's Avatar
Originally Posted by David Rick View Post
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
Thank you for the really informative post, David. Interesting read. I totally get the need to avoid hum. And best practice is best practice. Every industry finds its own particular sea level. I guess where I'm coming from, as an end user and not an expert in electronics, is that the front-to-back distance on my power supplies (which are for new mics, I should add — not talking vintage) isn't much greater than the width left-to-right. Seems like it should be possible to design a PS that is perhaps housed in a wider chassis to provide the necessary separation to avoid hum, and puts the switches and LED on the front.

I don't know. Like I said, I'm mostly just grousing.