48VDC Phantom Power switch - seeking advice - diyAudio
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Old 2nd January 2012, 05:18 PM   #1
CZ101 is offline CZ101  United States
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Default 48VDC Phantom Power switch - seeking advice

Hi,

I am trying to add switchable phantom power on each input channel of a Yamaha PM-1000 mixing console.

In looking for a suitable switch, like for example a toggle switch to be mounted on each input channel, I've noticed that switches are typically rated for something like:

5A 120VAC/28VDC (or 2A 250VAC)

and this is sometimes qualified in manufacturers' datasheets as:

Maximum DC Voltage: 28VDC, etc.

My question is: if the switch manufacturer does not state a maximum DC voltage, or if the maximum DC voltage is expressed in terms of the maximum allowable current (5A 28VDC, as above for instance), is it safe to assume that incrementally higher DC voltages (like 48VDC) are allowable - given a proportional decrease in the amount of current (48VDC 2A max, for instance), or are any higher-than-rated-for voltages, even with much lower total current (25mA for a couple of condenser mics) not a good idea?

In other words, for a switch (like a toggle switch) -

does a 28VDC, 5A rating imply a 48VDC, 2A rating? or even a 48VDC, 100mA rating?

Thanks for the help everyone!
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Old 2nd January 2012, 05:53 PM   #2
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Basically any swiych will do. Phantom circuits don't really draw more than 15mA.
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Old 2nd January 2012, 08:47 PM   #3
CZ101 is offline CZ101  United States
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Thanks for the help.
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Old 3rd January 2012, 12:46 AM   #4
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Just FYI....
Switch's are de-rated for DC.... An AC switch rating: The switch can switch at any point along the wave (might be zero, might be wide-open). DC is always switching at maximum voltage, maximum current. (I made a ton of $$ replacing switch's in Makita grinders.... the welders would run them on the 120vDC off the welder exiciter..... the switch's would work until JUST out of warranty).
So... even IF you really overmaxed the switch, As long as it was ON when the power came on, there would be no strain on the contacts.
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Old 6th January 2012, 10:57 PM   #5
CZ101 is offline CZ101  United States
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Ok, here is my next newbie question:

In this circuit,

Click the image to open in full size.

which can be found at this website,

would C9 introduce any possible distortion into the balanced mic signal when the phantom switch is open - in the absence of 48VDC biasing the + side of C9, or is this nothing to worry about?

Would making C9 a non-polar cap (like a film cap) reduce the possibility of distortion (if any)?
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Old 7th January 2012, 04:51 PM   #6
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Default Suggest you go to the THAT Corp website

and look at the application documents for their preamp chips. There is about as thorough description of phantom powering that you will find anywhere, including the protection circuits necessary, should, say an input be grounded with no mike connected.

Charles
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Old 7th January 2012, 05:01 PM   #7
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After reading your thread I have difficulty in understanding exactly why you want to turn off the Phantom at each channel. The phantom will work just fine with a microphone not requiring phantom power.
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Old 7th January 2012, 05:22 PM   #8
imix500 is offline imix500  United States
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... Unless he plugs a ribbon mic into it.
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Old 7th January 2012, 05:55 PM   #9
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I'll give that one

There are some good ribbons available nowdays that use phantom too. I'd look toward a different mixer if it were me.
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Old 8th January 2012, 12:13 PM   #10
CZ101 is offline CZ101  United States
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Quote:
Originally Posted by Original Burnedfingers View Post
After reading your thread I have difficulty in understanding exactly why you want to turn off the Phantom at each channel. The phantom will work just fine with a microphone not requiring phantom power.
Partly because I have some line-level gear (balanced) that I want to run through the board, and partly because I'm of the sort that would rather eliminate something from a signal path if it is not necessary.
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