Phantom Power Toggle Switch Issues

[JohnRoberts]

Just do it.

The 47 will reduce the in- and out-rush current , only if your mic is asking more than stored and it is audible , go back to 100.

I suspect he just did it over four years ago....

JR

 

[Evertide]

I suspect he just did it over four years ago....

JR

Using the most sold virus... W..

 

[Matt Syson]

Using a resistor to bleed off the charge on the cap would be 'polite' and the LED would achieve that in this circumstance. The tolerance for 48 Volt phantom is generally stated as plus or minus 4 Volts and I think it is only a handful of specific mics that actually care if it is a touch on the low side. The MATCHINGof the 6K8 resistors is important but the value isn't. 100 Ohms in series with the switch wiper is hqndy to reduce burning of the contacts.

 

[nsalis]

Been having a particular problem pop up on two (maybe three) separate preamp builds, and you know what the man said....

"Once is happenstance, twice is coincidence, three times is enemy action!"

Basically, the issue is the failure of the phantom power switch. It will work fine for a little while, then eventually just quit, leaving me with phantom all the time or not at all.

The first time was when I was working on my last tube preamp project. I had TWO switches fail. I didn't think too much of it because those two switches were dug from a box of junk parts, so who knows what shape they were in? So far so good on their replacement. The second instance involved a solid state build. Soon after using it a few times, the brand new switch failed. I have a possible third instance in that a two channel version of the solid state unit hasn't lost either phantom switch yet, but when I switch them on, I swear they make a slight popping sound instead of the usual click. I imagine what I'm hearing is arcing going on inside the switch.

So, naturally, I'm thinking that these toggle switches just aren't capable of switching 48VDC. I've read in a couple places that the typical off-the-shelf miniature toggle isn't designed to handle that, but can switch AC all day long. I've also read in a couple places that that's essentially bollocks. I'm inclined to believe that, since I have another preamp that I built from a kit every bit of twenty-five years ago, that uses the same type of switches for phantom power, but has never failed.

I've attached a schematic of the power supply I'm using for the solid state preamp. Could it be that the phantom stage is pushing too much current? Would simply adding a current limiting resistor to the 48V output solve the issue? I thought about putting the phantom switch on the AC side of the phantom supply circuit. That would likely work for a single channel unit, but not for a two channel unit.

"So, naturally, I'm thinking that these toggle switches just aren't capable of switching 48VDC."

The relevant standard for the design of a phantom power supply for microphones is set out in DIN EN IEC 61983. This standard states that the series resistances of the 48 VDC voltage source on pins 2 and 3 should each be 6.8k Ohm, and that the maximum current, which is allowed to flow should be 10 mA. This is a relatively small current that can only be switched reliably if the switch is designed for such currents. In practice this means that the switch contact must be hard gold-plated with a gold plating of at least 3um, preferably 5um. Most toggle switches in circulation have contacts made of silver cadmium. Such contacts are intended for switching mains voltage, i.e. 110 VAC to 230 VAC. Every time you switch, a small spark is created that burns away oxidation on the silver cadmium oxide layer. The low current of a 48 VDC phantom power supply cannot burn away such oxidation. Therefore, toggle switches rated for 250 VAC are not suitable for switching phantom power for microphones.
If they are used and if they have not already oxidized during storage, after a few days at the latest they will have an oxide layer that prevents the 48 VDC voltage from being switched reliably with just a few millivolts of current.

Look for a switch with hard gold plating as mentioned above. An example is here:

https://www.mouser.ch/ProductDetail/NKK-Switches/A11AH-GA?qs=KqcCwYXy5KHnrXlE3uHFcg==

or

https://tinyurl.com/mr4b9fhz

 

[JohnRoberts]

As has been shared already it's not the voltage but the current that burns out switch contacts and Phantom current is modest, only mA. They can get stressed if trying to discharge across large electrolytic caps resulting in potentially amps of current.

As has also been shared already a small series resistance can protect against such switch damage.

JR

 

[Mihobro]

Just remembered, in most phantom power supply designs, (Telefunken from the 60's) the Pre 6.81K resistor voltage is actually 56 volts, and the 6.81 resistor drops it down to around 48V when correctly loaded.

 

[JohnRoberts]

Just remembered, in most phantom power supply designs, (Telefunken from the 60's) the Pre 6.81K resistor voltage is actually 56 volts, and the 6.81 resistor drops it down to around 48V when correctly loaded.

While the 6.81k series resistors are pretty much standard, the voltage drop across them varies a lot between different microphones and sundry phantom powered accessories depending on their current consumption.

AFAIK the nominal unloaded voltage is 48V.

JR