tt patchbay- mic connections to preamp phantom powered

[jaschoon]

hi all:

i am planning to buy a tt patchbay to connect up my studio gear. i have read a number of threads regarding the connection of microphones requiring phantom power through a patchbay to a mic pre. i apologize if it seems like i may be digging up the earth on this subject yet again, but i hope that i might get clarification on things ive read.

it sounds like patchbay connections can send intermittant power spikes that can damage either the connected microphone or, if i understand correctly, the pre.

my question...

is this still the case if the microphone is connected with pp turned off, then turned on afterward- or does running a mic with phantom power through a patchbay to a mic pre always present a potential issue to the device(s)?

thanks in advance

[Enlightened Hand]

Originally Posted by Hugh Robjohns from SOS

...Patching microphone circuits from the studio floor to the desired mixer channel is one of the most useful aspects of a jackfield, although there are potential problems since, if phantom power is being supplied from the desk through the jackfield, patching a mic can damage the console.

As the jack plug is inserted into the jackfield its tip will momentarily connect with the ring contact in the socket, while the ring of the plug touches the sleeve contact. This short-circuits the phantom supply, bridging the +48V line straight to the earth return and, although the phantom supply should cope, the resulting voltage spike can cause irreparable damage to the input circuitry of the mixer channel. The mic input stage can be destroyed outright (particularly with older types of electronically balanced inputs) but it is more usual to find a gradual degradation in performance as various circuit components deteriorate.

Also, unless the patchbay labelling is extremely clear it is all too easy to plug a line source into a microphone input accidentally. If you are lucky the result will be a loud and horribly distorted signal, but it is possible for the phantom power on the mic sockets to destroy the output stage of the line source you are trying to connect!

There are a couple of solutions to these problems. Ideally the phantom power should be turned off while patching, although many mixers hide the phantom switches on the back of the mixer where you can't get at them easily. Many of the better budget mixers (such as the latest XDR Mackie consoles, for example) have mic inputs which have been specifically designed to withstand hot-patching of mics. However, this still doesn't protect against plugging line sources into mic sockets.

If you are concerned about this kind of accident, a reasonably practical alternative (and one I personally recommend) is to patch the mics on a dedicated XLR patchbay -- Canford Audio, for example, can supply suitable panels. XLR sockets are designed so that the earth connector (pin 1) contacts first, with the two signal pins connecting fractionally later, thus avoiding any chance of the phantom supply being shorted. The contacts are also much better suited to delicate microphone signals, and you cannot accidentally plug a line-source jack plug into an XLR socket! In most broadcast studios the usual practice is to supply phantom power direct to the studio wall points, which is an alternative method of avoiding having phantom power across the jackfield....

ENTIRE PATCHBAY ARTICLE very useful. Hope it helps.

[DrFrankencopter]

Re: tt bays and phantom

Mic pres won't have a problem with hot patching phantom. Most mic's won't have too much of a problem with it either. Ribbon mic's are best not subjected to this kind of treatment though.

The biggest problem will be the abuse your monitors will take if you forget to mute them while hot patching.

In general it's best to run an xlr bay for your mic lines, but if you're careful you can make a tt bay work.

Cheers

Kris

[jaschoon]

thanks very much.

enlightened: that was one of the articles that i read. i gathered that as long as i make a patchbay connection between the mic and pre with phantom off then switch it on im ok. problems arise if pp is on and i jump between the two. is this true?

drfranken: i understand what you mean. im really careful to connect then pp my ribbons. also, as long as i turn down my monitors i should be ok on that end- right?

thanks much for your thoughts.