Crazy idea? Phantom supplied patchbay

[Matt Syson]

The current you should 'allow for' is 14 milliamps for each mic circuit (48 Volts through a pair of 6K8's in parallel) which would be the situation if the mic input were shorted to ground. OK there would be no signal either in this situation but it makes it robust against the damaged mic cable that happens to be plugged in for a day while you are doing something else.
25+25 ac is probably unnecessarily high as it will be lightly loaded so 22+22 is probably sufficient and less wasteful of heat.
Many BBC studios (TV) had phantom power applied at the wallboxes so appearing on the XLR but capacitors and resistors removed the phantom for the wiring run to the desk so that the desk /control room patchfield sees no DC. At least a couple of Calrecs had phantom power switches for each channel on the patch panel. Those mini EAO switches are very delicate and fiddly, necessary for a 40 channel patch strip.
While I didn't see the PUK UA8000 I was responsible for factory testing (and commissioning) 3 or 4 more. An upgrade to the Abbey Road desk (from 48 to 64 channels) and getting the heat out of the UA8000 in Stockholm (Abba's studio). Calrec Q series have phantom applied on the EDACs in the rear of the desk so on the mic circuit input.
Ramped phantom supply is a bit pointless as you never know if someone is about to plug or unplug a mic in the 'heat' of a session and all reputable manufacturers will 'design in' the ability to cope with a high voltage (possibly 48 Volt) spike. Interestingly only Studer (so far as I have seen) mention the necessity to demagnetise mic input transformers periodically to 'reset' the distortion that can occur by users 'spiking' the inputs with accidentally (presumably one leg of signal to ground short). Mr Lundahl has also commented that random spiking causes increased distortion even on line level transformers.

TL783 has the advantage of good ripple rejection and nearly bulletproof reliability which will also 'protect' the mains transformer and rectifier if you consider them part of the 'design'. They will also 'shut down' if you overload them or have insufficient heatsinking.

 

[Brian Roth]

In 1977 when I designed the Maranatha desk, the client's studio made up custom mic input wall panels which included a small toggle switch for each XLR input that supplied phantom at the panel. Hence, no need for phantom switching at the desk. There was no mic patching in that room.

Patching mic lines through a TT patchbay is a bad idea, IMHO.

Just something to ponder....

Bri

 

[abbey road d enfer]

In 1977 when I designed the Maranatha desk, the client's studio made up custom mic input wall panels which included a small toggle switch for each XLR input that supplied phantom at the panel.

We had exactly the same system at Barclay Studios. It was the assistant's job to turn them on. Actually we had not many phantom powered mics. We had a large number of U47/67 and a truckload of RE16's, some RE20's. The few U47FET saw little usage, like the RCA ribbons.

Patching mic lines through a TT patchbay is a bad idea, IMHO.

+1

 

[Newmarket]

Isn't it time for someone to come in and talk about using a SMPSU for phantom 48V

 

[gyraf]

I don't see the problem in using a switchmode supply, as long as it's well filtered - all my three Studer desks does that (981, 982 and 903)

/Jakob E.

 

[NOON]

Another issue that hasn't been brought up is the change in impedance of the mic input. Another set of 6k8 resistors in parallel with the existing mic input will reduce the impedance. How much depends on the original input impedance, only about 250ohms with a standard 2k mic input. Most mics this will make zero difference but with some dynamic and ribbon mics it can make a difference to the 'flavour' by retuning the RLC resonant circuit of the mic capsule + transformer < > preamp input circuit. Could be good or bad or just different..

 

[Greg Norman]

Stumbled across this today. Sorry if this was covered
If you don't remove the consoles phantom drop resistors (2x 6.81k guys), you'll create a voltage drop when phantom's off on the board, delivering roughly 24v to the mic. 24 supposedly should be fine with the phantom power standard being 12-52 volts, however, the current is limited by the divider. Also, as noted above, the impedance load on the mic drops (two sets of 6.81k drop resistors presents a parallel impedance of 1702.5Ω, which is a little higher than a typical preamp input impedance, which when added in parallel drops the load on the mic to below 800Ω, changing the performance for some mics).
I was recording at a 'world famous studio' which recently shoehorned a half-normalled patchbay between the mic panel and fancy vintage console to allow for external preamp patching! Other than the ground sharing issues, the two preamps in parallel caused a lot of wasted time chasing down distortion and noise on a few trusted mics. After realizing what was happening, the temporary "solution" was dead patching the bottom row, console preamps... I'm still kind of in awe that no one picked up on this at the place, and the people I was describing the problem to seemed not to comprehend the issue Though, they were the ones that put the wrong thing in to begin with.