Phantom power destroying outputs?

A

andrew gissing

Sophomore
Jan 11, 2011
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Perth Western Australia
www.stylusra.com.au
Hi,

One for the tech heads among us (JR ?)

Two questions:

On a console route a signal on channel 1 to Aux 1, then the output of aux 1 back into the console on say channel 2. Turn up the aux 1 on this channel to create a feedback loop.

The signal is for all intents and purposes unusable because of the feedback, but can this cause component damage in the console ? Ie increasing voltage ?

Second question is what i'm really interested in though: Turn down aux1 on the second channel to remove feedback and now turn on phantom power. Or, with phantom power on, repeatedly plug in/out of channel 2. This has the effect of sending 48v DC to the Aux 1 output plug on the console.

Can this cause component damage to the output stage ?

The same situation could occur when one auxillary or output of one console is fed into an input of another console.

I've got a repair tech who says it can on my midas verona. I'm not doubting him but wondering if this is a common situation; are components usually designed to withstand 48 volts being fed to it ?

When I see the tech i'm also going to ask if this is the possible cause of my helix eq failing badly. The gig where it failed was one where my gear was setup by someone else and I noticed (but didn't think anything of it at the time) that the output of one helix channel was plugged into an input of the console; and the corresponding aux output went straight to stage rather than through the eq.

Andrew
 
Re: Phanton power destroying outputs ?

It is very common practice to place a series cap on the outputs of devices.

This does a couple of things-it blocks any DC offset that may occur on the output from getting to the next device in the chain.

It will also stop any DC that might occur on the output from getting back into the output stage.

However these "blocking caps" are generally (it has been a long time since I have looked at them-since I don't do component level repair much anymore) on the order or 35V or so. So the 48V phantom could possible break them down.

I have never seen any damage-but that doesn't say a whole not-since this is not a common "setup". I usually make sure a line output goes into a line input.
 
Re: Phanton power destroying outputs ?

The mic preamp input caps that block the 48V phantom supply can charge up and accumulate some serious energy. This energy can deliver amps of current and break stuff when discharged very rapidly by a low impedance...

So don't do that.

Causing positive feedback in the electrical path generally does not sound very good, so it can injure your reputation. The circuits don't much care.

JR
 
Re: Phanton power destroying outputs ?

Thanks all for your words but esp to Ivan and JR.

I don't wire up PA system's that way; Long story short charity gig where they had their own people setup and I came later. Best intentions and all but i've learned from this. I'm kinda inbetween being a small guy and a big guy. Not big enough to issue contacts like the big guys do. But I can see now that on occasions where I or my guys won't be at a gig, I'll now be asking for some kind of proof that my gear will be insured by them; and they will accept responsibility.

And to re-inforce, I don't make a practice of feedback; I thought i'd ask about the electrical side.

And Timo, yes; verona console supplying the 48v in my case too.

Andrew
 
Re: Phanton power destroying outputs ?

For a little extra texture on this... the release of smoke from external connections is usually from the rapid discharge of fully charged phantom caps. Many of these same connections would probably survive, turning on the phantom "after" being plugged up, since that is generally current limited to mA not amps through the same path. Of course as Ivan mentioned many outputs are not engineered to tolerate 50v DC.

So perhaps turn off phantom while patching up the system. Seems like people should already be doing that or perhaps patching with no power at all applied.

JR
 
Re: Phanton power destroying outputs ?

The SSM 2142 line driver, a very common line driver in the '90s and perhaps later, does not tolerate phantom voltage on its output. I once worked on a Crest console that had been used to feed a BetaCam (with phantom turned on, of course :roll:) from a matrix output, and it had let the smoke out. The funny thing was that the smoke came out of the power supply decoupling resistors feeding that part of the console, not the output driver itself. As far as I could tell, the SSM 2142 still worked once phantom was removed, although I replaced it anyway. Here's the link to the SSM 2142:

http://www.analog.com/static/imported-files/data_sheets/SSM2142.pdf

Crest built in provision for output transformers on the PC boards, but I've never seen a console (Century Series) that had that option. Has anybody?

BTW, here's a tip for you console designers out there :razz: : Metal film resistors might be great for audio, but aren't so great for power supply decoupling. When you exceed the power dissipation on a metal film resistor, its value goes down, increasing the current draw (The resistors in the Crest started off at 10Ω. By the time I got to look at it, the value was down to about 5Ω each). Traditional carbon film resistors behave like a fuse, and open up once they've smoked a bit. I've run into this problem a few more times in other gear, when tantalum power supply bypass capacitors shorted. The current draw wasn't enough to blow the line fuse, but it did cook the custom $100 toroidal power transformers (I guess Rupert made his name designing audio circuits, not power supplies).

GTD
 
Re: Phanton power destroying outputs ?

Had a GL3 tied in to a house rig using the mono out. We didn't know that the channel the house tech plugged in to had phantom on it. we were on stage when we saw the smoke coming out of the board...... needless to say the mono out on the desk never worked again. (no more monitor cue out on XLR had to use headphone out after that!)

Kim x
 
Re: Phanton power destroying outputs ?

Geoff Doane said:
The SSM 2142 line driver, a very common line driver in the '90s and perhaps later, does not tolerate phantom voltage on its output. I once worked on a Crest console that had been used to feed a BetaCam (with phantom turned on, of course :roll:) from a matrix output, and it had let the smoke out. The funny thing was that the smoke came out of the power supply decoupling resistors feeding that part of the console, not the output driver itself. As far as I could tell, the SSM 2142 still worked once phantom was removed, although I replaced it anyway. Here's the link to the SSM 2142:
Click to expand...
I wouldn't know never used one.
http://www.analog.com/static/imported-files/data_sheets/SSM2142.pdf

Crest built in provision for output transformers on the PC boards, but I've never seen a console (Century Series) that had that option. Has anybody?
Click to expand...
Transformers protect against a long list of mistakes... unfortunately they cost more and generally deliver less fidelity, unless you pay a ton.
BTW, here's a tip for you console designers out there :razz: : Metal film resistors might be great for audio, but aren't so great for power supply decoupling. When you exceed the power dissipation on a metal film resistor, its value goes down, increasing the current draw (The resistors in the Crest started off at 10Ω. By the time I got to look at it, the value was down to about 5Ω each). Traditional carbon film resistors behave like a fuse, and open up once they've smoked a bit. I've run into this problem a few more times in other gear, when tantalum power supply bypass capacitors shorted. The current draw wasn't enough to blow the line fuse, but it did cook the custom $100 toroidal power transformers (I guess Rupert made his name designing audio circuits, not power supplies).

GTD
Click to expand...

I am unsure about such broad generalizations, I used to use good quality carbon film (ROHM) that would turn red hot and only shift around 10%.

They make flame proof resistors specifically for use in such applications, where you want them to fail open and not catch on fire.

thank you for thinking of us, tho.

JR

PS I recall in the early days of wireless mics, they would routinely get smoked by phantom until the manufacturers figured out how many field failures they could avoid with simple adjustments to their design .
 
Re: Phanton power destroying outputs ?

01v96 outputs and A&H GLD outputs tolerate phantom power. Soundcraft Spirit 8 outputs don't. :)

One of the many benefits of my upgrading from Mixwizard/01v96 boards to my new GLD is individually switched phantom power, which means it's off unless specifically necessary for a certain input. I know that's not particularly impressive to you big dogs, but individual phantom switches is new to me, and therefore cool.
 
Re: Phanton power destroying outputs ?

I don't recall ever seeing a PCB copper failure that shorted two PS traces together from abuse or electrical stress (a possible manufacturing flaw that would never work in first place). To create a short you basically need to grow trace material laterally. If you don't see a solder bridge on a top or bottom layer, I would look elsewhere for the short.

That specific failure is a fairly common IC failure. Or perhaps shorted decoupling capacitors, shorted pass transistors, etc.

A dead short between + and - 15V is commonly through an IC that is connected to both. For caps or transistors, you would need pairs of failed parts which is less probable.

or not...

JR
 
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