Blasting an output stage with phantom power?

[Sammas]

Hey gents!

I recently acquired a Neve 542 suitcase mixer. Its a nifty little beast! I am currently constructing a new road case for it.

A thought occurred to me though, the PSU supplies phantom power to all 8 xlr inputs at a time. There is no independent switch per channel - which I can add... the real problem is these XLR inputs are used for both the mic and line inputs of the channel... so naturally there exists a very real potential to blast the output stage of a set of audio converters with a phantom power.

Will this be a problem?

What about if the output stage is unbalanced and the phantom power output is essentially grounded through one of its 6.8k resistor?

Would a better idea be to add a second set of inputs via an additional DB25 connector (I have already added one for direct outputs) for line inputs? I can decouple the line inputs from the microphone inputs using some DC blocking capacitors?

 

[abechap024]

Most decently engineered da outputs *should* have dc blocking caps rated at 50v or higher. That being said my Apogee Ad8000 doesnt have caps in the input, don't want to find out about the output!

I would say you probably won't ever regret putting in the extra line inputs.

 

[JohnRoberts]

If there is opportunity to have phantom power applied, there are ways to protect the output circuits. Equipment makers often learn this the hard way. I recall when wireless microphones were relatively new, There were lots of such phantom induced failures until the manufacturers got the designs robust and protected.

Standard output protection can include clamp diodes to power rails, build out resistance, caps, etc.  Not very elegant for minimalist circuit paths, but failing in the middle of a gig is buzz kill for commercial products. Even if the customer made an obvious mistake, well designed gear can just shrug it off.

JR

 

[gyraf]

We've had the problem in our B-studio, as the ssl4000 has permanent phantom on mic inputs - and these are present in the patchbay. This means that you can inadvertently plug a line output into a mic input.

It seems that this is a problem for many units that has the SSM-type integrated balanced output driver - specially TC's units seems to die instantly when looking into phantom. Same problem for some Lexicon products - but I don't recall changing output drivers in other units..

Jakob E.

 

[MagnetoSound]

Is the 48v really still there when you switch the channel to LINE, so the switch is simply a pad?

 

[tv]

I had similar problem with active bass preamp - on smaller gigs, plugging direct into mixer line input, there was a very real chance you could get zapped with a phantom power. (Happened to me twice, my amp's line-out got zapped badly).

So I had to invent a simplistic and clever solution for my situation, which included some clever scheming with output impedances etc. It still works, 20 years after.

Sadly, I don't think this solution would be applicable to your situation.

(but you could try with protection diodes and a small buildout resitance in series with "vulnerable" outputs - perhaps as an "go-between", standalone inline unit).

 

[Kingston]

RME HDSP analog output also dies when seeing phantom. I suppose most of their other product line too since (AFAIK) they've the same output driver design. There's electrolytic output caps, but they're to protect the outside world from the output stage DC, and not the other way around. I suppose a bipolar cap in this same position would protect from phantom. Beware!

 

[mjrippe]

The mic/line switch in the 34128 modules simply adds a resistive pad for the line in.  In fact, phantom power is not even shown on the schematic for the module.

 

[Sammas]

Yeah, they are pretty simple modules.

There is a 48v pin on the PSU connector that connects directly to a bus bar running above the input XLRs. The phantom power is tied from the bus bar straight to pins 2 & 3 of the input XLR via 6.8k resistors. According to the manual, the PSU has a switch to turn the 48v off. Mine little desk appears to use a non-original PSU though.

I like the idea of bipolar caps... mainly because I have a shitload of 100uf 63v sitting on the shelf. Will it be an adequate solution?

 

[gswan]

Maybe you should re-engineer it so that the line input comes into the module after the mic transformer, with a pad and its own set of caps. The line input does not need to go through a mic transformer and the phantom power will be on the other side so it will never be seen by the line input.

 

[Sammas]

Maybe you should re-engineer it so that the line input comes into the module after the mic transformer, with a pad and its own set of caps. The line input does not need to go through a mic transformer and the phantom power will be on the other side so it will never be seen by the line input.

It is possible, but basically requires too much invasive work.
The Neve 542 is what it is... but, it actually is a pretty damn cool piece of equipment. I have implemented direct outputs on each channel by isolated a spare pin on the edge connector of the modules. It is very non-invasive. The modules can still be removed from the desk, etc. Re-engineering the input requires a lot more work, including hacking too much of what is currently there.

 

[abechap024]

Polarized output caps wouldn't block the DC coming into the unit? Because they would be reversed biased?

 

[MagnetoSound]

According to the manual, the PSU has a switch to turn the 48v off. Mine little desk appears to use a non-original PSU though.

Seems like you just need to add the switch then - and don't forget to use it.

Maybe put a little sticker on the desk somewhere to remind you.

 

[CJ]

48/6.8k=7ma,

short resistor, i^2 becomes .007 times .007 = .000049 time 6800 = 0.332 watts,

so use 1/2 watt or better resistors,

7 mils won't fry anything, it might pop a fet input, but it will not fry an output transformer, and remember, the transformer leads should be connected to pins 2 and three, which hopefully are at 48 each, so no current will flow thru the xfmr sec,