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Author Topic: A quandry from the Mackie manual.  (Read 7785 times)

Dave Potter

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A quandry from the Mackie manual.
« on: April 16, 2011, 01:25:51 pm »

From pg13 of the Mackie PPM608 manual
http://www.mackie.com/products/ppm608/pdf/PPM608_OM.pdf

"Never plug single ended (unbalanced) microphones into the mic input jacks if the phantom power is on."
I never thought about it, (or  done it) but it makes sense because you don't want to short out the phantom.  but then a couple of lines down.


"These 1/4" jacks share circuitry (but not phantom power) with the mic preampss..............."

Can any one explain this to me?
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Gordon Brinton

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Re: A quandry from the Mackie manual.
« Reply #1 on: April 16, 2011, 01:48:21 pm »

From pg13 of the Mackie PPM608 manual
http://www.mackie.com/products/ppm608/pdf/PPM608_OM.pdf

"Never plug single ended (unbalanced) microphones into the mic input jacks if the phantom power is on."
I never thought about it, (or  done it) but it makes sense because you don't want to short out the phantom.  but then a couple of lines down.


"These 1/4" jacks share circuitry (but not phantom power) with the mic preampss..............."

Can any one explain this to me?
The first quote is part of the description for the XLR jack on page 12.
The second quote is from the description for the 1/4" jack.

The second quote simply means that phantom power does not go to the 1/4" jack. It goes to the XLR jack only.

If you happen to have an unbalanced mic, but it is wired for use with an XLR connector, then pins 1 and 3 are most likely soldered together within the connector. This will short out the phantom power if turned on.

(I'm no expert, but this is how I understand it.)
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Dave Potter

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Re: A quandry from the Mackie manual.
« Reply #2 on: April 16, 2011, 02:02:19 pm »

Well that certainly fits Gordon. I took "single ended" to mean a 1/4" TS jack.  I can't think of any modern mics that would be unbalanced AND XLR, but I suppose they need to cover the basses.

Cheers.

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John Roberts {JR}

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Re: A quandry from the Mackie manual.
« Reply #3 on: April 16, 2011, 04:36:47 pm »

The problem is not shorting out the phantom power, which is (or should be) designed to tolerate that. The problem is connecting an unbalanced mic to phantom power, which means the phantom supply current will flow through the mic element to ground. This is not good for the mic.

That same mic connected to phantom power with balanced wiring, will not draw any phantom current since both ends of the mic element are sitting up at phantom voltage.

JR   
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Dave Potter

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Re: A quandry from the Mackie manual.
« Reply #4 on: April 17, 2011, 05:59:06 am »

Yes Joun.  I understand the science.  I just can't conceive the the circumstances where an unbalanced mic could become connected to an XLR, without considerable home build effort. 
Having said that, my day job does involve the phrase "industrial stupidity". The warning is a good call.
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Ian Stuart

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Re: A quandry from the Mackie manual.
« Reply #5 on: April 17, 2011, 09:27:32 am »

Hi Dave,

In case you're wondering, When you engage phantom power on a device, it simply generates the 48v positive signal and puts it on pin 1 of the XLR. From a circuitry perspective you can easily ensure that this won't travel down any TRS connections since it's either a different jack plug or in the case of a TRS-XLR combo jack, on separate connecting lugs.

Phantom power is fairly fool proof, apart from the aforementioned gotchas I can only really think of T-powered microphones being something you'll really need to watch for, but I'm sure there are a few others.

Hope this answers your question.
Ian
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Tim McCulloch

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Re: A quandry from the Mackie manual.
« Reply #6 on: April 17, 2011, 10:56:50 am »

Yes Joun.  I understand the science.  I just can't conceive the the circumstances where an unbalanced mic could become connected to an XLR, without considerable home build effort. 
Having said that, my day job does involve the phrase "industrial stupidity". The warning is a good call.

This might be from "back in the day..." but using XLR connectors for unbalanced signals was just as common as using it for balanced.  Ribbon mics, like the old RCA 77, were unbalanced and one could easily "blow up" the mic by applying phantom power.

There are still lots of unbalanced mics out there, but the ones we need to be concerned with are the vintage dynamics and ribbons that will be killed by +48v., and they are the ones most likely to be wired with XLR connectors.  Cheap current dynamic mics typically have 1/4" connectors (and most owners won't cry like an RCA owner will, right before he hands the PA company the restoration bill, if the connector was replaced with an XLR).

This is the same thing that comes up when building cables for MP3 players and similar devices... wire the sleeve of the 3.5mm TRS to pin 3 and the tip or ring to pin 2.  Leave pin 1 unconnected.  That way a mixer with global phantom or some pilot error won't destroy it.

Have fun, good luck.

Tim Mc
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John Roberts {JR}

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Re: A quandry from the Mackie manual.
« Reply #7 on: April 17, 2011, 11:59:03 am »

Hi Dave,

In case you're wondering, When you engage phantom power on a device, it simply generates the 48v positive signal and puts it on pin 1 of the XLR. From a circuitry perspective you can easily ensure that this won't travel down any TRS connections since it's either a different jack plug or in the case of a TRS-XLR combo jack, on separate connecting lugs.

Phantom power is fairly fool proof, apart from the aforementioned gotchas I can only really think of T-powered microphones being something you'll really need to watch for, but I'm sure there are a few others.

Hope this answers your question.
Ian

Whether you are wondering or not, Phantom power is NOT applied to pin 1. +48V phantom supply voltage is applied to both pins 2 and 3 through separate 6.81k resistors. Pin 1 is connected to ground to return the phantom power current back from the mic circuitry to the console power supply.

JR
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kristianjohnsen

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Re: A quandry from the Mackie manual.
« Reply #8 on: April 17, 2011, 12:36:10 pm »

Well that certainly fits Gordon. I took "single ended" to mean a 1/4" TS jack.  I can't think of any modern mics that would be unbalanced AND XLR, but I suppose they need to cover the basses.

Cheers.

A client bought an Alto package with a mixer, two active speakers and three dynamic mics.  The mics would ruin the channel they were connected to every time the global phantom power was switched on...  Before long the mixer had very few working channels.  Great package!
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Dave Potter

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Re: A quandry from the Mackie manual.
« Reply #9 on: April 17, 2011, 05:20:53 pm »

Whether you are wondering or not, Phantom power is NOT applied to pin 1. +48V phantom supply voltage is applied to both pins 2 and 3 through separate 6.81k resistors. Pin 1 is connected to ground to return the phantom power current back from the mic circuitry to the console power supply.

JR
Well that just about wraps it up for industrial stupidity.  It happens.  Since the early 80s I've had an ammo box for mics with the correct wiring for TRS and XLR - Just in case!
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Re: A quandry from the Mackie manual.
« Reply #9 on: April 17, 2011, 05:20:53 pm »


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