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Author Topic: A quandry from the Mackie manual.  (Read 7776 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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Ian Stuart

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Re: A quandry from the Mackie manual.
« Reply #10 on: April 18, 2011, 08:16:33 am »

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

Who cares? Do I build consoles, preamps and/or microphones? No. The point that I wanted to make was that the 48v positive is applied to one pin which is most commonly separate from any TRS jacks. I feel that it makes little difference to OP anyway. Sorry if I offended with my "Industrial Stupidity"... Wankers.
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Mac Kerr

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Correcting wrong information
« Reply #11 on: April 18, 2011, 11:10:10 am »

Who cares? Do I build consoles, preamps and/or microphones? No. The point that I wanted to make was that the 48v positive is applied to one pin which is most commonly separate from any TRS jacks. I feel that it makes little difference to OP anyway. Sorry if I offended with my "Industrial Stupidity"... Wankers.

You got corrected because what you posted was completely wrong. In these forums the difference between correct and incorrect information actually matters. Sorry if that offends you.

Mac
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John Roberts {JR}

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Re: A quandry from the Mackie manual.
« Reply #12 on: April 18, 2011, 11:11:53 am »

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

Who cares? Do I build consoles, preamps and/or microphones? No. The point that I wanted to make was that the 48v positive is applied to one pin which is most commonly separate from any TRS jacks. I feel that it makes little difference to OP anyway. Sorry if I offended with my "Industrial Stupidity"... Wankers.

I for one, care about the accuracy of answers. Many people don't know and look to forums like this for information.

In this case how phantom power is connected actually matters, because it explains the unusual circumstance, that how the microphone is wired matters in the context of phantom supply.

Phantom power is applied common mode, to both pins 2 and 3. As long as the microphone is wired balanced, the voltage is the same at pins 2 and 3, so no current flows inside the dynamic mic. If one leg of the mic is grounded, the phantom current flows through the mic.

JR

PS I have no explanation for the Alto unit that self destructs one input at a time when global phantom is applied to a dynamic mic. Clearly a faulty design. 
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Dave Potter

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Re: A quandry from the Mackie manual.
« Reply #13 on: April 18, 2011, 01:44:30 pm »

Quote
PS I have no explanation for the Alto unit that self destructs one input at a time when global phantom is applied to a dynamic mic. Clearly a faulty design.

Or a faulty mic transformer and whimpy resistors on the phantom channels.

Anyway;  Enough is enough.  Lets walk away from the arguments and get on with important things........ Will phantom power run my toothbrush?....... ;)
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Greg_Cameron

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Re: A quandry from the Mackie manual.
« Reply #14 on: April 18, 2011, 02:28:40 pm »

I can't think of any modern mics that would be unbalanced AND XLR, but I suppose they need to cover the basses.

Since the world is full of adapters of pretty much every conceivable configuration combined with operators that know little to nothing about audio or electronics, someone is bound to do it wrong. And this is one case where it's not tough to conceive. Let's say you have one of these Rat Shack beauties here:



And let say you just bought yourself one of them fancy Midas XL88s because you have a new hobby called "PA" for doing back yard parties and such and all ready have this cool top notch mic you got from Rat Shack 30 years ago and you want to do some karaoke. So you get yourself one of these from the local Bango Center:



Then you go ahead and flip on the phantom power for that channel 'cause "if it's 'phantom power,' that will make things more powerful."

And then there's a problem.

Greg

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John Roberts {JR}

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Re: A quandry from the Mackie manual.
« Reply #15 on: April 18, 2011, 04:39:20 pm »

Quote
PS I have no explanation for the Alto unit that self destructs one input at a time when global phantom is applied to a dynamic mic. Clearly a faulty design.

Or a faulty mic transformer and whimpy resistors on the phantom channels.

Anyway;  Enough is enough.  Lets walk away from the arguments and get on with important things........ Will phantom power run my toothbrush?....... ;)

I didn't know this was an argument.

I guess it is possible to find resistors small enough that they are technically overloaded by 1/3W, but even if they burned up (they won't), that channel would probably still work with not phantom mics.

Not even sure how transformers could be bad that could break stuff.
========
yes, phantom power probably does provide enough current to run your electric toothbrush. If not enough to run it full time, surely enough to charge it up.
-------

Thinking about this a little more, I have a theory and a question for Kristian. Were those mics with one input leg shorted, plugged in "after" global phantom power was turned on. That could make a big difference.

If yes, that shorted input to ground, was driving the phantom DC blocking cap that was sitting fully charged up to 48V, down to ground in an instant. When that happened, the other end of that cap that was sitting at roughly 0V, was instantly driven to -48V. Since the mic input circuitry can't go to -48V when operated from only a -15V supply, something has to give.

Good design practice is to provide clamp circuitry to absorb such transient currents that could amount to several amps (depending on blocking cap size and ESR). These momentary shorts are more common in large recording studios with patch bays, so professional recording inputs are well protected, I suspect the Alto had inadequate protection for this user fault, if any protection at all.

BTW, it always good practice to "not" hot plug mics in with phantom power on, for this very reason.

Maybe now I'll walk away...

JR 
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Ian Stuart

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Re: Correcting wrong information
« Reply #16 on: April 18, 2011, 11:19:44 pm »

You got corrected because what you posted was completely wrong. In these forums the difference between correct and incorrect information actually matters. Sorry if that offends you.

Mac

It doesn't offend me at all Mac, what offends me is the presentation of said correction. There is a tactful way of correcting someone and labeling my post 'stupidity' is not a tactful way of doing that.

I appreciate that my knowledge has been challenged and I learnt something from this today, I also take back what I said about not caring. However I don't deserve to be called stupid because I accidentally slipped the wrong information whilst trying to be helpful to the OP.
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Greg_Cameron

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Re: Correcting wrong information
« Reply #17 on: April 18, 2011, 11:43:55 pm »

However I don't deserve to be called stupid because I accidentally slipped the wrong information whilst trying to be helpful to the OP.

I read through the thread. I didn't see anyone call your post stupid, just incorrect.

Greg

[edit: corrected context]
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Adam Wh3tham

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Re: Correcting wrong information
« Reply #18 on: April 19, 2011, 12:15:54 am »

However I don't deserve to be called stupid because I accidentally slipped the wrong information whilst trying to be helpful to the OP.

I read through the thread. I didn't see anyone call your post stupid, just incorrect.

Greg

[edit: corrected context]

+1 didn't see a stupid from JR or anyone. Tuff love. Get used to it.
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kristianjohnsen

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Re: A quandry from the Mackie manual.
« Reply #19 on: April 19, 2011, 09:12:32 am »

Quote
PS I have no explanation for the Alto unit that self destructs one input at a time when global phantom is applied to a dynamic mic. Clearly a faulty design.

Or a faulty mic transformer and whimpy resistors on the phantom channels.

Anyway;  Enough is enough.  Lets walk away from the arguments and get on with important things........ Will phantom power run my toothbrush?....... ;)

I didn't know this was an argument.

I guess it is possible to find resistors small enough that they are technically overloaded by 1/3W, but even if they burned up (they won't), that channel would probably still work with not phantom mics.

Not even sure how transformers could be bad that could break stuff.
========
yes, phantom power probably does provide enough current to run your electric toothbrush. If not enough to run it full time, surely enough to charge it up.
-------

Thinking about this a little more, I have a theory and a question for Kristian. Were those mics with one input leg shorted, plugged in "after" global phantom power was turned on. That could make a big difference.

If yes, that shorted input to ground, was driving the phantom DC blocking cap that was sitting fully charged up to 48V, down to ground in an instant. When that happened, the other end of that cap that was sitting at roughly 0V, was instantly driven to -48V. Since the mic input circuitry can't go to -48V when operated from only a -15V supply, something has to give.

Good design practice is to provide clamp circuitry to absorb such transient currents that could amount to several amps (depending on blocking cap size and ESR). These momentary shorts are more common in large recording studios with patch bays, so professional recording inputs are well protected, I suspect the Alto had inadequate protection for this user fault, if any protection at all.

BTW, it always good practice to "not" hot plug mics in with phantom power on, for this very reason.

Maybe now I'll walk away...

JR

Unfortunately, I have a hard time recalling the details on this one - but if I had to guess I would say that the mics were plugged in while phantom power was switched on.

Thanks for your insight! :)
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John Roberts {JR}

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Re: Correcting wrong information
« Reply #20 on: April 19, 2011, 09:53:17 am »

You got corrected because what you posted was completely wrong. In these forums the difference between correct and incorrect information actually matters. Sorry if that offends you.

Mac

It doesn't offend me at all Mac, what offends me is the presentation of said correction. There is a tactful way of correcting someone and labeling my post 'stupidity' is not a tactful way of doing that.

I appreciate that my knowledge has been challenged and I learnt something from this today, I also take back what I said about not caring. However I don't deserve to be called stupid because I accidentally slipped the wrong information whilst trying to be helpful to the OP.

If that is your idea of knowledge, I guess I was challenging it. I'm sorry if I appear harsh to you, I was not trying to hurt your feelings, but more concerned about informing the larger audience. If misinformation is not corrected promptly, it becomes somebody else's incorrect knowledge. 

I'm glad your learned something, I did too. Apparently Alto mixers are so poorly engineered that their own mics kill them. 

JR


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John Sabine

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Re: A quandry from the Mackie manual.
« Reply #21 on: April 19, 2011, 11:55:09 am »

I've actually gotten some inexpensive microphones in that had unbalanced XLR cables shipped with them so yes, they do exist.




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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Dave Potter

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Re: A quandry from the Mackie manual.
« Reply #22 on: April 21, 2011, 03:25:25 am »

The word "stupidity" was mentioned only by me, and the context was very clear:- 
One does not need to BE fundamentally stupid to DO a stupid thing.  I had already stated that I work in an environment of the creativity stupid - despite the best attempts on designers.  We all do the stupid things.  Its our right as human beings.  The incorrect wiring post only confirms this.
The only questions that remain are: "Is any one dumb enough to blow up a mixer, smart enough to understand the warning?"  and "What can designers possibly do about it?"
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John Roberts {JR}

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Re: A quandry from the Mackie manual.
« Reply #23 on: April 21, 2011, 10:41:49 am »

The word "stupidity" was mentioned only by me, and the context was very clear:- 
One does not need to BE fundamentally stupid to DO a stupid thing.  I had already stated that I work in an environment of the creativity stupid - despite the best attempts on designers.  We all do the stupid things.  Its our right as human beings.  The incorrect wiring post only confirms this.
The only questions that remain are: "Is any one dumb enough to blow up a mixer, smart enough to understand the warning?"  and "What can designers possibly do about it?"

The is a well understood failure mechanism, and explored in new and not so new (but not very old) AES preprints.


References
1)
Bortoni, Rosalfonso and Kirkwood, Wayne, “The 48V Phantom Menace Returns,” Audio Engineering Society preprint from the 127th AES Convention, October 2009; www.aes.org.
2)
Hebert, Gary and Thomas, Frank, “The 48 Volt Phantom Menace,"  Audio Engineering  Society preprint from the 110th AES Convention, May,  2001; www.aes.org.

As I mentioned the failure mode is managed for in professional recording consoles where because of patch bay use the inadvertent input short circuit is far more common.  The lower frequency of this fault, or perhaps inexperience of Alto's designer, means that mixer was unprepared for the stress.. That killer mic would not break every console, but make a few uncomfortable...

I don't consider an operator hot plugging a mic into phantom as stupid (perhaps a little inexperienced), as this is a rather subtle design quirk of phantom power. I do consider it very stupid for a manufacturer to make such a mic with one input shorted as this is a disaster just waiting to happen.

While Kristian didn't say, I hope the mics were not also made by Alto, that would be inexcusable, and they would surely have figured it out their mistake pretty quickly. I suspect some music dealer or distributer put the killer mic together with a weak mixer, in a "how cheap of a system can we assemble" exercise.
 

JR

PS: A common newby design mistake is to make the input phantom caps rather large, thinking bigger is better, but the transient fault current increases linearly with cap value, so the protection needs to be scaled up along with cap values, or smoke can get out. 
 
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Re: A quandry from the Mackie manual.
« Reply #23 on: April 21, 2011, 10:41:49 am »


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