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Ribbon Mics and Phantom Power
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mstar
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PostPosted: Tue Dec 08, 2009 11:31 am    Post subject: Re: Ribbon Mics and Phantom Power Reply with quote

Nice info! thanks...
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cfMC
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PostPosted: Tue Jan 11, 2011 9:55 pm    Post subject: Re: Ribbon Mics and Phantom Power Reply with quote

ribbon mics

http://www.wired.com/rawfile/2011/01/gallery-ribbon-mics-part-1/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+wired%2Findex+%28Wired%3A+Index+3+%28Top+Stories+2%29%29&pid=568&viewall=true
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Derrick
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PostPosted: Mon Jan 31, 2011 5:25 pm    Post subject: Re: Ribbon Mics and Phantom Power Reply with quote

But I read in Mix Magazine where Wes Dooley said if DC through a winding magnetizes the core material of the output transformer and it becomes less linear. So are we really out of the water with the phantom used with ribbon mics issue?

Also, I was reading someones educated guess that: "DC can't get through a transformer's windings, but switching transients can. So by itself 48V of phantom power can't kill a ribbon mic but my guess is that when you hit the 48V phantom power switch this creates a huge 0 to 48V switching pulse which is transmitted through the isolation transformer right to the ribbon element. To make matters worse, chances are the switch is not denounced so the pulse wouldn't look like a perfect square wave but a damped oscillation which means the value could overshoot 48V until the transient waveform settles at a nominal DC value (48V). Say goodbye to that vintage ribbon mic if this happens!" What's the consensus on this?
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aitikin
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PostPosted: Mon Jan 31, 2011 8:00 pm    Post subject: Re: Ribbon Mics and Phantom Power Reply with quote

Derrick wrote:
Also, I was reading someones educated guess that: "DC can't get through a transformer's windings, but switching transients can. So by itself 48V of phantom power can't kill a ribbon mic but my guess is that when you hit the 48V phantom power switch this creates a huge 0 to 48V switching pulse which is transmitted through the isolation transformer right to the ribbon element. To make matters worse, chances are the switch is not denounced so the pulse wouldn't look like a perfect square wave but a damped oscillation which means the value could overshoot 48V until the transient waveform settles at a nominal DC value (48V). Say goodbye to that vintage ribbon mic if this happens!" What's the consensus on this?


That's an interesting idea, however what if you never switch the phantom power off?
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willyk
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PostPosted: Tue Feb 01, 2011 6:31 pm    Post subject: Re: Ribbon Mics and Phantom Power Reply with quote

Shiny Box. We love you.

Will from Burl Audio here, in Santa Cruz.

Had a brief chance to check out the shiny box ribbon mic.

Thanks for clearing this up! I needed that.

All the best.
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Darlington Pair
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PostPosted: Thu Feb 03, 2011 11:16 am    Post subject: Re: Ribbon Mics and Phantom Power Reply with quote

Derrick wrote:
But I read in Mix Magazine where Wes Dooley said if DC through a winding magnetizes the core material of the output transformer and it becomes less linear. So are we really out of the water with the phantom used with ribbon mics issue?

Also, I was reading someones educated guess that: "DC can't get through a transformer's windings, but switching transients can. So by itself 48V of phantom power can't kill a ribbon mic but my guess is that when you hit the 48V phantom power switch this creates a huge 0 to 48V switching pulse which is transmitted through the isolation transformer right to the ribbon element. To make matters worse, chances are the switch is not denounced so the pulse wouldn't look like a perfect square wave but a damped oscillation which means the value could overshoot 48V until the transient waveform settles at a nominal DC value (48V). Say goodbye to that vintage ribbon mic if this happens!" What's the consensus on this?


Well, I might agree with you on the switching transient if your phantom power isn't soft start because AC signals can pass through a transformer, a well built and regulated power supply should not have this problem, but we live in a world that things aren't always as well built as they should be.

But, are you saying that passing DC through a transformer ruins transformers? I just can't buy that, if that's the case we've all been ruining our transformer coupled condenser mics and active DI's for years. Any magnetization of the core is negligible at best.
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Derrick
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PostPosted: Thu Feb 03, 2011 1:47 pm    Post subject: Re: Ribbon Mics and Phantom Power Reply with quote

Again, it was Wes Dooley of AEA in a Mix Magazine article... not me saying that. He's pretty reputable though and the article wasn't out of line so just sayin.
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Darlington Pair
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PostPosted: Thu Feb 03, 2011 3:51 pm    Post subject: Re: Ribbon Mics and Phantom Power Reply with quote

Derrick wrote:
Again, it was Wes Dooley of AEA in a Mix Magazine article... not me saying that. He's pretty reputable though and the article wasn't out of line so just sayin.


Didn't mean to offend you. AEA mics are truly incredible and the man obviously knows what he's doing building them. But I think this might be a case of Mojo versus practical application. I'm just trying to point out that transformers are very commonly used as a DC block in lots of very high end equipment, interstage coupling, output transformers on tube amps rely on hundreds of volts of DC, and so on

I'm just trying to say that I don't buy the DC ruining a transformer mojo, obviously it's just safer to not take the chance.
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Derrick
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PostPosted: Thu Feb 03, 2011 4:56 pm    Post subject: Re: Ribbon Mics and Phantom Power Reply with quote

I understand. And while neither of these two were my direct comments, I wanted to offer them here as a point/counterpoint to the subject. I like a healthy dose of all sides being represented when a point is being stated as gospel Wink Without having researched much into either, I couldn't really argue one side or the other. I just assume not play around with sending 48v into a $2200 ribbon.
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Andy Peters
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PostPosted: Thu Feb 10, 2011 2:01 am    Post subject: Re: Ribbon Mics and Phantom Power Reply with quote

Derrick wrote:
But I read in Mix Magazine where Wes Dooley said if DC through a winding magnetizes the core material of the output transformer and it becomes less linear. So are we really out of the water with the phantom used with ribbon mics issue?


Arrrrgh .... with phantom power, the 48 V is applied to both pins 2 and 3. So if there is a transformer, with the mic element on the primary side and the secondary connected to the XLR, then that transformer secondary is connected to pins 2 and 3. And since pin 2 and pin 3 are at the same potential -- both at 48 V -- there is zero voltage drop across the secondary coil. This means that there's no DC current in the secondary, which means it's not possible to magnetize it or whatever.

Quote:
Also, I was reading someones educated guess that: "DC can't get through a transformer's windings, but switching transients can. So by itself 48V of phantom power can't kill a ribbon mic but my guess is that when you hit the 48V phantom power switch this creates a huge 0 to 48V switching pulse which is transmitted through the isolation transformer right to the ribbon element.


Realize that there is a 6.81k resistor between the phantom supply and pin 2, and another (hopefully identical value) 6.81k resistor between the supply and pin 3. The usual (and correct) way to switch phantom power is to use a single-pole switch between the 48 V supply and the "top" of those resistors. This means that when you throw the switch, at the same time both pins 2 and 3 see that 48 V, which again, there is no voltage difference between the pins, so there's no "pulse" that goes through anything.

Quote:
To make matters worse, chances are the switch is not denounced so the pulse wouldn't look like a perfect square wave but a damped oscillation which means the value could overshoot 48V until the transient waveform settles at a nominal DC value (48V). Say goodbye to that vintage ribbon mic if this happens!"[/i] What's the consensus on this?


See above.

-a
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ShinyBox
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PostPosted: Fri Feb 18, 2011 1:22 pm    Post subject: Re: Ribbon Mics and Phantom Power Reply with quote

Thanks Wil,

I've been obviously slacking and not following this thread Very Happy

So it's been a few years since I made the video, and there are a couple things I've come across that I should mention.

Firstly, there are some ribbon mics out there that have a grounded center tap configuration on the ribbon output transformer. This is mainly going to be older ribbons( RCA 77DX seems to have this when I look at the coutant.org site). Bob Ohlsson pointed out that this is potentially problematic, as they may or may not have been wired with the center tap grounded (the mics came unwired back in the day, so each studio wired their own mics).

As to DC magnetizing the core of the output transformer, it would depend on the core size, and the makeup of the transformer lamination(different materials have different permeability, and some materials are less prone to saturation), but theory aside, let's look at real world examples. How many consoles were made where phantom power is always on? Quite a few in my experience. Many of these consoles have been used for decades, with phantom on, plugging in ribbon microphones every day.

IMO, experience has played out that with a few odd exceptions, ribbon microphones will ignore the presence of phantom power and will perform just fine. Does this mean go hog wild with wanton disregard for the microphone? No =) Treat your mics like a valuable tool, with some care( like you treat any other valuable microphone). Turn phantom off if you can, patch before plugging into the microphone, and keep a cover on your mic when not in use. If you notice phantom power is on and are concerned about it, unplug the mic first, before you turn it off. It shouldn't hurt the microphone, but hey, your treating the mic with care, remember?

And when it's time, get your mic re-ribboned. I purposely keep my re-ribboning fees as low as possible, as I want people to not be afraid of using their microphones.

Hope this helps, and thanks for the dialogue on this.

Regards

Jon
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Derrick
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PostPosted: Fri Feb 18, 2011 4:57 pm    Post subject: Re: Ribbon Mics and Phantom Power Reply with quote

So those of us with RCA-77DX ribbons should check if the center tap is grounded and if so, disconnect ground there?
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ShinyBox
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PostPosted: Sat Feb 19, 2011 2:56 pm    Post subject: Re: Ribbon Mics and Phantom Power Reply with quote

I would say yes, it's worth checking out.

With the center tap grounded, you have path for current to flow, which is going to accentuate any potential issues a great deal.

Regards

Jon
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SafeandSoundMastering
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PostPosted: Mon Jul 30, 2012 9:26 am    Post subject: Re: Ribbon Mics and Phantom Power Reply with quote

As a general rule I have always erred on the side of caution and ensured I have not applied phantom to any ribbon mics. I only wish I had worked in rooms good enough to warrant figure of 8 patterns more. Good subject.

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Jim Williams
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PostPosted: Mon Jul 30, 2012 10:13 am    Post subject: Re: Ribbon Mics and Phantom Power Reply with quote

As Andy said, if the voltage is matched, no current flows. This is dependent on the matching of the 6.81k resistors. Any mis-match and current flows.

Many cheapo preamps don't do that, some don't use 1% parts either. If you want to be safe, match them yourself. I use a 4 digit meter and match them to one ohm. That also improves the mic preamp CMRR specs.

Do that and maybe a few millivolts of difference remain. Not enough to worry about.
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