ibvee

Phantom stopper for protection of unbalanced microphones
« on: March 15, 2011, 08:38:03 AM »
Hello all,

I'm an assistant at a very nice studio, which recently bought some vintage unbalanced microphones. This studio has some outboard preamps in the control room, and 2 Digi-pre's which are in the machine-room.

The Digi-pre's can be default-patched to some of the microphone inputs in the liveroom. These units remember their settings of the last protools session, and maintain those settings until changed in protools, or on the front of the unit. This is also the case with the +48V.
This means that a (visiting) engineer, unaware of this 'feature', could patch one of our unbalanced microphones, or vintage ribbons, directly into a phantom powered input.

In order to prevent any damage to the microphones, I thought of the following: We rewire all the vintage ribbon mics, and the unbalanced mics, with a 4-pole XLR, so it's impossible for anybody to plug them into the normal microphone inputs. We make little converter boxes, from 4-pole XLR to standard 3-pole XLR. Inside these boxes, we make this:

Mic sig (or +)   ---  ||+ 100uF/63V   --- --- XLR +
                                                      |
                                                      |
                                                  1k Ohm
                                                      |
                                                      |
                                                    GND


Mic GND (or -)   ---  ||+ 100uF/63V   --- --- XLR -
                                                       |
                                                       |
                                                   1k Ohm
                                                       |
                                                       |
                                                     GND

Will this be enough to protect the microphones? When +48V is applied to this circuit, the condenser won't be charged right away, so the first spike will pass through, could this spike be big enough to ruin a microphone?

Any thoughts about this would really help!

Regards


okgb

Re: Phantom stopper for protection of unbalanced microphones
« Reply #1 on: March 15, 2011, 12:25:26 PM »
this discussion may interest you

http://www.groupdiy.com/index.php?topic=41416.0
GKB Audio / Greg Boboski

ibvee

Re: Phantom stopper for protection of unbalanced microphones
« Reply #2 on: March 15, 2011, 01:19:10 PM »
this discussion may interest you

http://www.groupdiy.com/index.php?topic=41416.0
Thank you, I already found that thread. Useful information.
However, the studio has 2 vintage RCA44's, a vintage RCA 77DX, and a few balanced microphones. As I understand, the problem with phantom on ribbons is when they're plugged in while there's 48 volts on the other side. During the plugging some nasty spikes might end up on the ribbon. We don't want the risk.
And if I'm correct, phantom power on an unbalanced microphone, no matter if it's a dynamic or a ribbon, will be a big problem. Right?

I also found this schematic:
http://www.actsaudio.com/images/phantom/schematic.gif

Anybody here has any experience with this?

Thank you.

MagnetoSound

Re: Phantom stopper for protection of unbalanced microphones
« Reply #3 on: March 15, 2011, 01:37:09 PM »

Your diagram is not too clear, but you want the 1k resistors on the MIC (negative) side of the caps, if their purpose is to prevent the negative plates from charging to any significant voltage.
Dan

I don't think people realize what an embarrassment of riches this place is   -  Paul Gold

ruffrecords

Re: Phantom stopper for protection of unbalanced microphones
« Reply #4 on: March 15, 2011, 01:59:42 PM »
However, the studio has 2 vintage RCA44's, a vintage RCA 77DX, and a few balanced microphones.

To the best of my knowledge both the RCA 44 and 77 are balanced. Furthermore, the ribbons are both connected via transformers. This means they can safely be connected to systems with phantom power.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

okgb

Re: Phantom stopper for protection of unbalanced microphones
« Reply #5 on: March 15, 2011, 03:05:36 PM »
The story i read was that certain kinds of transformers
[ center tapped secondary ? ] with a MIS-wired xlr
cable could voltage to the ribbon .
Please someone correct me
GKB Audio / Greg Boboski

zebra50

Re: Phantom stopper for protection of unbalanced microphones
« Reply #6 on: March 15, 2011, 03:19:02 PM »

To the best of my knowledge both the RCA 44 and 77 are balanced. Furthermore, the ribbons are both connected via transformers. This means they can safely be connected to systems with phantom power.

Cheers

Ian

Ian is absolutely right - for studio work they can and should be wired balanced, low-Z.

Sometimes you find the odd one that has been wired unbalanced, so they just need rewiring.
Ribbon microphone services
http://www.xaudia.com
Microphone blog

ibvee

Re: Phantom stopper for protection of unbalanced microphones
« Reply #7 on: March 15, 2011, 04:21:38 PM »
Thanks for the info on the RCA's, didn't know they're transformer balanced, that's convenient.

But we also have some old microphones which just have an unbalanced out, and can't be rewired. That's my biggest concern.

zebra50

Re: Phantom stopper for protection of unbalanced microphones
« Reply #8 on: March 15, 2011, 04:30:16 PM »
Hi IBVee

Can you say which vintage unbalanced mics you have that you're worried about?

There may be better solutions than adding capacitors, and we can help better if we know which mics you are worried about.

Many unbalanced mics are high impedance, so you probably will get better performance from those by connecting to an instrument level input, or by using a matching transformer. In either case you'll get a better result than just adding a couple of caps.

Cheers!

Stewart
« Last Edit: March 16, 2011, 03:48:58 AM by zebra50 »
Ribbon microphone services
http://www.xaudia.com
Microphone blog

ruffrecords

Re: Phantom stopper for protection of unbalanced microphones
« Reply #9 on: March 15, 2011, 07:38:25 PM »
Thanks for the info on the RCA's, didn't know they're transformer balanced, that's convenient.

But we also have some old microphones which just have an unbalanced out, and can't be rewired. That's my biggest concern.

Then if possible you should re-wire them balanced. Can you say what make/models they are?

Cheers

ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'


ibvee

Re: Phantom stopper for protection of unbalanced microphones
« Reply #10 on: March 16, 2011, 05:26:29 AM »
Some of them are quite obsolete. For a few I'm almost certain they can't be rewired, the others I'm not that sure. I'll be in the studio this afternoon, so will check and post a list of the microphones.

Some of them have this old switchcraft (?) connector:
http://angela.com/images/view.aspx?productId=1056

Can't really see how to rewire these to balanced without messing to much with an old and original microphone. But if it's meant for a high impedance input, it's easiest to just use these with a DI right?

ruffrecords

Re: Phantom stopper for protection of unbalanced microphones
« Reply #11 on: March 16, 2011, 06:21:42 AM »
Can't really see how to rewire these to balanced without messing to much with an old and original microphone. But if it's meant for a high impedance input, it's easiest to just use these with a DI right?

Most DI boxes are designed for relatively high level inputs but your high impedance mics will still have a relatively low signal level so you will need to watch noise levels if you use a DI box.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

ibvee

Re: Phantom stopper for protection of unbalanced microphones
« Reply #12 on: March 16, 2011, 07:06:31 AM »
So I guess a few transformer boxes with the right transformers would be the way to go if they can't be rewired?

abbey road d enfer

Re: Phantom stopper for protection of unbalanced microphones
« Reply #13 on: March 16, 2011, 01:09:13 PM »
So I guess a few transformer boxes with the right transformers would be the way to go if they can't be rewired?
You would be adding a transformer to a transformer...woud work, but not very nice.
I'm talking about those high impedance dynamic mics.
I guess there are two nicer solutions.
One is to take off the transformer and access directly the low-Z capsule. You would need to make a breakout box or replace the connector. Now, if you want idiot-proof phantom protection, you need those caps.
Another solution is to have dedicated hi-Z preamps. These could be DIY'ed or derived from an existing product.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: Phantom stopper for protection of unbalanced microphones
« Reply #14 on: March 17, 2011, 01:10:59 PM »
Hi ibvee,
A friend of mine had the same trouble with Shure Unidyne 580. This mike has unbalanced Hi-Z output (it has step-up transformer inside). So, he removed transformer and made balanced wiring of capsule.

Another solution is to use good DI Box.

ibvee

Re: Phantom stopper for protection of unbalanced microphones
« Reply #15 on: March 31, 2011, 04:31:07 PM »
Well, as all the ribbons appear to be balanced, and some (like the RCA44's and the 77DX) for sure even transformer balanced, I guess the problem only concerns some of the microphones with the switchcraft plug I posted before: http://angela.com/images/view.aspx?productId=1056 and some old microphones with a jack connector. I guess it's possible to use a good quality DI box for the ones with a high Z output, but what to do with the low Z ones? Or is it OK to assume that all those microphones are high Z?

It concerns some old Turner mics, a few Shure (a.o. an old unidyne 55), and a pretty cool Olson 169 stereo mic.
« Last Edit: March 31, 2011, 04:52:35 PM by ibvee »

zebra50

Re: Phantom stopper for protection of unbalanced microphones
« Reply #16 on: April 01, 2011, 04:54:49 AM »
The Olsen looks to be high-Z (50K). There is some info here, including a schematic. You should grab the photo before they vanish.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&_trksid=p4340.l2557&item=270725567476&nma=true&rt=nc&si=kNNlpnZ9lYEbjv%252BY4m7lOxpWvTM%253D#ht_2386wt_998
Ribbon microphone services
http://www.xaudia.com
Microphone blog


 

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