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Wiring unbalanced to XLR being aware of Phantom Analog Processors (HW)
Old 13th April 2018
  #1
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Wiring unbalanced to XLR being aware of Phantom

I have a mic which is unbalanced and I would like to wire it to XLR.

Usually I would have Ground on Pin 1 and the signal on both pin 2 and 3.

But what happen if I use it with a mixer with phantom power on? would this damage the mixer? Is there a better way to wire unbalanced to balanced?

I know I could use a DI box but this is not an option

thank you
Old 14th April 2018
  #2
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Actually there is a mistake in my explanation: usually you would wire ground (pin1) and cold (pin3) together and leave the signal to hot (pin2)
Still is this way safe for phantom power and mic? Or better leave the cold (pin3) unconnected?

Thank you
Old 14th April 2018
  #3
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Leaving pin 3 unconnected may stop signal from flowing at all (if your mic input uses a transformer), and it won't stop phantom power from trying to flow through the unbalanced mic.

BTW, applying phantom power across an unbalanced mic is more likely to cause damage to it than the mic pre. Once again, if the mic pre uses a transformer, you run the risk of magnetizing it, which will increase distortion, but that is fortunately only a temporary effect. OTOH, if the mic diaphragm is connected directly to the output (no transformer in the mic), you might destroy the very tiny wires in the voice coil. It would also certainly cause distortion.

You might get away with connecting the mic between pins 2 (hot) and 3 (shield). That leaves the mic without a ground reference which might be noisy, so I would connect pin 3 to 1 with a capacitor. Probably 10 uF @ 63V would be sufficient, with the positive end to pin 3.

A DI is still the best answer, but you ruled that out to start.

Geoff
Old 14th April 2018
  #4
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Richard Crowley's Avatar
 

Assuming that "XLR" implies a typical balanced/differential input, you MUST connect the signal between pin 2 and pin 3.
And assuming a low-level signal (as from a microphone) you really need the shield (pin 1) to protect the signal from hum/noise, etc.
So a typical unbalanced signal would connect the shield to pin 1 (ground) and pin 3 ("cold"). And the signal ("hot") to pin 2.

However, you (correctly) add that you should accommodate phantom power if present at the microphone input.
You did not mention whether your microphone is dynamic or condenser.
If it is condenser, then you are probably on the wrong path here and will likely toast it into a useless lump of coal.
If it is dynamic, the traditional way to do this is to use a transformer.

However, you could use capacitors to block the DC phantom voltage from getting to the microphone.
You would need a DC blocking capacitor INSTEAD of a hard connection between pin 3 and ground/pin 1
And another DC blocking capacitor between the microphone signal and pin 2.

Perhaps a more suitable method would be to use an active circuit inside the microphone as used in most condenser mics.
That would provide a balanced/differential output from the mic which would be much more suitable to connect to an XLR input.
Old 17th April 2018
  #5
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Quote:
Originally Posted by Richard Crowley View Post
Assuming that "XLR" implies a typical balanced/differential input, you MUST connect the signal between pin 2 and pin 3.
And assuming a low-level signal (as from a microphone) you really need the shield (pin 1) to protect the signal from hum/noise, etc.
So a typical unbalanced signal would connect the shield to pin 1 (ground) and pin 3 ("cold"). And the signal ("hot") to pin 2.

However, you (correctly) add that you should accommodate phantom power if present at the microphone input.
You did not mention whether your microphone is dynamic or condenser.
If it is condenser, then you are probably on the wrong path here and will likely toast it into a useless lump of coal.
If it is dynamic, the traditional way to do this is to use a transformer.

However, you could use capacitors to block the DC phantom voltage from getting to the microphone.
You would need a DC blocking capacitor INSTEAD of a hard connection between pin 3 and ground/pin 1
And another DC blocking capacitor between the microphone signal and pin 2.

Perhaps a more suitable method would be to use an active circuit inside the microphone as used in most condenser mics.
That would provide a balanced/differential output from the mic which would be much more suitable to connect to an XLR input.
well it's not dynamic or condenser but it is a crystal (piezo) contact type - It eventually could go as well with a ribbon though (like a reslo which had an unbalanced jack out)
Old 17th April 2018
  #6
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Richard Crowley's Avatar
 

Piezo needs protection from phantom voltage also. Is there some compelling reason that you must use unbalanced cabling? Why not connect the transducer (whatever kind it is) as balanced and run conventional balanced cabling and XLR connection? You haven't revealed the big picture here, so we don't know what other options might be available.

Since you are asking about phantom power and XLR, we assume that you want to connect this to an ordinary mic input (vs. an unbalanced instrument input).
Old 17th April 2018
  #7
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how would I balance the piezo which has only 2 cables? the big picture is connecting a piezo to an XLR without damaging the preamp if by any chance has the phantom power on (damaging a piezo is not a big issue as it's cheap)

Quote:
Originally Posted by Richard Crowley View Post
Piezo needs protection from phantom voltage also. Is there some compelling reason that you must use unbalanced cabling? Why not connect the transducer (whatever kind it is) as balanced and run conventional balanced cabling and XLR connection? You haven't revealed the big picture here, so we don't know what other options might be available.

Since you are asking about phantom power and XLR, we assume that you want to connect this to an ordinary mic input (vs. an unbalanced instrument input).
Old 17th April 2018
  #8
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Richard Crowley's Avatar
 

I don't know what you mean by "piezo has only 2 cables"?
Are you talking about a pickup component with two terminals where you can attach wires?
Or are you talking about some kind of packaged item with a long, attached unbalanced cord?

If you have a piezo pickup with two terminals, then simply attach those two terminals to XLR pin 2 and pin 3.
And use a shielded twisted-pair cable with the shield connected to XLR pin 1.

Much depends on details not revealed in your questions.
Old 18th April 2018
  #9
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Thread Starter
a piezo disc has two outputs. The crystal and the metal which is ground and hot on a normal connection / ok ? ? ?

if I attach the signal to pin 2 and the ground (disc) to pin 3 would I have signal? also would it be safe for both the disc and the preamp is there is phantom power +48Vdc implemented?

[And use a shielded twisted-pair cable with the shield connected to XLR pin 1] What you mean? should I connect what to pin 1?



Quote:
Originally Posted by Richard Crowley View Post
I don't know what you mean by "piezo has only 2 cables"?
Are you talking about a pickup component with two terminals where you can attach wires?
Or are you talking about some kind of packaged item with a long, attached unbalanced cord?

If you have a piezo pickup with two terminals, then simply attach those two terminals to XLR pin 2 and pin 3.
And use a shielded twisted-pair cable with the shield connected to XLR pin 1.

Much depends on details not revealed in your questions.
Old 18th April 2018
  #10
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Richard Crowley's Avatar
 

A piezo element (typically disk shaped, but could be other shapes, also) has two terminals.
The signal is produced between these two terminals.
There is nothing about either terminal that requires you to connect it to ground.
You can simply connect the two piezo terminals to XLR pin 2 and pin 3
And use a shielded, twisted pair cable (like a microphone cable) to take your balanced signal to the XLR connector.
Pin 1 of the XLR connector connects to the shield of the cable.
And the shield/pin 1 doesn't connect to anything at the piezo end.
Now it may be advisable to connect the shield to something at the piezo end shield it, but we don't know enough about your application to offer any significant comment.

If your piezo disk was connected between pin 2 and pin 3 then it will be immune from any effect of the 48V phantom power. As long as you do NOT connect either terminal to XLR pin 1 ground. There is no phantom power BETWEEN pin 2 and pin 3.

Last edited by Richard Crowley; 18th April 2018 at 03:43 AM..
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