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Old 1st October 2008, 03:03 PM   #1
diyAudio Member
 
Join Date: Oct 2008
Default Passive phase inversion?

First, hi to everyone here as this is my first post.

I have searched the forum but all posts seems to relate the active circuits and crossovers which doesn't help me.

I have a Rode video mic for my video camera and it sends a preamped mono signal down both sides of a stereo audio cable with a mini jack end. It works ok in the camera but there are a few enthusiasts who would like to use it away from the camera with an extension cable.
Problem is, it's noisy as hell.
Anyway, I was thinking that seeing as there is identical signal on the tip and sleeve, is there a relatively easy circuit that could invert one side before the extension without the need for batteries?
We could then invert one side again digitally once the audio is captured to the computer, and combine the signals to cancel out the added noise like a balanced signal.

I have a leem passive DI that creates a balanced signal from an unbalanced signal and looks like a fairly simply design, but alas my knowledge of circuit design is very little.

Any hints?
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Old 2nd October 2008, 04:23 AM   #2
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I openned the box to the Leem and it's just a small transformer but I don't know what type I'd need or how to wire it?
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Old 2nd October 2008, 02:02 PM   #3
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http://sound.westhost.com/project35.htm

Perhaps the schematics there will help you understand.

It's really very simple - since the transformer means you have no electrical connection to ground, you can wire up the input side of the transformer normally, and then wire the output side "backwards", and you have a passive phase inverter.
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Old 2nd October 2008, 06:28 PM   #4
sreten is offline sreten  United Kingdom
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Location: Brighton UK
Hi,

You need to be specific about the actual problem. What is noisy ?

Balancing a cable only reduces interference from the cable, and
it has to be a very long cable for this to be of major annoyance.

Check that the input is a proper low noise microphone input.
I've fitted microphone transformers to noisy inputs in the past.

/sreten.
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Old 6th October 2008, 02:38 AM   #5
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Thanks for the replys guys. To be honest, the mic preamps in the camera a pretty noisy and when the mic is mounted on the camera it's definatley not 'noise free'. But when using using an extension cable to get the mic on a boom pole, the noise level increases dramatically and it's this added noise I want to eliminate. Thicker shielded cable makes a little difference but not much.

Seeing as those transformers are expensive and I don't use the D.I, I'm going to desolder it and wire it between the mic and the extension. I let you know how it goes.
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