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Author Topic: Use of Mixing Console quiz - Need incorrect answers  (Read 2799 times)

thedisillusion

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Use of Mixing Console quiz - Need incorrect answers
« on: April 06, 2013, 06:28:48 AM »

Hi All

I am currently making a quiz for use of a mixing console and am having trouble thinking up some incorrect answers for the following question

12. Why do we need common mode rejection?

If anyone has any suggestions they would be greatly appreciated :)

Many thanks

Ed
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Tim Halligan

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Re: Use of Mixing Console quiz - Need incorrect answers
« Reply #1 on: April 06, 2013, 11:24:53 AM »

Mixers aren't common...they're uncommon.

Cheers,
Tim
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Rick Sutton

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Re: Use of Mixing Console quiz - Need incorrect answers
« Reply #2 on: April 06, 2013, 11:40:13 AM »

 Because the hi hat doesn't get along with the ride cymbal. They used to be married.
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Jim Williams

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Re: Use of Mixing Console quiz - Need incorrect answers
« Reply #3 on: April 06, 2013, 12:36:09 PM »

Audio signals are waveforms. They alternate up and down, hence alternating current. When one side is up, the other side is down.

Common mode noise is just that, it's not a alternating waveform but continuous noise interference. This is why it resides on both sides at the same time period. It's like the white water between waves on the beach.

CMRR or common mode rejection ratio is the ability of a balanced electronic circuit to cancel out that noise while allowing the audio waveforms to pass. It is measured in decibels. Most opamp based line input stages offer about a -40 db CMRR spec. Match up the inputs and source resistance and you can up that 20 more db's. Match up the current gain of the input transistors and you can up that another 20 db's.

In mic preamps this is an important spec if you want to avoid picking up dimmer noise, rf and other noise sources. With a feeble signal, that noise is more noticable. Great designs do better than -90 db CMRR 20~20k hz. With the best input transformers CMRR is frequency dependent. Most are good at below 100 hz but fall off above that. The best Jensens only do -70 db CMRR at 20k hz.

It is also an important spec in mixer sum circuits. Some offer balanced bussing with -80 db CMRR when single ended sum designs (most Neve, Trident, MCI's etc.) offer no CMRR reduction on the mix buss.

You can test this with a cell phone. Stick it up on top of the mic preamps, hear anything? On my console with balanced bussing I can lay the cell phone on the master strip with no interference. Do your mic preamps or console do that?
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Jim Williams

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Re: Use of Mixing Console quiz - Need incorrect answers
« Reply #4 on: April 07, 2013, 12:59:32 PM »

After reading this I may have caused more confusion. The secret sauce of CMRR is phase cancellation. "Normal mode" audio signals swing up and down. Noise tends to be common mode, injecting signals on both inputs at the same time period. With phase cancellation those common mode noise sources are simply cancelled out, sort of like what you do in software to find artifacts.
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John Roberts {JR}

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Re: Use of Mixing Console quiz - Need incorrect answers
« Reply #5 on: April 12, 2013, 03:18:48 PM »

Hi All

I am currently making a quiz for use of a mixing console and am having trouble thinking up some incorrect answers for the following question

12. Why do we need common mode rejection?

If anyone has any suggestions they would be greatly appreciated :)

Many thanks

Ed
A. "To reject common sounding notes."

A. "So only uncommon mode sounds get reproduced."

A. "So only the best sound gets through".

If you want wrong answers, just ask your students.

JR
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