6400 Balanced mic input?

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  • Updated 6 months ago
  • Answered
  • (Edited)
In testing the mic input, on the 6400 it was discovered that the ring connection was connected to case ground as well as the sleeve.

That being said, how can you call the input Pseudo-balanced?
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Bill -VA3WTB

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Posted 6 months ago

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Wayne

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fuzzy logic :-)
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KC2QMA_John

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I thought only the 6500,  6600 & 6700 have Balanced inputs?
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Tim - W4TME, Customer Experience Manager

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Tha is correct.  They have a truly balanced input.
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Wim

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It's like fake news, it's fake balanced ;-)
(Edited)
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Bill -VA3WTB

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Then in most cases a isolation transformer should be used?
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Wim

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If you don't have any RFI problems then I would not worry about it ...
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Bill -VA3WTB

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RFI is not the reason for this post as you can see.
It looks to be completely un-balanced.
This is an on going discussion on air, I am wondering about comments from those that have good insight to this.
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Steven Linley

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I have a Heil Pro micro IC ( Electret) the mic uses a 3.5mm mono plug. I made an adapter that allowed me to connect the tip to a TRS (Tip Ring Sleeve) 3.5 mm using sleeve and tip only.
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Bill -VA3WTB

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Flex says it is Pseudo-balanced but it does not seem so.
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Bill -VA3WTB

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Can Flex comment on this? How is it balanced? on the 6400
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W7NGA

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Probably means there is some minimal common-mode rejection. If it isn't spec'd it isn't much.
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Mike KE4U

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Hi Bill - Your tests verify the design.  The sleeve is grounded for the cable sheild termination and the mic- signal is grounded at the A/D converter. Here's the reference in the user guide:

5.3.19 Mic Connector

The MIC connector accepts a 1/8-inch (3.5mm) stereo (TRS) plug and provides a pseudo-balanced

microphone input. Pseudo-balanced means that the mic (+) and mic (-) lines are balanced all the

way to the input of the A/D converter, where the mic (-) line is DC grounded. .....

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Bill -VA3WTB

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Then the test show un balanced.
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Mike KE4U

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Right!  If the 6400 input circuitry was a true differential balanced input, the impedance of both of the microphones signal leads would be the same and you would not 'see' a resistance to ground on either lead as measured at the rear apron jack.

I am guessing that the reason  FLEX opted to connect the mic- signal to the cold side of the A/D's differential input was for these advantages:
1. That likely places a reasonable resistance between the unbalanced equipment ground and a balanced microphone
2. This appraoach could still reduce ground loops 
3. Thios approach accomodates both balanced and unbalanced microphones
4. This approach allows for the provisioning of phantom  / bias power for electret mic elements.

I never really knew about the term "pseudo-balanced" in the audio world. Because the impedances to ground at the source end of the hot and cold sides is different, the line isn't properly impedance balanced -- it's 'pseudo-balanced'.

Think I'll go have a cold one and try to get my head unwrapped from this concept ;-)

(Edited)
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Tim - W4TME, Customer Experience Manager

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Official Response
Technically pseudo-balanced input it is an unbalanced input into the codec, but it is wired as a balanced input from the connector to the input of the codec where it is converted to an unbalanced input (mic- and ground are bonded together).  This prevents multiple ground points in the input circuit when the transducer input (mic) is wired for a balanced input.  The advantage is that it makes the unbalanced input into the codec less susceptible to RFI.
(Edited)
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Bill -VA3WTB

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Tim can you help answer this...

One last question for Tim ...”looking at the 1/8 inch mic jack on the rear of the 6400...if metered with a DVM in resistance mode, should both the TIP and Ring be well above ground potential with nothing external connected to the jack?” 
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Tim - W4TME, Customer Experience Manager

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Ring will be at ground as the MIC- is grounded at the codec input.
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Terry AB2UE

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What i would like to know since the mic[-] is grounded, would a 13.8vdc power supply that has a grounded DC[-] output[my Astron does] cause a problem ?  
Do the Flex Radio service technicians use 'floating or grounded' DC[-] output power supplies ?