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Re: [TowerTalk] Balun Recommendation

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Subject: Re: [TowerTalk] Balun Recommendation
From: "Paul Christensen" <>
Date: Tue, 24 Apr 2012 08:10:42 -0400
List-post: <">>
> True servo balanced inputs, with no pin 1 problems usually have good CMMR
> long as the balanced lines  are short.

Jim, I think you mean servo-balanced *instrumentation* inputs where three 
single op-amp stages are used?  In a good instrumentation design, CMRR stays 
high regardless of the source Z and common-mode frequency.   The function of 
the "servo" (i.e., the 4th op-amp) does nothing for CMRR.  Rather, it is 
used to dynamically minimize DC offsets, allowing for DC inter-stage 

> Make em longer, and the CMRR
> specs will drop off.  In extreme cases, by using xfmrs on each end, the 
> will improve by a whole bunch.

See comment above, but I'm with you on the use of high quality input 
transformers (e.g., Jensen or CineMag).  To get CMRR as consistent as those 
transformers with an instrumentation input requires a design that maintains 
high CMRR not only across the audio spectrum, but well above it to minimize 
RFI/EMI common mode input noise.  You can be much sloppier with a 
transformer input design.

> Equipment makers  will try and avoid the use of
> xfmrs like the plague.    I can see why.  At $75.00  a pop, you will have 
> a lot of $$
> invested in  xfmrs.

I've only seen a handful of really good instrumentation input designs and 
they don't cost a lot more to design right -- and still well under the cost 
of a Jensen or CineMag transfomer.

> problem you can think of.   I think the icom 7800 has a fiber input on the 
> rear panel.

Loved the '7800/'7700 S/PDIF connections.  Wish everyone supplied those. 
For a time, much of the shack audio was converted to light, and switched 
with a Z-Systems router.  Ended up selling the '7800 and '7700 and the 
router to try other stuff.

Paul, W9AC


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