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Re: [Amps] RF on audio using 374A

Subject: Re: [Amps] RF on audio using 374A
From: Jim Brown <>
Date: Thu, 15 Sep 2011 10:36:57 -0700
List-post: <">>
On 9/14/2011 9:39 AM, Jim Barber wrote:
> Sure, but assuming the balanced input of the Flex/mic/cable combo 
> doesn't have formal a pin one problem to start with,

That's a VERY optimistic assumption. :)  Virtually all computer gear, 
ham gear, and consumer audio gear is built with Pin One Problems. Before 
Neil Muncy published his work, so did almost all pro gear, including 
broadcast gear. Now, I'd guess that less than half of  current pro audio 
products have Pin One Problems, but it's taken 16 years to get here.

> then shouldn't it be more resistant to common-mode RF than the 
> unbalanced front-panel connector? I'm just looking to locate where the 
> RF is entering the system. 

Assuming that the balanced input is balanced at RF, yes.  That is also 
an optimistic assumption.  Bill Whitlock, a superb audio and RF engineer 
who is also a Fellow of the AES, showed in 1994 that a balanced 
interface can be analyzed as a classic Wheatstone Bridge. He showed that 
the noise immunity of a balanced interface depends ENTIRELY upon the 
balance of the impedances in the circuit at the frequencies of interest. 
That includes the balance of the output stage, the balance of the cable, 
and the balance of the input stage. It's one thing to make a stage 
balanced at audio, but a far more difficult thing to also make it 
balanced at RF. You can find his work at the Jensen Transformers website 
-- he is the owner and chief engineer.  His paper on this was published 
in the June 1995 issue of the Journal of the AES, which is available at 
most engineering libraries, and which can be ordered from the AES for 
about $10. It can also be downloaded (for a small charge) from the AES 

73, Jim Brown K9YC
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