[TenTec] Audio Hum on Omni-VII

[Robert Close]

Jim, no mention of differential amplifiers? No mention of the difference
between balanced and unbalanced methods of signal distribution?  "600ohm"
audio is still very much used, albeit under different names. The 600 ohm is
a somewhat arbitrary number, and when devices were inefficient it was more
efficient to match the impedance for maximum transfer of energy without
introducing phase distortions.    Your "solution" works , in that it reduces
the problem to inaudible levels, which is probably "good enough", but it
doesn't address the root cause of un-balancing a balanced feed by using a
"coaxial" connecting wire. (one conductor in a shield), and how the common
mode problem rears its ugly head, as well as the ground-loop problems.
Isolation of grounds is always effective when we are talking about
ground-loop issues cause by unbalanced connections.  Stick in a transformer,
balance the audio feed, make the ground independent of the signal, and poof,
"no problem".   Nice website you have, but since you opened the door to
discussion of audio, be a bit more specific in your statements, or nuts like
me are going to send you email.  I am not as cool as you, since I don't
"consult" in the audio field,  I just design and "do" for the last 25 years.
One thing I do know, is once you add RF into the equation a lot of the audio
"solutions" fall apart if the root cause of the problem is not addressed.
73










, 

-----Original Message-----
From: tentec-bounces at contesting.com [mailto:tentec-bounces at contesting.com]
On Behalf Of Jim Brown K9YC
Sent: Thursday, September 18, 2008 11:56 AM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] Audio Hum on Omni-VII

On Wed, 17 Sep 2008 22:32:00 -0500, Dudley Hurry wrote:

>You have a ground loop to the laptop..  I bet that you will lose the hum 
>if you break the ground from the radio to/from the laptop sound card..   
>Better isolation from the radio to the sound card.  

>You might even try an extra ground from the laptop to station ground,  
>but it is most likely a audio loop,  isolation transformers should take 
>care of it.. 

I'm an audio systems design consultant and consultant on EMC in audio 
systems (that is, hum, buzz, and RFI). There is much misunderstanding 
about the unbalanced interconnection of ham gear. Much of what is written 
about it is WRONG, including in some ARRL publications. You do NOT need 
"isolation," you need simple BONDING between the laptop and the radio. 

To see what's REALLY going on with your hum and buzz, and to fix your 
problems without a transformer and using nothing more than plain ordinary 
copper wire, see 

http://audiosystemsgroup.com/HamInterfacing.pdf 

and the Chapter on Solving Problems in the Shack in 

http://audiosystemsgroup.com/RFI-Ham.pdf

73,

Jim Brown K9YC
Audio Systems Group, Inc. 
Chair -- AES Technical Committee on EMC



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