[BC] Re: Humming pre-amps

[RLO2L@aol.com]

In a message dated 4/7/2007 11:35:12 A.M. Central Daylight Time,  
webmaster at radiopower.org writes:

I just  took the time to re-calibrate all of our preamps as you suggested 
with  maximum pre gain and minimum output level and guess what, they all work  
reasonably well now! Even the telephone hybrids sound cleaner. I can't believe  
I have fought this problem for so long and it was such a simple fix.  The real 
issue here is that my studio is about half mile away from a  whole farm of 
radio transmission towers, we are at the highest point in the  city. This must 
magnify the problem, thus the need to tweak the gains  this odd way. I've been 
running the pre stage gain around 6 which is by no  means low and probably 
normal but I never thought to crank the gain to 10 and  put the send stage levels 
low. I'm 45 and humbled by your knowledge. How much  should I paypal for your 
invaluable albeit elementary advice?  


Shelby



Shelby:
 
1. I would not run the pre-amps at full tilt boogey levels, as you "could,"  
if you hit the inputs with enough level cause them to go into clip, and this 
is  NOT what you want. Use a good 100 Mhz. (min) 'scope and take a look at the  
output signals, to make sure you are not running the pre's into or near  clip.
2. Post this on the list so that everyone does not think you are trouble  
shooting an RFI problem but one of simple gain structure settings, at least  
initially. This is like peeling an onion. Each level reveals something which may  
be entirely different!
3. Some pre's have sufficient gain from DC to light to amplify everything  
(almost including sunlight), just kidding about the sunlight.
4. Avoid snake oil solutions to common and well documented problems.
5. take a look at _www.audiosystemsgroup.com_ 
(http://www.audiosystemsgroup.com)  and look  for Jim's white paper (downloadable) on _Power and  Grounding 
For Audio and Audio/Video Systems -- A White Paper for the Real  World_ 
(http://www.audiosystemsgroup.com/SurgeXPowerGround.pdf) 
6. Get a copy of Sound System Engineering (Third Edition) Chapter 3, Page  90 
and read 3.15 Gain Structure Revisited. Everyone doing any kind of sound  
system should have a copy of this book in your library!
 
These things are so important, I am going to post this to entire  list.
 
Regards,
 
Russel L.  O'Toole, P.C.
R&M Consulting

31 Ponderosa  Drive
Romeoville, Illinois 60446
815-372-1990 voice
630-699-2965  cell
815-886-9232 fax
KD5OAE General



************************************** See what's free at http://www.aol.com.