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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

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Old 3rd December 2012, 10:39 PM   #31
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Join Date: Sep 2005
Hi

I want to do this the old-fashioned way, the Analog way. No tubes no digits, no nothing, just intrinsic solutions to the real problem.

I was thinking a lot about this long distance ~ 40 feet or 12meters of signal transport with about 100dB of signal to noise ratio. (A hundred thousand to one !!) Why do I need such huge signal to nose ratio? Maybe I don't, but that is my goal. At low level, normal listening level with NPR on FM with low volume, my wife's morning routine. The volume is set at about 3mW in the speakers. No hum should be audible with the ear to the speaker. Why? because the speaker is next to the breakfast table (tea and croissants). No hum or noise should be heard. When I really play loud at 30W into the speaker no hum or noise should be heard then either.

I assume 3mW to have about 55dB of dynamic range the range from 3mW to 30W is 40dB plus the previous 55dB equals 95dB. I need 100dB signal to noise ratio, period.

When I hook up about 12 meters of dual coax Audio (RCA) cables I can hear 60Hz related signals in the speaker at any volume, I expect that this noise should be reduced by a factor of about 40dB to be inaudible. I make the assumption that the hum and "noise" is due to the 1 Ohm resistive connection between my power amplifiers and the Audio processing center. I want to electrically increase this shield resistance by a factor of 100 to 100 Ohms at most probably 120Hz and above, because a 60Hz sine-wave is practically inaudible. The 1 Ohm resistance becomes a differential mode resistance, I can prove that.

That calls for a 100 Ohm reactance in the cable shields (100 times the resistance of 1 Ohm), reducing audible hum by a factor of 100. That calls for a 133mH inductance in the shielded Stereo Audio cables. I can achieve this by winding some shielded Audio cables on a tape-wound magnetic toroid. This toroid can not be a ferrite toroid because ferrites are difficult to get with more than 10uH per turns squared. This calls for 120 turns, which is impractical. This must be done using a High mu iron core with about 3.5uH per turn squared. That is practical because only about 20 turns of the dual Audio cable should be required.

This is obviously close to a design document, but in desperation I reach out to learn if anyone ever has been able to translate the cryptic specs on toroids to the simple trerm of uH per turn squared. I can handle those specs, but those convoluted specs used by the magnetic guys leave me in the "dust", probably intentionally so that I have to deal with the "application engineer" who obviously knows more than I after my 60 years in the industry.

Frustrated

Hans J Weedon.

Last edited by HJWeedon; 3rd December 2012 at 10:49 PM.
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