Long Distance Interconnects - diyAudio
 Long Distance Interconnects
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 9th January 2004, 04:50 PM #1 Cham   diyAudio Member   Join Date: Jan 2004 Location: Houston, TX Long Distance Interconnects I have a situation where I have approximately 50 feet between my tube preamp and tube amplifiers. Bruce Rozenblit of Transcendant Sound tells me that even with his very low output impedance Grounded Grid Preamp, I will be rolled off 6 dB at 20 KHz by the time that I get to the amps. My questions are: Does anyone remember the formula for that rolloff? Is there a way around moving the amps to my already overcrowded equipment shelves? Do I need to equalize the cable runs (35' vs 50') to the amps to at least equalize the rollof between channels? I've thought of going to transformers to run balanced low impedance lines. We routinely run hundreds of feet of pro line level cable in radio. For critical home use however, I'd rather avoid iron and / or silicon in the audio path. Any help would be appreciated, especially that formula. Bob
 10th January 2004, 11:19 AM #2 nonik   diyAudio Member   Join Date: Jan 2004 Location: Rijckholt Long cable roll-off Hi In the drawing below, consider the generator on the left as the actual output of your pre-amp while the resistor next to it is the output impedance of the same. on the right is the input impedance op the power-amp. In between the cable of wich, most likely, only its capacity is relevant here as its resistance will be probably 100000x smaller than the input impedance. Also we may assume that the input impedance of the power-amplifier will be much higher than the output impedance of the pre-amp. The roll-off will be caused mainly by the capacitance of the cable. -3dB will occure where the reactance of that capacitor (cable) equals the output impedance of your pre-amp. So determine the output impedance of your pre-amp. -connect a signal generator to the input of the preamp with any signal that will give fair output f.e. 1 Volt reading on a voltmeter at 1kHz. at the output of the pre. -Connect a variable resistor ( potentiometer) to the output and reduce its value untill the voltage drops to 0.5 Volt. -Measure the resistance of the pot wich now represents the output-impedance. to the output, CableCapacity from mnf. specs Reactance is Xc ..............1 Xc = ----------- ........2 (pi) f c (ignore the dots in the formular) Andre
nonik
diyAudio Member

Join Date: Jan 2004
Location: Rijckholt
Re: Long Distance Interconnects

Forgot the schematic
Attached Images
 cable.jpg (5.4 KB, 153 views)

 14th January 2004, 06:30 AM #4 neutron7   diyAudio Member     Join Date: Dec 2003 Location: Toronto Canada Maybe you could build a small low output Impedance driver circuit. question is if it would degrade the sound more Than the problem it is fixing.
 14th January 2004, 05:29 PM #5 Cham   diyAudio Member   Join Date: Jan 2004 Location: Houston, TX Thanks to andre for the Formula! The suggestion of a low impedance driver makes sense only to a point. Burces preamp already is only 200 Ohms. Much lower than that is silicon territory. I'm thinking that I have to find a place for the power amps near the preamp and some large gauge speaker cables. Bob
 14th January 2004, 07:00 PM #6 sreten   diyAudio Member RIP   Join Date: Nov 2003 Location: Brighton UK 200 ohms ? then he's talking out of his hat ! 200 ohms will drive ~ 40 nF @ 20Khz for 3DB loss. This is equivalent to 400 metres of 100pF/metre cable. sreten.
fdegrove
diyAudio Senior Member

Join Date: Aug 2002
Location: Belgium
Hi,

Quote:
 200 ohms ? then he's talking out of his hat !
The Zo of a circuit doesn't tell you the whole story...

I agree with BR, it won't drive the cable without loss...even if and when it's low capacitance...
Let's get REAL on this.

Cheers,
__________________
Frank

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