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Old 9th January 2004, 04:50 PM   #1
Cham is offline Cham  United States
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Location: Houston, TX
Default 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
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Old 10th January 2004, 11:19 AM   #2
nonik is offline nonik  Netherlands
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Default 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
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Old 10th January 2004, 11:21 AM   #3
nonik is offline nonik  Netherlands
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Default Re: Long Distance Interconnects

Forgot the schematic
Attached Images
File Type: jpg cable.jpg (5.4 KB, 150 views)
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Old 14th January 2004, 06:30 AM   #4
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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.
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Old 14th January 2004, 05:29 PM   #5
Cham is offline Cham  United States
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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
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Old 14th January 2004, 07:00 PM   #6
sreten is offline sreten  United Kingdom
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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.
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Old 15th January 2004, 02:12 AM   #7
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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,
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Frank
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