|
Bottlehead Forum Welcome! Need support, you got it. Or share you ideas and experiences. |
|
|
|
Bottlehead Forum Welcome! Need support, you got it. Or share you ideas and experiences. |
|
In Reply to: 2k or 50 Ohms? posted by Jim Carlon on January 9, 2005 at 16:16:25:
A great source of confusion - good question!Any signal source functions as a voltage source in series with a resistance, called the output resistance. The load has a resistance also. In modern designs, the source resistance is made small relative to the load resistance. This preserves most of the voltage, independent of the actual load resistance.
The source can put out a certain maximum voltage, and it also has a certain maximum current capability. To stay within the current capability, the load resistance must be larger than some value. For example, the Foreplay can produce a maximum of 20v, and its cathode follower cannot produce more than 2mA, so its specified minimum load impedance is (Ohm's Law) R = V/I = 10k ohms. This minimum load resistance is still much larger than the equivalent internal source resistance, 800 ohms for the Foreplay. The Marchand is specified to have 50 ohms internal resistance, and be able to drive a load of 2000 ohms or greater.
Note that all this has nothing to do with "matching" resistances! That idea comes from the early days of the telephone business. If the source has no current limit, then the most power that can be extracted from it occurs when the load resistance equals the source resistance. This was important when telephones did not have amplification - the power generated by the microphone was the only power available for the earpiece, so you had to couple as much as possible to the earpiece in order to hear anything. Once the telephone system acquired tubes, that was no longer necessary, but at the same time lines became longer. Long lines (over a mile or so) have reflections unless the source resistance, line characteristic resistance, and load resistance are all identical. So matched resistances were still needed by the phone system.
Hope that helps!
This post is made possible by the generous support of people like you and our sponsors:
Follow Ups
- Re: 2k or 50 Ohms? - Paul Joppa 10:30:33 01/10/05 (1)
- Whew! Thanks! - Jim Carlon 11:30:44 01/10/05 (0)
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2024, Audio Asylum, all rights reserved ]
