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Re: Transmission line theory in S'phile

Posted by Lew (A) on October 25, 2001 at 12:44:54

In Reply to: Re: Transmission line theory in S'phile posted by George Kubitsch on October 25, 2001 at 02:21:55:

Ralph or Mark or any of several other people can tell you more than I can, but here goes: The Atma-sphere amplifiers with which I am familiar have a switch on the front panel, indicating "high Z" or "low Z" input. The "high Z" input switch position gives either a 200K ohm input impedance for balanced operation or a 100K ohm impedance for un-balanced operation of the amplifier. When the switch is in the "low Z" position, balanced mode only, then there are a pair of 300 ohm resistors bridging each of the two "hot" pins of the XLR input jack to the ground pin. Thus the pre-amplifier will see a 600-ohm load in balanced mode. Use of the low Z input mode with a pre-amplifier that has an output impedance equal to or less than 600 ohms (e.g., the MP1 and MP3 but also a few other balanced-out units) allows one to drive the amp in accordance with transmission line theory, assuming one's interconnect ALSO has a ~600 ohm or lower "characteristic impedance". In a nutshell, transmission line theory suggests that driving a high Z device with a low output Z device, especially using wire that has a Z significantly higher than that of the signal source, results in reflections of energy and signal back into the signal source, creating distortion. (Typically, manufacturers design for at least a 10X DIFFERENCE between the output Z of a pre-amp and the input Z of an amp in order to avoid high frequency roll-off due to capacitance; hence the optional "high Z" input impedance of Atma amps. So, the this is definitely an area of controversy.) But read the article in S'phile for a more in depth explanation of TL theory and characteristic impedance.

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