[BC] Re:111C repeat coils

[Paul Christensen]

> this, cutting distortion by a factor of 20. Even better, connect the 5534
opamp directly to the output transformer through a 50 ohm resistor and use
these output transistors for another purpose (below).

Good suggestions.  A directly-driven transformer from a 553x through a 
50-ohm series resistor is probably fine if the total Z seen by the 5532 is 
greater than 1K-ohm.  As I recall, the original Signetics spec sheet showed 
600-ohm as an absolute minimum, but if at all possible, even the best 
Bi-polar and Bi-FET op-amps should probably see a Z of no less than about 2K 
across the audio spectrum.

One alternative between the op-amp and transformer is to use a better 
high-current driver stage in the feedback loop (as Dana points out) 
consisting of the classic symmetrical pair of the MJE 170/180 series 
transistors with diode bias temp. compensation.  At one time, Orban offered 
a line-driver servo upgrade to one of its products and we purchased the 
modules by the dozen so that we could add them to other equipment that 
brought out an op-amp directly to the outside world or through a 
transformer.  For intra-studio use, I've never had much of an issue with 
using an op-amp without a current driver, but over any type of extended 
distance, it can make a significant difference in audio performance and 
equipment reliability when a driver is used.

At distances of over one hundred feet of typical TP, it makes sense to start 
treating the cable as a transmission line, using similar Z matching 
techniques like dropping Z to between 50-150 ohms over the run to minimize 
high frequency loss due to distributed cable C and minimize noise 
susceptibility on the destination end..

About 15 years ago, Richard Cabot with Audio Precision wrote a great article 
in BE that established a basis of using an output Z of 47-55 ohms was the 
ideal transmission Z for extended distances of TP.  If anyone has retained a 
copy of that article, I would like a copy.

Paul


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Paul Christensen, CPBE
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