Line level Input and Output Impedances - Legacy and Modern Gear

[ruffrecords]

Getting back to that transmission line business. We should remember that whatever the line's impedance is, it only holds for a narrow band of audio frequencies.

The bottom line of Jim's paper is that, as long as your cable is less than two thousand feet long, all you need to consider is its total series resistance and parallel capacitance. Its characteristic impedance is immaterial at audio frequencies.

This is a completely separate issue from the effects of loading on input and output transformers.

Cheers

Ian

 

[gridcurrent]

The VU meter committee picked 600 Ohms, perhaps to coordinate with telco practice. But also because rectifier voltage drop hurts more for lower impedances.

Of the three (+/-1) broadcast networks, two went to 600 Ohms and one went to 150 Ohms. It is not clear why. Probably each CE's personal preference and no need to co-ordinate between.

Trivial history,  a CBS engineer told me back in 1972 that they wanted all their equipment at 150 ohms to reduce confusion. 
The Neve console Columbia Broadcasting used during George McGovern's coronation in Miami that year had all the output transformers strapped for 150; it was also fitted with Cannon UA connectors in lieu of XLR's.

 

[PRR]

> CBS engineer told me back in 1972 that they wanted all their equipment at 150 ohms to reduce confusion.

I'm confused.

 

[MatthewF]

I've just come across this thread while considering the best way to drive some 1176s (with 600R inputs) from unbalanced mixer inserts driven from TL07x opamps.

I would suggest you piggy-back a THAT 1246 and a THAT 1646.

This was my first thought too. These products should make life easy:

https://www.sparkfun.com/products/14002
https://www.sparkfun.com/products/14003