Old School Mixers,Echo Sends and Buses?

[xpulsar]

  Does any one know of any examples of how old 600:600 mixers summed their mix buses and echo sends. From my understanding they would wire the input mic/line amps output transformers for 150ohm or 600ohm to drive all the passive echo and mix buses? So that all of the total loading of these would look like a 150 to 600 ohm load? They would use constant impedance attenuators to maintain these loads?
    Can anyone shed some light on how they may went about having a console with 8 inputs summing to L/R along with 3 echo sends all maintaining the 600hm matching idea?

 

[lassoharp]

Search around for the old UA tube console schematic - part of it is somewhere on this forum.  You'll see the plate load being split before the output transformer for driving or partial driving of the echo send.

Reference the Radiotron handbook for passive mixing designs using attenuators in low impedance networks.

 

[xpulsar]

Thanks for the info! I'll start there.

 

[emrr]

input amp / 600R build out / 600R attenuator; T or ladder / 1-off-2 bus switching / program amp / output switching.

how they may went about having a console with 8 inputs summing to L/R along with 3 echo sends all maintaining the 600hm matching idea?

very few ever actually summed to L/R, and virtually none ever had even one echo send, let alone three. 

Echo sends, if existing, would be some sort of bridging input into another amp for output drive.  Returns would simply be additional input channels.

In rare cases you see the bus switching set up as L/C/R.  The center channel usually feeds an extra isolation amp and then circles back and outputs two signals to the L/R channels.  So, yes, middle information passing through an additional stage. 

In 10 years I have collected six total 600 dB pan pots.  Yes, they are that hard to come by, and I have never seen one as a stock feature on a console. 

You can download the Collins 212 console manuals for sure; start there for typical vintage setup.