Remote Recording Mixer using Vintage Tubes

[Bill Wilson]

This unit was designed by a close friend of mine & used it in his recording & sound business (late 40's-mid 50's). He gave me the schematic & a Kenyon output transformer. He used the diagram for the Western Electric 22D remote broadcast amplifier. I have included the page on the 22D from my Audio Facilities by Western Electric catalog; probabley dating from the late 40's or very early 50's.

The input transformer can be any unit that has 50, 150, 250 ohm input impedance; with a secondary from 40- 80k ohms impedance. The output transformer must have a primary of approx. 10k ohm impedance, that has a center tap with secondary winding w 600 ohm, & 250 ohms impedance with the ability to handle +20dbm.

The schematic shows 3ea. 30 ohm impedance daven rotary controls having 2db per step, but falling logrithmacly at near the off or closed position. This was due to his using Western Electric Mikes, that had an output impedance or 30-50 ohms. A more up to date implementation would use 250 ohm daven type stepped attenuators. The output switching can be ignored as you would not need facilities for a second telephone order line to communicate with master control at the radio station. You are free to use this circuit as you see fit as I am now the copyright owner.

 

[emrr]

Thanks Bill!  That's an interesting circuit to see. 

What else can you tell us about the labeling?

Academy Remote Amplifier - Model 1

It follows the usual design ideas of the era. 
Today almost no one would want the passive mixer front end, which also gives a matching load to the mic, or mics. 
The gain structure makes sense for the time and purpose, today most people would probably benefit from having the first stage run triode for lower gain and noise.  A pentode front end lacking NFB like this can make a lot of noise, so if the gain isn't needed it's probably best avoided.  Even as a triode, the input transformer and first 6J7 is going to run 30dB or so into that 25K output attenuator.  Speaking of which, that is designed to roll off a lot of low end, being 0.1mfd/25K.  100K or higher would make more sense for that coupling cap. 

 

[Bill Wilson]

The Academy Mixer was patterned after this unit. Westrn Electric 22D remote mixer. My friend Stan Maxwell started Academy Sound & Recording after discharge from the Navy in 46. He sold background music service to stores in town & did location recording w a Presto 6N disc recording setup.

This is strictly my personal opinion, but I think this is a good representation of professional equipment from a bygone era.