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3 outputs into 1 input question/TROUBLES

S

SFM-Ptgal

A few days ago I took a schematics wich shown how to connect 3 cart machines (triple decker) into one input (balanced-balanced)
I made a resistive joint with 150ohm resistences connected to each + and - output. All of them converged to another 150ohm b4 the input.

Result:
ITC Delta triple decker: no issues at all. I've just noticed an overall gain increase.

Harris triple decker: ISSUES- losses when more then a deck is on playback mode. If the 3 decks are running I have a decrease of almost 15db on the output.

Why am I having troubles with the Harris when both machines (ITC and Harris) have 600 ohm transformer balanced outputs? ::)
 
Its been a while since I worked on those machines. I do remember that one or the other may have had relays or light-dependent resistors in the output circuit that effectively disconnected the output from the amplifier when the deck is stopped. This action would remove the amplifier's source load from the output (and any amplifier hiss which is the reason for doing it). I think the Harris is doing this and it's changing the load on your combining circuit and making the levels change. Check the Harris schematic.
 
I think ncradioeng is probably on the right trail.

I would:

1. get rid of the last two 150 ohm resistors before the input.
2. If the console inputs are high-z, I would put a 600 ohm resistor from + to - across the console input so the cart machines see a 600 ohm load.
3. raise the series resistors at the output of each deck from 150 ohms to something higher. Try 1K in each leg.

The level will still fluctuate depending on how many decks you have going, but this should even it out some.
 
I'm not sure what the "on" resistance of the LDR would be. I would solder a 47-ohm resistor across the LDR's resistor leads to be safe and then cut a trace feeding a light lead of the LDR so it won't activate. This way it's easy to undo the modification if it doesn't work.
 
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