[sdiy] OT: Audio line levels

[Michael Ruberto]

Byron,

>How much of a dinosaur is this?  Is it old enough that there's some room in 
>the chassis to as some new circuitry?  And is technology under the hood 
>something you could reverse engineer?  Those transformer-isolated inputs 
>are designed for microphones, not line outputs, and have a rather low input 
>impedance...the 1.2K and 1.7K settings are on the edge of hostility to some 
>output stages.
>

The specs show 1.2K to 1.7K on the transformer inputs. This will be a 
problem since the unbalanced outs of my synths need to go into an input with 
a much higher impedance.
I don't have the mixer home yet, I'm picking it up tomorrow so I can't say 
how much room there is inside. It's physically a large beast at over 66 
inches long and 34" deep. The channels can be removed individually too so I 
think modding should be possible. I'm not certain but I think the unit is 
from the late 70s or early 80s so I imagine the circuitry is pretty basic 
with mostly discreet design. Should be easy to reverse engineer. Or at least 
as hard as my 1975 Hammond spinet that I hot- rodded.

>If it's a common transformer into non-inverting opamp stage, one way to add 
>a line input is to add a jack that bypasses the transformer, feeding the 
>input opamp.  This also avoids the possibility of accidentally putting 
>phantom power into a synth output.  The jack could be a switching one, so 
>it bypasses the transformer input, or you could add a toggle switch.
>

This could work but if the gain switches employ the old opamp in the 
feedback loop of the input opamp I may have probs with impedance again.

>Of course, the RCV jack is already built out to 20K input impedance, so 
>it's maybe a better place to interface.
>

I agree. This is what I'll try first.

>If you'd rather not mod the thing, hunt around (Ebay...) for a "bump box," 
>-10 to +4 converter.  They're pretty common in broadcast installations, 
>because most broadcast gear is strictly +4, but -10 front end gear is 
>becoming more & more common.  ART and Henry Engineering are two vendors I 
>can think of right now.
>

I've looked up some of these and it won't be financially feasable for me to 
buy the quantity I'll need for all the outputs of my gear. The cost will be 
more than I paid for the mixer itself. I may buy one and reverse engineer it 
so I may then build my own multi-channel version. I'll be needing at least 
24 channels.


M. A. Ruberto

>
>Do you know the input impedances?  Many solid-state outputs get noisy when 
>asked to drive a 600-ohm input.
>

>Also, correctly balancing & unbalancing can prevent some noise 
>headaches...but needs to be handled on a case-by-case basis, depending on 
>the type of circuit.
>
>Byron Jacquot
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