These are passive. Actually a strategically placed class A buffer in each input would be a good thing but you don't NEED it.

Assuming you don't get a pile of junk... They do make nice line mixers if you set them up right. These are often 8 or 16 input mixers on panels using a single bus extrusion. There are little pins that stick out with white (most of the time) single wires going to switches. The pins are actually legs off the free ends of the "drop-on" resistors. You would come off the red pins that are the buss itself and take that to a make up gain amplifier. Ideally you want a 1272 on each buss for makeup gain.

There are a number of variations on what you do about terminating the buss if it's short.

Ideally the buss impedance should be somewhere between 200 and 300 ohms so you may need to put a loading resistor on buss to pull the impedance down.

So... here you go...

The drop-on resistors in the buss extrusion would probably be 7K5 or 15K.

Let's say the drop-on resistors are 15K

Let's say you want 24 inputs. Passive loss summing busses have to have the drop-on resistors terminated all the time - to zero volts or to a relatively low impedance source...

So the buss impedance without loading is 15K divided by 24 (24 equal resistors in parallel) or 625 ohms.

If you throw a 1200 ohm transformer primary across that...

To find the equivalent impedance you invert the values and then add them - then invert the result.

For example take 625 ohms and 1200 ohms in parallel.

Inverted... 1/625 +1/1200 = ~ 3/1200

Inverted again... this is 1200/3 = 400 ohms

the general formula...

1/Rtotal = 1/R1 + 1/R2 + ... + 1/Rn

The answer is always a lower value than the smallest resistor

You could throw another 400 ohms of resistance across that and get the impedance down to 200 ohms

...OR

Maybe we wire the transformer primary to 300 ohms (10468 transformer)...

For two resistors (only) you can do (R1 x R2) divided by (R1 + R2)

For the 625 ohms and 300 ohms.

R1 x R2 = 180000

R1 + R2 = 900

180000 / 900 = 200

now... figure the internal "zero" of a console is -4dBu (-8dB below operating level of +4)

a -4dBu signal is attenuated because the buss is a voltage divider and adder.

20 Log X

X = (buss impedance) divided by (drop-on resistor + buss impedance)

that works out to -37dB

start at -4dBu (top of drop-on) and drops by -37dB to reach -41dBu

that's why it's called passive loss

then you need 45dB gain to get to +4dBu

old - useful and may be correct - can't remember

http://www.technicalaudio.com/zoop/passiveloss.jpg