Dave's passive summing question

Hello, mr. Dave, and all other gurus heere.

I'd like to use Dave's idea from here
http://www.twin-x.com/groupdiy/albums/userpics/balancedmxr.pdf

But I have one question.

If I use small numbers of inputs there is almost no problem, but if I use for example 16 channel there is a little problem. If I use only first channel (for example) and all other are muted, all pairs of 10k resitors (total 20k) are connected to + and - pin of balanced line.
In this case if 15 stereo channels are muted, the total resistance between the balanced line is 20kohm / 15left+15right = 660 ohm!!!
This 660 ohm give me a lot of  signal less.

The question is why when the corresponding channel is not used, the resistor pairs are connected?

Here is a picture to illustrate my question:


click for a larger image

Thanks for the support.

Niki

I'm no "NY Dave" but I believe the design concept was to feed this output to a low Z mic input. By back grounding the inputs the output level and source impedance remains more constant, and more compatible with a mic input as number of inputs changes. If the divider changed with number of sources assigned, you might overload the mic input when only a few inputs are assigned, and not have optimal source impedance for the preamp input.


JR

Nikolay said:

Hello, mr. Dave, and all other gurus heere.

I'd like to use Dave's idea from here
http://www.twin-x.com/groupdiy/albums/userpics/balancedmxr.pdf

But I have one question.

If I use small numbers of inputs there is almost no problem, but if I use for example 16 channel there is a little problem. If I use only first channel (for example) and all other are muted, all pairs of 10k resitors (total 20k) are connected to + and - pin of balanced line.
In this case if 15 stereo channels are muted, the total resistance between the balanced line is 20kohm / 15left+15right = 660 ohm!!!
This 660 ohm give me a lot of  signal less.

The question is why when the corresponding channel is not used, the resistor pairs are connected?

Click to expand...

This passive mixer assumes the source resistance of any input is very low - almost equivalent to shorting the 10K input resistors to ground. This means that it does not matter if an input is selected or muted there are effectively 10K resistors on both inputs to ground. This means the bus impedance - all these resistors in parallel - is almost constant and also therefore that the bus loss is constant. This means you can add a constant gain to the output to restore the level. This mixer is supposed to have a large bus loss - if you look as Dave's info you will see that with 16 inputs and the output slugged for 200 ohms, the bus loss is 40dB. The idea is you use a low noise mic pre to provide the 40dB gain.

This, in essence, is how all passive mixing works.

Cheers

Ian

Thanks guys for the support
I understand the idea

> If I use small numbers of inputs
> if I use for example 16 channel
> ........a lot of  signal less.

A 16-in passive mixer "must" have 16:1 loss.

So you "must" have 1:16 gain after.

This mixer is actually designer for even greater loss. For more than 2 or 3 inputs you generally need gain after mixing. And it is designed to be Balanced Line. What is the handiest balanced-in gain-box in a studio? A microphone amplifier. Mike-amps have HIGH gain and need low impedance sources. The mixer is designed for over 40db loss, so that line level minus mix-loss plus mike-amp gain gets you back near line level, and the mike-amp sees less than 200 ohms.

If you use fewer inputs, do you want less loss? Then you must reduce the gain following the mixer every time you change the number of inputs.

The idea is that you can have 1 to 16 inputs, cut in or out at any time, and the gain from each input to output stays the same.

This may not be "best". Often when you mute 15 inputs you want the one input to come up. But how much? A little, NOT 15 times higher. Not any fixed amount: it depends on the musical reasons. Maybe none when a LOUD lead voice goes solo. Maybe a lot when a minor voice jumps in.

> resistance between the balanced line is 20kohm / 15left+15right = 660 ohm!!!

No, because there is Rshunt. For 16 inputs you use about 238 ohms. Therefore for 16 inputs all the 20K sources are loaded with 660||238= 175 ohms. And if you do not short the un-used inputs, the mix network is loaded with 238 ohms. The difference is only 3db.

If you only need 2 inputs, use the 2*10K mix resistors, no Rshunt, into a balanced input which is over 20K and has gain of 2 or 3. I could use my little limiter, which has balanced line input and 3:1 extra gain beyond what I would need for one input. But for more inputs, the amount of recovery gain becomes large, it starts to make sense to cut all the way to a strong mike level, then use a mike-amp to bring it back up.

There are also "colored" mike-amps which some producers love so much they want "everything" to be mushed together in their favorite mike-amp.