Passive summing bus output impedance change?

I've been using NYDave's passive summing bus design for a few months now, and it sounds great, but I'm wondering how difficult it would be to drop the output impedance from around 1k ohms to more like 350 ohms for better matching with mic preamp inputs. As I understand it, strapping a resistor over the balanced outputs may accomplish this, but I have no idea of how to calculate the value needed or how to calculate its effect on the output level, which I assume will drop as well.

By way of reminder the summing bus is a balanced 8-channel mono design (of which I made two for 16 inputs as 8 stereo pairs) and uses 3.92k bus resistors.

Can anyone provide the formula for dropping the output impedance and level?

Thanks so much and peace,

JC

It is a sin to waste audio power.

But if you must:

> formula for dropping the output impedance and level?

Don't need no stinkin formula.

The output looks like a 1K resistor. Suppose you put another 1K resistor in parallel, what do you get? 500 ohms. Now three 1K resistors: 333.333 ohms, which is exactly equal to 350 ohms (for any practical purpose).

So hang two 1K resistors (or one 470 or 560 ohm resistor) across the output. Bingo, 350 ohms.

You also get about 10dB less voltage level. (There is a formula for that but bah on formulas.) This means you are wasting 10dB that you have to make-up with 10dB more amplifier, and 10dB less signal to noise.

If you are stuck with amplifiers that have 10dB more gain than you need, and are not crying about noise, this will work.

Since the "typical" mic preamp input--if there is such a thing--is about 1500 ohms, the lowest-loss way to do it would be just to connect them together. You'd only lose 4.5dB that way.

However, many mic preamps will not behave properly unless their inputs are terminated with a 150-200 ohm source, or at least something close to it. Stepping down 1K to 200 is a 5:1 impedance ratio. You could use a transformer having a 2:1 turns ratio, for an impedance ratio of 4:1, which would give you a 250 ohm output Z--"close enuf.". You'd only lose 6dB this way, and gain the advantage of having a true "floating balanced" output.

Shunting the output with a resistor will work but is the least desirable way to do it unless, as PRR pointed out, you already have a surplus of gain in your make-up amp and can afford a degraded S/N.