Variable Impedance Circuit between Mic and Preamp?

[FlowersForHuman]

I am interested in the possibility of building a passive circuit to go between my mics and preamps that would allow me to adjust impedance for different tones.
Is this a good idea/possible?

I'm still kind of new to DIY electronics but I can solder and I know some basics. I was thinking that dual gang linear pots like these could work but I'm not sure about the ratings. My initial concerns are these:

How do I limit the load range so it doesn't top/bottom out?

What would be the ideal range for ribbons to condensers?

How do I achieve such a range if not simply buying the right pots? Don't I have to take into consideration the preamp load combined with the pot?

How does phantom power factor in? Am I going to end up with a Darwin award?

[Andy Peters]

I am interested in the possibility of building a passive circuit to go between my mics and preamps that would allow me to adjust impedance for different tones.
Is this a good idea/possible?

Anything you put in front of the preamp works with the preamp.

If you're really into doing this, dig out the schematic for your preamp, and figure out how its input impedance is set. Usually, it's a pair of resistors connected to op-amp inputs or to transistor bases. Perhaps replace the resistors with a ganged pot.

But beware: as you increase the resistance (decrease the loading), the amplifier gets noisier because higher-value resistors are noisier than lower-value parts. Putting a high-gain stage after a higher-value resistor means a noisier preamp.

On the other hand, too low a value could load the mic and cause distortion because the mic (especially something with an active output) may not be able to drive such a hard load. And of course 0 ohms is bad, as it's a dead short

How does phantom power factor in? Am I going to end up with a Darwin award?

Phantom power is provided through 6.81k resistors, which are basically in parallel with load resistors in the preamp. So you won't kill yourself (not enough current) but any resistance you put between the mic and the preamp limits the current available at the microphone.

-a

[The Scum]

Everything you and Andy just wrote implies that you're adding series resistance in line with the mic. Practically, it makes more sense to add additional load in parallel with the preamp input.

Some of this was discussed here a while back. First, read the Stamler article and understand what he was up to.

http://messageboard.tapeop.com/viewtopic.php?t=76757

The trick: load the mic by putting a resistor in parallel with the preamp, between pins 2 and 3.

Is this a good idea/possible?

Possible, yes. Good idea? Depends on your definition of good.

How do I limit the load range so it doesn't top/bottom out?

Use the parallel resistor equivalent equation: Re = 1/((1/R1) + (1/R2)).

At high values of the load resistor, the preamp input impedance will dominate. As the load resistor drops, it'll suck more current, and begin to swamp the input. Eventually, it reaches 0, drawing all available current for no voltage rise - pins 2 and 3 are shorted together, effectively a mute circuit.

The practical version - use a 25K pot, in series with a 200 Ohm resistor.

What would be the ideal range for ribbons to condensers?

Ideal? From what perspective? From a power transfer perspective, you want your additional load to be as large as possible, so all power is transferred to the preamp.

Condensers have active output stages, and they might not be particularly influenced by extra loading...active outputs are there to prevent the output level from varying into different loads.

How does phantom power factor in? Am I going to end up with a Darwin award?

If your resistance is between pins 2 and 3, there's no DC potential between them, so very low risk. And as mentioned above, phantom is coupled through 6.8K resistors, so even if they short to ground, there's not much current available...14 mA. The coupling resistors might get a little warm, that's all. If the 6.8K resistors aren't well-matched, then there's will be a little DC across the pot, making it crunchy when you turn it. Not destructive, just ugly.