Variable impedence on pre-amps? What's that good for?

[Ryan Silva]

I just got a Focusrite Twin Track, and saw that it had variable impedence on each pre. So I found out the impedence of all my mics, and indended to use these settings. Now after testing it, I found that it does a lot to color the sound. Has anyone played around with impedence, and if so any tips in practice.

[mertmo]

Now after testing it, I found that it does a lot to color the sound

you already the answer you seek. Use the different settings to get different sounds from the same mics, and/or find which setting sounds the best to your ears on a given mic...

i'm oversimplifying a little. It's for matching "correct" settings on mics, like you figured out, but the fun part is having the different options.
have fun!

[Ryan Silva]

Well if it's just for playing around with great, but is there any danger to my gear at extreme settings? I have watched enough power amps blow as a result of incorrect impedance, maybe on such a small a mount of power this is not an issue. I hope so.

Thanks

[Dot]

It's good for putting more little blinky lights and knobs and switches on the mic pre to make it look cool.

Bottom line: it's bullshit.

[Family Hoof]

If you mean loading the mic, no I don't think that could hurt it. Especially not a dynamic. Unlike speakers and amps where max power transfer requires impedance to be the same on both ends, a mic wants the preamp impedance to be at least 10x greater so that the tiny little voltage it's putting out is not dissipated across the mic's internal resistance and has a chance to get amplified before significant loss. If you set the variable impedance low enough you can load the mic and while this may be considered bad it can sound cool.

THis is all based on my knowledge of math, by the way, not my experience with variable impedance preamps because I don't have very much. I do believe that preamp input impedance has a lot to do with whether or not it'll make a particular mic shine. I think many people overlook this and that variable input impedance is a great idea. It was used more often many many years ago and I'm glad to see a resurgance.

[joel hamilton]

I love this option. You can load the hell out of the front end of a pre with a good mismatch, or you can let some nice open sounds "sail through" and get the most out of your microphones, and the pre.

To say it "colors the sound" is like saying mic choice affects the recording.

[Tim Farrant]

It's good for putting more little blinky lights and knobs and switches on the mic pre to make it look cool.

Bottom line: it's bullshit.

Hey Dan, please clarify, why do you think it's bullshit?

Some mics will change sound more than others as you vary the load impedance. Notably transformer coupled condensor microphones will vary the most as the load changes the top end frequency response. A low impedance load on the transformer will cause the top end to roll off, the reverse for higher loads. This may be useful for dulling the sound of bright mics. Dynamic mics will also display tonal difference with load. There is no right or wrong setting, and it's near impossible to damage the mic with extreme settings, although you will start loosing a few db's of level with low impedance settings.

Cheers
Tim

[Professor]

'Cause they can charge more.


Just kidding.
As you've already noticed, changing the load changes the sound of the microphone. Changing the load changes the demand placed on the transformer (if the mic has one) the preamp electronics, right on up to the capsule. The lower the load, the more demand, and the harder the microphone is working. But we're not talking about so much work that the microphone is in danger, just working harder.

I have two ViPre preamps in the school studio, which stands for "Variable Impedence Preamp". I've loved these since first hearing them at the AES in LA 6 years ago, and am glad to have them. The settings available are 300, 600, 1200, 2400 Ohms. It seems in practice that most mics have a 'favorite' setting where they perform their best, while some can operate almost anywhere. The RE-20s seem to work best at 600, while the RCA-77 thins out at 1200 & 2400 and thickens up down at 300.
I usually find that the signal gets hotter as the impedence goes down which is contrary to Tim's response there, but seems to make sense and is certainly noticeable in practice. Of course, this makes it tricky to compare settings since we tend to hear the 'louder' signal as 'better'.
The ViPre can also bypass the selectable impedence and go to a non-transformer input, though I can't remember where the impedence is set there. The coolest thing though is the variable rise time on the amplifier circuitry which is like adjusting the attack time on a compressor - it does wonders to soften hard or brittle attacks, but that is another subject entirely.

So don't worry about trying to memorize all of your mic impedences. Simply plug in the mic, and turn the switch to see what sounds best for each mic, instrument and song.

-Jeremy

[Meriphew]

It's good for putting more little blinky lights and knobs and switches on the mic pre to make it look cool.

Bottom line: it's bullshit.

I have a Groove Tubes Vipre that has variable impedance and it's very functional and very useful for getting the most out of your mics.