Are you a fan of ribbon mics?

[veracohr]

 

The AEA preamp designed to optimize their mics has an input impedance of around 18,000 ohms, compared to your typical preamp with an input impedance of 1,500 to 2,500 ohms. I have a Mackie 800R with an input impedance switch and found that a CAD Trion 7000 that I had for review sounded best when loaded with 500 ohms. Royer mics, since they designed them to work with the kind of gear that most people have today, seem to work best with an "ordinary" preamp.

 

 

Is there some kind of general guideline for pairing mics with preamps in regard to impedance? I would think the goal would be maximum signal transfer from the mic to the preamp, which would call for a high input impedance. Is this incorrect thinking? Is the goal maximum power transfer, and thus impedance matching?

[MikeRivers]

 

Is there some kind of general guideline for pairing mics with preamps in regard to impedance? I would think the goal would be maximum signal transfer from the mic to the preamp, which would call for a high input impedance. Is this incorrect thinking? Is the goal maximum power transfer, and thus impedance matching?

The goal is definitely not maximum power transfer. There isn't enough power to worry about. Maximum voltage is a good idea, and in fact, the Gordon, a very fine sounding (and also very expensive) preamp has an input impedance of a couple of megohms. It works best on transformerless condenser mics that are relatively insensitive to loading as long as it isn't too much (low impedance = more load). It doesn't do much for an SM57, however, which, determined by by experimenting, seems to be happiest when loaded by about 650?.

 

There's no real guideline. If you have a preamp with a fixed input impedance, you use what you got. If you can adjust it, you play around until the mic sounds best in that application. There are a few "knowns," for instance the SM57 above, and old RCA ribbon mics as well as new AEA ribbon mics that like at least 10k?.

 

There's also a matter of how capacitive or inductive the impedance is. A transformerless input with a resistor across the input terminals is going to sound different with a given mic than one with an input transformer that gives the same impedance at the mic terminals, both because the transformer itself contributes something to the sound, plus the inductance affects how the mic is loaded at different frequencies.

 

You can't win this one easily, but if you have a particularly bad combination, you either change the preamp or the mic. Or you love it for what it is.

[veracohr]

 

The goal is definitely not maximum power transfer. There isn't enough power to worry about. Maximum voltage is a good idea, and in fact, the Gordon, a very fine sounding (and also very expensive) preamp has an input impedance of a couple of megohms. It works best on transformerless condenser mics that are relatively insensitive to loading as long as it isn't too much (low impedance = more load). It doesn't do much for an SM57, however, which, determined by by experimenting, seems to be happiest when loaded by about 650?. There's no real guideline. If you have a preamp with a fixed input impedance, you use what you got. If you can adjust it, you play around until the mic sounds best in that application. There are a few "knowns," for instance the SM57 above, and old RCA ribbon mics as well as new AEA ribbon mics that like at least 10k?. There's also a matter of how capacitive or inductive the impedance is. A transformerless input with a resistor across the input terminals is going to sound different with a given mic than one with an input transformer that gives the same impedance at the mic terminals, both because the transformer itself contributes something to the sound, plus the inductance affects how the mic is loaded at different frequencies. You can't win this one easily, but if you have a particularly bad combination, you either change the preamp or the mic. Or you love it for what it is.

 

 

I just figured that (ignoring for the moment the reactiveness of the impedance and the effects of a transformer on the sound) if you want maximum voltage going into the preamp, you would want a uniformly high impedance on all preamps. Say, 1Mohm or something like that. So regardless of whether you're using a 150 ohm mic or a 600 ohm mic, you're in the clear. Why then do some preamps have relatively low input impedances, like the 1.5k-2.5k you mentioned? It seems like preamp input impedance should be a no-brainer and not have all this variation.

[VideoRacer]

>>There's no real guideline...You can't win this one easily, but if you have a particularly bad combination, you either change the preamp or the mic.

 

Pretty much why I said you should experiment. Reading your comments about the high impedance, I wonder if there's a point of diminishing returns, or if it really just depends on the mic (and of course, how it interacts with your pre). I don't have input impedance figures for the Edirol FA-66 or the M-Audio thingie I tried, but the Behringer BCA2000 has a rated balanced impedance at ~2.2k, while the Rane DMS22 is rated a balanced 1k.

 

That really doesn't seem like a whole lot to me (not that it means anything electronically of course). Another preamp I'm looking at is the little Rane MS 1b, which in comparison has a balanced 10k input impedance. I wonder how my ribbons would sound through that.

 

AFAIK, both the DMS22 and the BCA2000 are transformerless inputs, yet how their input circuits differ may still play a role in how they interact with the ribbons. There was a significant difference in the sound of my Fatboy between these two. They both have 60db of gain, yet the Rane, an older preamp design, sounded quieter that the newer BCA2000's IMP

[Dexter Frank Jr]

Damn I really should get my m160 fixed, I love that one on guitar amps

[Ernest Buckley]

I`ve been blessed to have access to use an arsenal of mics from $100 mics to $5000 mics and the best mic I have used on my voice (meaning it sounded most like my natural voice) was a ribbon mic. As you may have guessed, it was not the most expensive one. So use your ears, forget the labels, the price tag, the proproganda, etc... and just listen...

 

And to answer the questions, yes, I`m a fan of ribbon.

[MikeRivers]

 

I just figured that (ignoring for the moment the reactiveness of the impedance and the effects of a transformer on the sound) if you want maximum voltage going into the preamp, you would want a uniformly high impedance on all preamps. Say, 1Mohm or something like that.

If you could ignore reactance and some other things, then a high input impedance would indeed give you a higher voltage at the preamp input, and therefore require less gain for the same level out of the preamp. The problem is that you can't ignore the real world. One reason why the Gordon preamp works with its very high impedance is because he takes very great pains with grounding and shielding so that common mode noise rejection will be excellent. It's easier to get better common mode noise rejection with a low impedance input. But lower voltage at the input means that more gain is needed to get a mic up to standard line level, and the trick with preamp design is to get the gain without too much noise. It's a design seesaw when you add price as a design parameter.

 

Why then do some preamps have relatively low input impedances, like the 1.5k-2.5k you mentioned? It seems like preamp input impedance should be a no-brainer and not have all this variation.

 

It's a practical compromise. A high impedance differential input designed like a medium impedance input would probably find that the mic cable makes a very good antenna.