Impedance Matching

[tkingen]

Having read a little about about impedance matching, I finally got around to testing the mic/load impedance settings that are available on the MAudio Tampa. They do make a difference.
Tonight I had time to test with two mics - Shure KSM32 and KSM44. They both have output impedances of 150ohms.
The available load impedances on the Tampa are 2400, 1200, 600 and 300. For some knucklehead reason I had it set at 2400ohms and never bothered to experiment. That was a mistake.
During the tests, as the settings got smaller, it seemed that the recorded signal was more of an an actual representation of the source. It seemed the most true at the lowest setting, 300ohms.
At first I thought that it was capturing more air in the signal. Now I'm thinking something different...it's not adding the lower midrange that's evident at 2400ohms. I also have a DMP3 and some people complain that it imparts a lower mid buildup across mulitple tracks. This is probably true, and the Tampa does sound similar when set at 2400ohms.
Anyway, this is a great revelation. My recorded tracks have felt and sounded a little too heavy handed. I'm looking for solid, transparent nuance and air in the signal. This new discovery looks promising!
Anyone else have opinions or experiences with multiple impedances?

Terry Kingen

 

[chessrock]

Impedence matching is very underrated and vastly overlooked.

Sometimes it can mean the difference between your mic sounding the way it's supposed to sound . . . and at the very least, it's a good way of shaping the sound without having to resort to EQ and/or other means.

 

[StevenLindsey]

How do we folks with RNPs or Syteks or Blue Tubes....cheap stuff such as that, take advantage of impedence matching? Is this a stupid question? What is the effect on the sound of a mic?

 

[sweetnubs]

I keep harping on this, especially in the "the sm57 sounds like shit" threads. Dynamic mics are especially sensitive to proper impedance matching. Do you know the impedance of your '57 and the preamp being used? It really does matter and will affect the sound of your mic. A good homework assignment for everyone. I get paid by the hour if you want me to write an article.

 

[chessrock]

How do we folks with RNPs or Syteks or Blue Tubes....cheap stuff such as that, take advantage of impedence matching? Is this a stupid question? What is the effect on the sound of a mic?

I wouldn't necessarily call the Sytek or RNP "cheap." Inexpensive or competitively priced relative to their value . . . yes.

One of the things that makes better pres "better" is the way impedence is matched. Using a good transformer like a Jensen is one of the best ways to go about it, but there are obviously other good methods. I'm not an electronics guy, so I don't totally know what the hell I'm talking about.

The effect on the mic goes something like this: A mic pre with a higher impedence at the input will have the effect of "loading the mic down," which makes the diaphragm more responsive to transients and higher frequencies. This could be a good thing on something like and sm57 and a snare, for example. Could also be a bad thing if it makes a vocal track sound unnaturally grainy or shrilly.

Either way, you're affecting the natural sound of that 57 . . . so under less-than-ideal loading conditions, you're not really hearing what a 57 is supposed to sound like.

As the input impedence starts decreasing, dynamic and ribbon mics start seeing loads they were moreless designed and tested for, so they're going to sound closer to how they were originally intended to sound. This will mean a subltle softening of it's transient response and rolling off of the high end. As they get even lower and lower, you'll get a rolling off of the highs and the lows and you're left with a honkier, edgier quality.

Like the nubs suggests, it's a good idea to have at least a basic understanding of how this stuff effects what you're working with.

 

[StevenLindsey]

Looks like I've got more homework to do. Thanks.

 

[crazydoc]

Here's my take on impedance, right or wrong.

Impedance and frequency are interdependent - if you change one, you change the other. Mics and pres are rated at nominal impedances at a given or perhaps a median frequency.
If you change the impedance of one or the other, you will change the frequency response of the system. So a mic that sounds good with one pre sounds bad with another, and likewise a pre that sounds good with one mic may sound bad with another.
So a variable input impedance preamp gives you another variable to play with to get the sound you want.

There probably is no one "correct" impedance to match a mic and a pre, but if you can't vary the impedance you're stuck with what you've got.

Maybe we need some variable output impedance mics also to play with.

 

[tkingen]

Well, the MAudio Tampa and the GT Vipre preamps have impedance matching features. Are there others?
How about a small box that can be put in line between the mic and pre?

Terry

 

[crazydoc]

The Studio Projects VTB1 has a switched dual impedance of 50/200 ohms.
http://www.humbuckermusic.com/studprojvttu.html

 

[crazydoc]

Here's another - variable 100 to 10k ohms.
http://www.summitaudio.com/images/2BA-221-big.jpg

 

[Rossi]

crazydoc is closer to the truth than chessrock, I think. There is no ideal impedance for a given mic. Well, actually it depends on your view point. Technically speaking the output impedance ought to be very low and the preamp's input impedance ought to be high so as to put as little load on the mic's output. In practice you'll find that some mics have a strong presence boost that you want to even out a little. A low input impedance will dampen the resonance of the microphone and its circuit. Impedance matching is basically a subjective thing.

Threre's also some mics that don't react to impedance switching at all. I recently had a Neumann TLM127 in my studio and an ART DMPA preamp with variable input impedance (150 ohms - 3 K). The Neumann's output impedance is only 50 ohms, so even at the lowest setting the ART's impedance was still 3 times that of the mic's output impedance. Also the Neumann doesn't use any frequency corrections, boosts, whatever in its electronic circuit, and the capsule's presence boost is very slight. So there's simply no resonances that the preamp could dampen. It sounded the same, no matter what the input impedance was.

A counter example is the MXL 1006BP. At the usual high impedances of >2 K it was very bright, a bit harsh, too. At lower impedances below 600 ohms it sounded much better. Less top end but much more intimate.

Dynamic microphones also are pretty sensitive to input impedances. A dynamic's output impedance is much more frequency dependent than a condenser's. So there's more interaction.

 

[Tonio]

Oh no! now I havta go out and get a variable impotent I mean
impedance micpre LOL he he.
I have always wanted one, but not many choices eh?
Chess and Rossi bring up very good points to ponder. Dynamics do react better than condensors for upgrading a mic pre, but they do seem to impart more HF and LF defining if you will..
Consendors seem to be more detailed in the mids.. e.g.
I have a Spirit Sx and Grace 101. When I first got the 101 dynamics(421, Ev408, Senne609, Audix D2) seemed alot more responsive (at least the wow factor) than Sm81, 414, TR40.
I think the 101 input is at 1600 ohms? Most of the condensors are around 100-200ohms dynamics?
It would seem that matching impedances would give you more
palettes to play with, which could be a good thing or confuse you more.
hmmm more things to ponder.heck just get a GR NV, has iron and 2 impedance inputs and youre' set.ha
can someone let me borrow one?

 

[meriphew]

I hav a GT Vipre - the impedance matching feature works great. I think the UA 2610 does imp matching to a more limited extent if I'm not mistaken.

________________
Post indie electronic
Meriphew
www.meriphew.com

 

[chessrock]

crazydoc is closer to the truth than chessrock, I think. There is no ideal impedance for a given mic. Well, actually it depends on your view point.

From the point of view of transparency, I think there is such a thing as "ideal." If ideally you want a 57 to sound like a 57 and a 441 to sound like a 441, then the ideal imp. would be one that allows a freq. response (curve) that corresponds relatively close to the published specifications (or the original intent of the designers).

From a tonal-shaping standpoint, anything goes. What's ideal is what sounds best, or what fits your application.

Frequency response and impedence are not directly corelated. Indirectly, they are . . . to an extent. Example: when you impede the mic's output more (higher impedence at the input of the mic pre), you load it down, which makes the mic's diaphragm more responsive. Which often/usually results in boosting the treble response . . . but not necessarily. You could try loading down a different mic, and the effects may be completely different -- you might even get the opposite effect.

 

[Rossi]

Hmm, I don't know what you mean by "making the diaphragm more responsive." The responsiveness of the diaphragm is a result of its construction and the polarization voltage it receives from its circuit. Both stay the same no matter what the input impedance of the preamp is.

It is the frequency response of the mic circuit that the preamp interacts with. Also I've yet to come across a mic that reacts to a low load impedance with treble increase. Theoretically this shouldn't be possible, and my experiences so far confirm what the theory says. BTW. There's often some roll of in the bass department as well, especially at very low impedances.

As for the designer's intention or published frequency curves, I don't care much. Nobody knows what the designers intended and published frequency plots are usually mere fiction. The closest you can get to what they *probably* intended is use high impedances, because that's de facto the standard. More interesting, however, is getting closer to what *you* intend the sound to be. And that may or may not include decreasing the preamp impedance. I don't think variable impedance is a must have. But it's a nice addition.

 

[chessrock]

You know a lot more about this stuff than I do.

I'm going by what one of the senior designers at Summit Audio explained to me (rather excruciatingly) over the phone last week, as I was hoping to do a freelance review for their 2BA-221. I suppose I could have heard wrong, or maybe they're just trying to give me a bunch of marketing mumbo-jumbo to confuse me and throw my review off.

 

[Rossi]

Hey Chessrock, I'm a freelance reviewer as well (for German Keyboards), and I definetely do hear a LOT of marketing mumbo jumbo. It's really hard to get at real information. You actually have to know your stuff pretty well and ask very specific questions, so they can assume they can't tell you just anything. A few will try nonetheless. Rule No 1: don't talk to marketing guys. Marketing guys give marketing answers. Try to get to the engineers themselves. That's not always easy, as engineers are shielded off by a protective wall of marketing guys. That's why it's important to ask questions so specific that marketing guys can't answer them.

I hear myself talk like a pro, but I'm fairly new to the business. I've still got to learn a lot myself.

 

[crazydoc]

These are some random thoughts on impedance - I can't insure their veracity. If there are any electrical engineers or rocket scientists out there , please jump in with your ideas/corrections.

Definitions:

The opposition of inductance and capacitance to alternating current is called reactance and is measured in ohms.
When this value is added to the resistance of a circuit the resultant sum is called impedance. Impedance is the total opposition to the flow of alternating current in a circuit. Impedance varies with frequency so nominal impedance measurements can refer to average or minimum values.
Impedance is a complex measurement of a device's resistance to the flow of electrical energy, and for microphones the impedance value refers to its output resistance at 1 kHz.

Inductive reactance increases with frequency: capacitive reactance decreases with frequency. To remember this, think of a blocking capacitor - it blocks DC voltage, which is the lowest possible frequency - zero Hz, and so passes no current (except leakage), and it has an infinitely high reactance. Think of a coil of wire - it passes DC completely (except for its mimimal resistance), and so has a low reactance - zero.

Source impedance (lower is better)-this is the impedance of an output and is a measure of how easily power will flow from that output.
Load impedance (higher is better)-this is the impedance of an input and is a measure of how much power the input will draw.

I think of input impedance as a split load on the source, part going through the signal path and part shunted to ground. If you hook a mic to a preamp of equal impedance, nominally half the signal will go through the signal path, and half to ground. Depending on the reactances involved (capacitive and inductive), different frequencies will be passed or shunted in varying amounts, which will color the frequency content of the signal out of the preamp.

Generally, the input impedance should be 10 times or greater than the source impedance, so that most of the signal goes through the signal path, and so that frequency variations are minimized. By varying the input impedance, it seems you could alter the frequency content of the signal. It's concievable that this could sometimes even improve the sound.

Again, these are just thoughts from the school of hard knocks and smoking circuits, and anyone with actual knowledge of these concepts should chime in and set the record straight.

Oh, and Chessrock, when you're done with that 2BA-221, could you send it over to me for some testing? It may take me a few years to arrive at my conclusions.

 

[NeoMagick]

Was looking for some reviews of the DMPA, and dug this up.

Can someone confirm that with a setup involving a preamp with no impedance variability and a compressor with variable impedance, adjusting the impedance on said compressor will only adjust the impedance between it and the preamp, and will have no direct impact on the mic?

i.e. -

Mic -> Preamp -> Compressor -> Deck

So the adjusted impedance on the compressor shouldn't alter the impedance given to the mic, because the compressor doesn't have a direct line to the mic without going through other hardware. I'm open to the possibility it'll change the sound coming from the /preamp/, but not directly from the mic.. I'm right in thinking this?

 

[venuesound]

Neomagick - you are correct. because the mic is on the other side of the preamp, the compressor will have no effect on the impedence of the mic, but might play hell with the preamp! Crazydoc's analysis is also correct. Alot of the mic pres use FET inputs, very high impedence compared to the mics which have output impedences in the 50 to 600ohm range. The greater the difference between the mic and the preamp (to a point of course) the less the mic loads down the front end of the preamp.
For some electronics the closer the two impedences, the better. What happens when loads and sources are not matched correctly in this case is a reflection. Some of the signal flows through the device and a portion is reflected back to the load. The better the impedence match, the more signal flow. There are times that matching impedence is very important, for example in antenna designs. Ideally the antenna and the amplifier have the same impedence so that the reflection coefficient is 0, and the Voltage Standing Wave Ratio (VSWR) is 1. But that is for a different forum......