Matching Microphones and Preamps: A Crucial Combination
Source: Pro Sound Web
Microphones and preamplifiers are the “chicken and egg” of audio. Want to start a discussion among audio folk? Ask whether mics or preamps are more important.
Later we’ll look at interviews with George Massenburg and John Hardy to get their take on transformer and transformerless preamps and solid state versus tubes. That’s the the main course.
But before we get to them, here’s a few appetizers.
For the entry-level engineer, getting better sound can be very frustrating. You may not be able to hear how great that new mic sounds until you get a better preamp.
Even then, if the mix bus or monitor section is the weakest link, the improvement won’t get to your ears. Even the cable can matter. Some colleagues and I were pained to find this out a number of years ago when comparing a Gefell mic with a Neumann U 87 and AKG C414.
In one studio, we got predictably different frequency responses, depending on whether we used the “house” cable, or Gotham GAC-3, or EMT 2022. Interesting, though, when we tried a similar test in another studio, the results were not so dramatic.
Why? Probably impedance differences. The first studio had a built-in wall panel and snake. The second didn’t. Was it the way the mics and snake interacted, or the way the snake and the first studio’s API console interacted. Or both?
The first studio used API console mic preamps, the second studio had an original issue Mackie 1604. Maybe the Mackie wasn’t open enough to pass the differences.
`Want a simple way to test how responsive your chain is to improvement? Listen to a “humble” Shure SM58 through your existing chain, and then using the same microphone cable, plug the SM58 into your prospective new preamp and come in at line level to your mixer.
If you can’t hear a world of difference, the prospective preamp isn’t that much of an improvement or—and this is a BIG or—something else in the chain is eating up the improvement.
To Tube or Not to Tube?
Tube circuits are sometimes elevated into mythological status, primarily because they were all we had before solid state came along.
But the truth is that a good solid-state preamp sounds much better than a poorly designed tube preamp, and a good tube preamp sounds better than a poorly designed solid-state preamp.
It’s a pretty simple quality issue.
Having said that, if you’re working with audio that has a lot of transient material—the result of pretty much anything you record where you hit something—a tube circuit can “round off” the peaks of those transients less objectionably than a poorly designed solid state preamp which clips the transients. The plate of the tube absorbs some of the loudest transients.
Of course, that “rounding off” of the transient peaks is part of the coloration of the circuit, and technically, it’s distortion.
Arguably, transformers are a throwback from the early Bell Labs days when the input and output characteristics of amplifiers required specific impedances, and because transformers are a great way to stop the flow of DC from one stage to the next.
In its simplest form, a transformer consists of two coils of wire—a primary and secondary coil, wound around a common core. Even though the primary and secondary wires may touch, their insulation keeps them from being directly connected.
Instead, the electrical energy is induced—picked up literally out of the air—because the two coils are so close. Transformers with metal cores also affect the transfer of the electrical energy.
Now, let’s sit back and let George Massenburg and John Hardy answer a few questions for us all.
Ty Ford: Some microphones have transformers at their outputs. Some are transformerless, some preamps have transformers at their inputs, some don’t.
Are there any rules that determine what happens to the sound when mixing and matching transformer and non-transformer mics and preamps, or are the individual circuits so different that simple guidelines can’t be established?
George Massenburg: Well, it won’t surprise anyone to know that I have different rules. Generally, I want to keep a signal as clean and transparent as possible for as long as possible.
I pretty much prefer mics with good output transformers or no transformers, like the Schoeps design.
I can’t really say that I like input transformers on mic preamps. Frankly, if I want a roll-off and low-frequency distortion I’ll add it to the degree that I want it, most often later in mixing.
John Hardy: Generally, either type of mic can be used with either type of mic preamp. Limiting this discussion to the interface between mic and mic preamp, it is mostly the interaction between the output impedance of the mic and the input impedance of the mic preamp that causes audible differences as the result of an “EQ” effect. If those impedances are the same at all frequencies (linear), there will be no EQ effect.
If the impedances are nonlinear—having some degree of inductance and/or capacitance in addition to the basic resistance—in or near the audio bandwidth, there can be an audible difference.
Equalizers use inductors and/or capacitors to create frequency changes. It is not the presence or absence of a transformer that matters, it is the linearity of the impedance.
It’s easier to have a linear impedance in a transformerless circuit, but a well designed transformer can have a linear impedance too.
Ty Ford: What are the advantages and disadvantages of mics with output transformers?
George Massenburg: I don’t like them for any reason. Even in live situations where one might reason that transformers would reduce interference over long distances, I avoid them where possible.
John Hardy: A transformer at the output of a mic can step the impedance of the mic up or down as required. It also blocks the +48 volt phantom supply from getting into the circuitry or capsule of the mic where damage could occur.
Page 3 of 3 It’s a similar situation at the input of a mic preamp.
If there is no transformer at the input of a mic preamp, capacitors are usually used to keep the +48 volt phantom supply voltage from traveling forward into the active circuitry of the preamp where it could cause damage.
Capacitors can cause phase shift at low frequencies. They can also smear the audio signal because of a problem known as dielectric absorption. Some capacitors are much better than others in this regard.
Transformers have their own potential problems, including phase shift and ringing at high frequencies, and core saturation at high signal levels and/or low frequencies. A well-designed transformer minimizes these problems.
On the positive side, a transformer coupled mic preamp has a much higher common mode impedance and a much higher breakdown voltage than most transformerless mic preamps, which results in the potential for a much higher common mode rejection ratio, and the ability to handle and reject much higher common mode signal levels.
This is extremely important where there are high levels of RF or other interference.
Ty Ford: Are there any ways to make better choices about mics and preamps than “Read the specs, listen, if you like what you hear, buy it?”
George Massenburg: The people who “listen” better do better work. There is no short-cut, nor is there a push-button answer here.
John Hardy: There are some recorded tracks available that demonstrate many mic preamps and mics, but since you were not there to hear the original performance, you do not know all of the details of the signal path, room effects, what the original sound source sounded like from the exact mic position, etc.
You must try things under your own circumstances, listening from the mic position and eliminating as many variables as possible.
Ty Ford: There are now tube mics that are quieter than some FET mics. Other than a tube’s absorptive capabilities as a result of plate saturation, have better components and circuit designs made the tube/solid state argument moot?
George Massenburg: Well, I don’t know anything about tubes, but I can tell you that discrete components haven’t evolved as far or as quickly as other semi technologies.
I would really like to have faster, higher-gain, higher-voltage transistors to use, but it just hasn’t happened.
John Hardy: There is certainly much confusion and misinformation regarding the supposed need for tube circuits to “warm-up” the often cold and harsh sound of digital circuitry. The cold and harsh quality is not the fault of solid-state circuitry. It is the fault of crappy solid-state circuitry that happens to sound cold and harsh because it is crappy.
A well-designed solid-state mic preamp can do wonders to warm things up. Actually, a well-designed solid-state circuit is probably bringing things back to “room temperature,” which I think is what most people really want. A tube circuit may be going beyond room temperature to a colored sound quality.
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