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Matching Microphones and Preamps: A Crucial Combination


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Matching Microphones and Preamps: A Crucial Combination

 

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.

 

Stronger Chains

 

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.

 

DIY Testing

 

`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.

 

Transformers

 

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.

 

Link: http://bit.ly/YwRSCS

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First the performer + performance and their environment, then the mic and where it is in relation to the sound source.  The preamp is far less important than those factors, esp nowadays when even prosumer preamps are plenty clean enough.  Preamps for "color" are a different matter, and generally not of much interest to location dialog recordists.  

 

philp

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There has been a huge increase in quality of components from yesteryear to today.  Also, many preamplifier designs that were advanced back in the day have been branded into the minds of today's designers and the design resources, so that the Hardy preamp design is really a flavor of the tried and true Jensen Twin Servo dual opamp design, also utilized by JLM Audio in their kits and countless others.

 

http://www.jensen-transformers.com/as/as083.pdf

 

Preamp designs such as these, have been modified, distilled, and packaged even sometimes on a chip, if not exactly, utilizing some of the same core design principles.  So when a designer makes a preamp today, even if "from scratch" more likely than not, he is leveraging the experiences of the decades of designers before him, and the fact that today's electronics are greatly superior to those from the past.

 

A company that makes cheap components, such as Presonus or Behringer, actually utilize designs that are based on excellent templates, perhaps using slightly cheaper components, which are much superior to the parts available back in the 60's to 70's.  So to some extent, even the entry level gear is performing on a level much elevated than entry level gear from past years, so the differences will not be so extreme.  In fact, it is the distortions and out of envelope performance characteristics that mostly differentiate one design from another and produce pleasing non-linear results.  Those differences are mostly audible approaching or perhaps slightly exceeding the designed performance envelope, so a Presonus preamp might not sound significantly different than a Neve preamp when used at moderate levels, but the differences will be more evident as the gain is cranked up or the performance becomes more dynamic.  It is ironic that Rupert Neve, when designing the 1073/1081/1084, would have been thrilled with the performance of the Presonus preamp, but now that the limitations of the 10## preamp are burned into our auditory consciousness, it is THAT sound we still seek.

 

There is not one preamp for everyone.  George Massenburg likes preamps that have a neutral frequency response, high slew rate, low noise, high gain, and low distortion.  Many people who work with rock music, as one example, welcome preamps that exhibit the exact opposite traits (except for self noise).

 

Solid state, tube... not an issue for me, it is the design itself that is more important, not the type of opamp or rectification used.

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The article has some glaring inaccuracies and some good things also. Keep in mind that the ultimate "receiver" that every one uses as a test instrument is a stretched piece of thin leather at the end of a resonate tube connected to three bones that are then coupled to a fluid filled filled coiled tube that derives signals by moving crystalline "hairs" that fire off various nerves that then go off to a processor that may be having a bad day. Also, by the time the owner of these dual receivers has enough wherewithal to buy a high end sound system, a lot of these nerves and hairs are broken off.

 

On the other hand, several professional papers have pointed out that emotion plays a huge factor in how sound is perceived. Basically, my $12,000 speaker cables sound better than a piece of zip cord and don't confuse me with facts.

 

I am a great believer in double blind tests; all else is equivalent to drinking "an amusing little wine". Double blind tests are rarely done because they are so ego shattering.

 

Best Regards,
Larry "Lead Ears" Fisher

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do condensers ever have them? 

 

Some can, depends on the design.  I just popped open my u87i, and noticed that it does have some sort of transformer in there.  I'm not an expert on the design, so don't know what function specifically it performs.

 

post-1336-0-91978900-1363623428_thumb.jp

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Dear All,

I am a keen audiophile hobbyist.

I have been since I was a boom op.

Having listened to the differences between interconnects,speaker cables and mains sources I could never understand the 'Professionals' I worked for who used mains when they could be on batteries,had a box of unmatching and different era XLR mic cables and different tails on their mic mounts.

When I started mixing I began earnestly looking at ways to make my dialogues 'better'.

If I could get the mic closer to the edge of frame,thus reducing gain and background noise I may gain 15 percent better dialogue.

If I could get the genny parked where the mics couldn't hear it I may gain another 10 percent.

If I could remove footsteps by using carpet wherever possible I may gain another 10 percent.

If I could record without hitting the limiters or using EQ I could gain another 5 percent.

If I could make sure all my equipment sounded as good as its potential allowed by choosing a high quality cable and connectors and using it throughout my kit,from mic cables,to tails,to interconnects so it all matched and run on batteries wherever possible I could potentially gain another 5 percent.

In the film/sound industry there are so many factors beyond our control that can have a huge impact on the sound we record

Ie- level of vocals,width of frame,location noise,costume noise

That I want EVERY single percent of what I CAN control.

This ethos came,I think,from being an audiophile and looking for tiny margins and adding them up.

I think I kind of stayed on topic here.....

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Congrats on all the many many practical steps you take to record the sound for your films--genny, carpet, limiters, proximity and even high-headroom preamps and other electronics --the effect of all that stuff is very provable and audible:  they DO add up.  (Congrats also on having the diplomatic technique to get the cooperation needed to get it all done.)  The audiophile cable-mystic kind of stuff has yet to ever be proved in repeatable non-observer-biased tests, no matter how many people Monster Cable sues over the issue, etc.  I do think that stuff makes buyers FEEL better about their stereo systems, and that is a desirable effect for sure.

 

philp

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Philip-

I know,I have had many discussions with my 2nd Boom op who comes from a science background and will always search for evidence and is adament a cable either carries a signal or does not and cannot have an effect on sound.

I stand by the fact I *think* I can hear the difference between my transparent speaker cable and piece of bell wire on my hi-fi- thus high quality mic cables all matching *must* make a difference.

But the bottom line is- I cannot prove it ;-)

But it makes my OCD bearable.

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Rich,

Thank you for your kind words.I am constantly looking for a sonic 'edge' and a way to try and redress the balance lost in our craft by the fact we have to fall into line behind the visual process of film making.

I don't see this as a negative,I see it more as a daily exciting challenge.

I can live with the things I cannot do anything about -

But the things I could have prevented ( a ballast on the set,a close up with foot scrapes on,a location dept generator suddenly appearing closer to the set than the electrical departments well placed silent genny)- well they honestly keep me awake at night.

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Philip-

I know,I have had many discussions with my 2nd Boom op who comes from a science background and will always search for evidence and is adament a cable either carries a signal or does not and cannot have an effect on sound.

I stand by the fact I *think* I can hear the difference between my transparent speaker cable and piece of bell wire on my hi-fi- thus high quality mic cables all matching *must* make a difference.

But the bottom line is- I cannot prove it ;-)

But it makes my OCD bearable.

Exactly my point, and I honestly see that as a good thing all around.  If it makes you feel better and more confident then you'll do an even better job.  I argue with the "audio scientists" frequently on this issue--the human factor, which I take very seriously.  

 

philp

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Love Simon's and Larry's posts!  But one quick note on cables.  The only difference between cables I've ever noticed was between normal mic cable (2 leads and a shield) and star quad cable (2 twisted pairs and a shield).  RFI and EMI are the most obvious culprits in mooshing up the sound we record.  Star quad goes a long way towards eliminating it.

Billy

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  • 2 weeks later...

" as do many other dynamic mics. "

dynamic mic's are transformers pretty much by definition...

 

" Do condensers ever have them? "

yes, there are even mic's available in transformer and transformerless versions. (AKG 414)

 

" today's new mixers are more concerned with bargains, than with quality. "

but not nearly so much as today's movie-makers

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Dear All,

I am a keen audiophile hobbyist.

I have been since I was a boom op.

Having listened to the differences between interconnects,speaker cables and mains sources I could never understand the 'Professionals' I worked for who used mains when they could be on batteries,had a box of unmatching and different era XLR mic cables and different tails on their mic mounts.

When I started mixing I began earnestly looking at ways to make my dialogues 'better'.

If I could get the mic closer to the edge of frame,thus reducing gain and background noise I may gain 15 percent better dialogue.

If I could get the genny parked where the mics couldn't hear it I may gain another 10 percent.

If I could remove footsteps by using carpet wherever possible I may gain another 10 percent.

If I could record without hitting the limiters or using EQ I could gain another 5 percent.

If I could make sure all my equipment sounded as good as its potential allowed by choosing a high quality cable and connectors and using it throughout my kit,from mic cables,to tails,to interconnects so it all matched and run on batteries wherever possible I could potentially gain another 5 percent.

In the film/sound industry there are so many factors beyond our control that can have a huge impact on the sound we record

Ie- level of vocals,width of frame,location noise,costume noise

That I want EVERY single percent of what I CAN control.

This ethos came,I think,from being an audiophile and looking for tiny margins and adding them up.

I think I kind of stayed on topic here.....

Wow simon, that s a great explanation. I want all my %!!

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