Mic Preamp Insider

Apr 1, 2002 12:00 PM, BY EDDIE CILETTI


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Despite the kicking and screaming that ensued during the Digital Revolution, the end result is the tasty Analog Renaissance we are currently enjoying. Analog tape still provides the ultimate in sonic airbag protection -- digital is far less forgiving -- forcing recording engineers to pay more attention to every phase, especially tracking. Design engineers, many of whom create gear for their own use, also re-educated themselves, hence the overwhelming number of new microphones and preamps, including those profiled in our report on tube preamps elsewhere in this issue.

This month, we'll examine some of the parameters for evaluating mic preamps, including listening tests, specification translation and a topology overview. While putting the finishing touches on this piece, I received an e-mail requesting my opinion on two preamps in the $850 range. The interactive investigation began with the customer lured by promises of "warm lows" and "silky highs." These and other words like "transparent," "clean" and "punchy" have made their entrance and exit early in this piece.

No review can tell you what to buy without first getting to the heart of your needs and desires. E-mail discussions tend to be brief and hot -- I felt the mailer was going in the wrong direction, but who wants to hear that? It is my hope that this article lays the groundwork for readers to conduct their own investigations.

It's almost impossible to talk about preamps without mentioning microphones. The relationship is symbiotic -- microphone-output levels determine how hard a preamp works, hence the brief detour. Examples of microphone sensitivity and preamp Equivalent Input Noise (EIN) were collected into two tables (no "weighting"), showing these and other specs at a glance.

Let the games begin!

Comparing preamps is no simple task, and Challenge Number 1 is simply assembling a number of units into the same time-space continuum. Challenge Number 2: Our ears are easily misled by level discrepancies. The ear perceives louder as better, so comparisons -- without first attempting to match levels -- are unfair if not outright invalid. By leveling the playing field (no pun intended), the results become more realistic. Embrace the scientific approach; calibrate levels to minimize the variables. That's easier said than done, because of Challenge Number 3: The output of one mic cannot be "multed" to several preamps due to impedance considerations.

Nashville engineer Lynn Fuston tackled all of the challenges during the process of creating a multi-CD series of preamp and microphone tests -- available at www.3daudioinc.com. Fuston's project also included consultations with several preamp designers until all were reasonably satisfied with the test procedure.


Each microphone type (particularly ribbon designs) has a fairly obvious sonic signature, so preamp surfing will require your undivided attention.

Let's assume that you have a known reference preamp and a "typical" gain setting, a starting point that has the unit operating in its most linear region -- good signal-to-noise ratio and ample headroom. Choose a microphone, connect to the reference preamp and feed pink noise through a speaker -- placement between the two must be fixed and repeatable " and then measure the preamp with a digital meter such as the NTI Minilyzer using the Type-C response setting. The mic can then be connected to each of the other preamps one at a time, matching the gain to the reference. (You can return the preamps to separate faders on a console, but this adds many more amplifiers and variables to the chain. Measure all the way through the chain.)

Before recording the results, check each preamp with your own voice while monitoring via headphones, flipping the Polarity switch to yield the "warmest" setting. This is the easiest nontechnical way to test and set polarity, while perhaps revealing one of those "mysteries" behind vocalists who can't hear themselves. (If the headphone signal arrives out-of-phase with the bone-to-eardrum conduction, then the sound will be thin and distant.) Start with "softer" acoustic sources -- like guitar, voice or piano -- repeating the same phrase over and over. Choose a comfortable monitoring and recording level with at least 12 dB of headroom -- if ya got more bits, use 'em -- and stay there.


Assuming consistent performance, listen back to each preamp. I believe the more matched the levels are, the more difficult it will be to hear the difference. This assumes that the preamp is being operated in its most linear (clean) range. Less complex sound sources -- like a nylon-stringed guitar as compared to wound strings -- may make it easier to differentiate between preamp-to-preamp nuances.

For the "airbag" test, start at a distance, banging a snare drum to achieve a similar level, and then move toward the mic until preamp headroom begins to run out. Optimize the level of the recording device one time, taking care not to overload the inputs, because some preamps can generate far more level than the average DAT or CD-R can handle. If you get this far, congratulations (and let me know)! For more excitement, try very high- and low-gain settings to see how preamps behave at those extremes. If it gets too complicated and you're exhausted, it's okay. (That's why Fuston made those evaluation CDs.)


Have you cruised the Net lately? There are prices and pictures galore, but just try to correlate the wealth of information. There is no Berlitz course for translating product specs (I looked), but Neumann does a great job, providing a stand-alone reference area.

Because online product literature can be easily updated, it should be. I found mistakes, discrepancies and vagaries. Manufacturers need to publish specs based on current measuring standards as well as provide background information about the test procedures. Otherwise, how could anyone compare? Assembling information was difficult enough; I can't imagine the challenge if I didn't speak the language.


If noise is an issue, look at microphone sensitivity first. Table 1 compares sensitivity for an assortment of large-diaphragm vocal mics. Sensitivity is specified in millivolts per Pascal (mV/Pa), typically with a 1-kohm load -- the only discrepancy is in the third entry, the AT-3035. At 25.1 mV/Pa, it has the highest output, open circuit, hence the bigger number.

AKG C2000B FET 20 mV/Pa 74 dB 20 dB, A-weighted
AKG C-12VR FET 10 mV/Pa


22 dB DIN 45412
Audio-Technica AT-3035 FET 25.1 mV/Pa (open circuit) -32 dB (ref. 1 V @ 1 Pa) 82 dB, 1 kHz at 1 Pa 12dB SPL, A-weighted
Audio-Technica AT-4060 tube 19.9 mV/Pa -34 dB (ref. 1 V @ 1 Pa) 75 dB 19dB SPL, A-weighted
Marshall MXL-2003 FET 16 mV/Pa 77 dB (ref. 1 Pa) 18 dB IEC 268-4
Neumann TLM-103 FET 21 mV/Pa CCIR-468-3: 76.5 dB DIN/IEC 651: 87 dB CCIR-468-3: 17.5 dB DIN/IEC 651: 7 dB
RØDE NT-1 18 mV/Pa FET


17 dB (unspecified)
Soundelux ELUX 251 tube 15 mV/Pa -36 dBv/Pa


27 dB, un-weighted; 17 dB, A-weighted
Soundelux U i95 FET 14 mV/Pa


71 dB, un-weighted; 82 dB, A-weighted

Table 1: Microphone sensitivity is measured using 1 kHz at a Sound Pressure Level (SPL) of 94 dB, equal to 1 Pascal (Pa).


A preamp in mathematical terms is a Voltage Multiplier. As such, it multiplies noise and the intended signal. Resistors generate noise, so do microphone capsules. EIN refers to the preamp's input circuit noise -- normally, the mic itself becomes part of the equation, but during the test, a 150-ohm resistor is typically substituted; its value must be published for this spec to be meaningful.

EIN = Gain + Noise

You can easily subtract gain from the published EIN spec to find the noise relative to 0 dBx, where "x" is the reference. A theoretically "perfect" amplifier (one that contributes a minimum of noise) will have an EIN of -129 dBu. This means that with a gain of 60 dB, the best possible noise floor will be -69 dBu. Most EIN specs are measured at or near maximum gain. Be suspicious if EIN is listed only as "A-weighted" -- okay if included, but not solo. Look at the fine print if EIN is over "-129 dB."

Note: 0 dBv is equivalent to +2.2 dBu, where "v" is referenced to 1-volt RMS; "u" is ref'd to 0.775 Vrms, and, when applied to a 600-ohm load, yields 1 milliwatt, hence the "m" ref, which is based on the ye olde telephone standard.

Table 2 compares a few preamp specs. EIN is fairly consistently reported. Only Grace Design uses a 50-ohm source impedance (ribbon mic) rather than the more typical 150 ohms for condenser and dynamic mics. The options are un-weighted (broadband response, typically 20 to 22k Hz) or A-weighted (filtered to replicate the ear's desensitized low- and high-frequency response). The old-fashioned method was to simply max signal-to-noise floor. The variables are source impedance (lower yields better numbers), max gain and max output.


The contents behind the front panel can be one or more of the following: tubes, transistors, transformers, discrete or IC op amps -- the choice and configuration pretty much sets the "tone" of the product. Any of the aforementioned topologies can be nearly indistinguishable when designed to achieve a specific goal and used conservatively, but in many cases, "vintage disciplines" are factored in for their character. Even the best designs can be compromised by poor production. Issues such as component quality, wiring, PCB layout and grounding are paramount to longevity, low noise, immunity from radio and television interference, and ultimately, compatibility with other gear.

/ (A-weighted)
Dual-mono ss
-124 dBu @ 45dB gain
53 dB
+31 dBv
Dual-mono tube
-125 dBu @ 45dB gain
60 dB
+32 dBu +15 dBm
MP-2NV ss + xfrmr
-125dBm, 150-ohm source @ 40dB gain
70 dB
+24 dBm
M-5 ss + xfrmr
-126dB, 150-ohm source
64 dB
+30 dBm
VT-2 tube
-127 dBm (tube-dependent); -124 dBm guaranteed
53 dB
+22 dBu +16 dBm
Flamingo ss
-128.5 dB typical; -129 dB
66 dB
+25 dBm
M-1 ss + xfrmr
-129dBu, 150-ohm source
60 dB 75-ohm load
+24 dBu
MP-2 ss + xfrmr
-129dBv, 150-ohm source (-132 dBv-A)
64 dB
+24 dBm
512c 212, 3124 ss
-129 dBu
65 dB
+30 dBu

Grace Design

201/801 xfrmr-less ss
-130dB, 50-ohm source @ 60dB gain
64 dB (70 dB optional)
+29 +26
Table 2: Mic preamp specs.
From top to bottom, in order of improved Equivalent Input Noise (EIN) performance. Note the two highlighted items are vacuum-tube devices with cathode follower outputs, and as such, their behavior under no load and 600-ohm load are noted. Also, a general product code is noted under the model number: "ss" is solid-state, "xfrmr" refers to the use of transformers, while "xfrmer-less" and "tube" should be self-explanatory.


Stepping up from the noise floor, the other end of dynamic range is the overload characteristic, of which there are three basic types -- sonic airbag, "soft" clip or "windshield" headroom. The transition between linear-clean and hard/nasty clipping can be wide or narrow. Remember, analog tape exhibits a gentle softening of transients that's very forgiving. Digital isn't, which is what your ear objects to (when it does). Single-ended Class-A tube and transistors circuits soft clip; however, the tube example that follows has a wider window when the output circuit is loaded by 600 ohms. Ultra-clean, transformerless designs are going to hard clip when driven hard. That's windshield headroom, just like discrete or IC op amps and digital (without "help" or DSP).

The sonic airbag is dramatically demonstrated in Fig. 1 by the single-ended Class-A vacuum-tube circuit in Fig. 2. Using a cathode-follower output stage, it will deliver the maximum sonic airbag effect. Note that in the two examples, the distortion is always round, not edgy. Two preamps in Table 2 are likely to be contenders. Can you find them?


Not all vacuum-tube designs have such a broad, nonlinear region. Pultec EQs and Universal Audio's 175 compressor have internally balanced amplifier designs that are very linear (as are Marantz and McIntosh power amps). Extending the linear region increases headroom but shrinks the sonic airbag. Remember, it wasn't the intention of vintage circuit designers to have their equipment red-lined.


Transformers are very much a part of the vintage/retro mystique, API and Neve being perfect examples, each known for a completely unique sound. Class-A Neve circuits don't have a wide nonlinear range like tubes. Depending on the load (600 ohms or not) or the Bias Adjustment -- the two are interactive -- the output amp can clip asymmetrically by generating the more musical, second-harmonic distortion before completely crashing into the power rail. API's 2520 discrete op amp uses a bipolar, Class-AB output stage that is linear to the rails. (Symmetrical clipping is great for maximizing headroom or for fuzz boxes, but for little in-between.) API's magic comes from the symbioses between the 2520 discrete op amp and its companion output transformer.

Note: An operational amplifier is basically a "black box" treated as a functional amplifier building block (no user-serviceable parts inside) with input, output and power connections. Op amps are essentially linear to the power rails, clipping symmetrically when they run out of juice, not a pretty sound, but then users are expected to know how to set levels!

Transformers are the "static" variation of the analog recording process -- head + tape = transformer. Tape's record and playback EQ curves further "enhance" the saturation characteristics, but that aside, as good as transformers can be, their distortion specs change with frequency. Table 2 includes links to both the Great River and John Hardy sites, where details are provided about how their high-quality Jensen Transformers perform at various frequencies.

A newer and nearly as famous discrete op amp is the Jensen 990 (used in John Hardy preamps). A natural progression from early transistor technology, the 990 coupled with a Jensen transformer has less "color" than the Neve or API combo. Audio started all over again when early IC op amps were introduced. They were no great shakes, but ICs reduced manufacturing costs and increased component density, making the early project studios possible as well as a vision into the future. Modern op amps are quite respectable compared to their '70s-era ancestors.

For more information about Jensen Transformers and the 990 op amp, visit www.jensentransformers.com and go to "/apps_sc.html."


Audio engineering is like cooking: To achieve superb results, ingredients are carefully selected, added at precise times, cooked or raw " every detail is important. And then there is fast fude. All geek matters aside, the emphasis should always be on the production. While it is now more possible to achieve extremely clear and transparent recordings, it seems that our ears like a little distortion just like our tongue craves a little fat " it truly is a matter of "seasoning to taste."

Visit the Eddie archive at www.tangible-technology.com.


Rod Elliott: Elliott Sound Products
www.sound.au.com/noise.htm or www.sound.westhost.com

Michael Hartkopf's Microphone Website

Glossary of Audio Terminology


David Josephson Engineering

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