M-Pulse: Digital Newsletter of M-Audio

Groove Tubes: Microphone Electronics

As we’ve seen in previous installments, the microphone capsule is responsible for translating sound waves into electrical signals. The other important part of the microphone is the head amp that conditions the sound coming from the capsule so that it can be transmitted through a length of cable to an external preamp or console.

Part of a head amp’s job is impedance conversion. (See “A Word About Impedance” for more information on impedance.) The average line-matching transformer found in dynamic or ribbon microphones has to convert on the order of several thousand ohms down to around 200 ohms

The condenser microphone presents a challenge of a different magnitude—converting a signal in the range of two billion ohms down to 200 ohms. This incredible leap is beyond the scope of most output transformers, requiring the addition of a specialized amplifier.

An output transformer and/or amplifier serves as a sort of translator and, in audio, we expect that translation to be excellent in order to maintain frequency response, dynamic range, signal-to-noise ratio and so forth. Just as a professional language translator costs more money than someone who took a few years of a foreign language in school, pro-quality output transformers and amplifiers cost more money than your garden variety ones. (A single transformer like those used in each channel of pro consoles and outboard preamps can cost more than a complete inexpensive multi-channel mixer.) Because the quality of this formidable translation is so critical in a completely professional microphone, all Groove Tubes mics employ high-quality Class A electronics in the head amp.

A Word About Impedance

“Impedance” essentially describes the resistance in a circuit. Water flowing through a pipe is a good analogy to electrons flowing through a wire. Let’s say you’ve got a pump designed to put out 100 pounds of pressure into an eight-inch pipe. If you double the size of the pipe to 16 inches, you get half the pressure. While the pressure is now only 50 lbs, there is no damage to the system. Halving the size of the pipe, on the other hand, yields twice the pressure that the system was designed for. As a result, back-pressure affects the pump, further reducing its efficiency and risking blowing it up all together.

The analogy in audio electronics is a 100-watt amp into 8-ohm speakers. While going to 16-ohm speakers is safe (although it reduces the power), switching to 4-ohm speakers is a fairly sure recipe for blowing up the amp. That’s why guitar amps designed to run into different speaker ratings often have output transformers with 4-, 8- and 16-ohm tapes that condition the signal appropriately.

You’ll also run across the terms “high impedance” and “low impedance.” Guitar pickups and most dynamic mics are high impedance, exhibiting impedance of many thousands of ohms. Low-impedance signals are around 200 ohms. While the high-impedance signals typically exhibit greater voltage, they can only be run through about 20 feet of cable before they begin losing high frequencies (or require amplification to overcome that problem). Low-impedance signals can typically be run much further without detriment.

Tubes vs. Solid State

The head amp can employ either tube electronics or less expensive solid state electronics. Before we can effectively compare these two technologies, it is important to understand some fundamental concepts. There are three main ways to measure how accurately an electronic circuit passes sound: frequency response, total harmonic distortion (THD), and dynamic distortion. Frequency response is the simplest to understand. Here we’re simply talking about whether any highs or lows are rolled off, or if any frequencies are cut or boosted to exhibit a non-linear frequency response. Both tube and solid state electronics can be made without negligible deficiencies in frequency response.

Regarding THD, all electronics induce some kind of harmonic distortion, i.e. harmonics that are not present in the original source. The nature of the harmonic distortion has more to do with the associated circuitry than with tubes versus solid state. Class A circuitry (where all amplifying components handle the entire signal waveform) tends to produce lower-order harmonics. On the other hand, Class B (where the positive and negative parts of the waveform are amplified by two separate devices) tends to produce higher-order harmonics. For this reason, Class A strikes most people as sounding warmer. (All Groove Tubes mics employ Class A circuitry.) That said, the differences in THD between our tube and FET mics are measurable but not audible.

That brings us to the third, more mysterious element called dynamic distortion—something that the industry didn’t even have the technology to measure until quite recently. Dynamic distortion refers to the accuracy or transparency over time, particularly critical regarding the transient at the very beginning of a sound. Take the recording of a finger snap, for example. You can roll off the highs and lows and/or introduce a good amount of distortion, yet still perceive the sound as a snapping finger. Change the dynamic, however, and that snap can quickly lose its characteristic sound. In general, accuracy in reproducing dynamics can make the difference between something sounding full and three-dimensional or flat and two-dimensional.

Ironically, the discussion comes down to measuring things that don’t matter and not measuring things that do matter. Tubes measure greater in THD than solid state. Yet while one can measure the difference between, say .01 percent THD and .001 percent THD—it’s pretty much impossible to hear that difference. On the other hand, it’s difficult to measure dynamic distortion quantifiably, but you can definitely hear it. Solid state electronics exhibit many orders of magnitude more dynamic distortion than tubes. This is a major factor contributing to why tube mics make recordings sound more life-like.

Next month we'll dive into the differences between tube and solid state electronics in greater detail.

This story is excerpted from our free Record Now: Choosing and Using Microphones guide, which covers microphone technology and techniques. It’s available either via download or at your local M-Audio dealer.

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