Dual Mono Microphone Preamplifier
Jim Jordan examines a dual mono microphone
pre-amp from Avalon and finds a true performer
Dual Mono Microphone Preamplifier
Jim Jordan examines a dual mono microphone pre-amp from Avalon and finds a true performer
The recording process begins at the microphone. Aside from y our choice of a trusty SMS7 or an old German classic, it is the microphone pre-amplifier that will ultimately determine just how good a recording can be.
In the electrical portion of the recording chain, the mic pre-amp sets the performance limits for the entire system. No matter how good the console signal path or outboard gear, the noise levels and distortion figures of the audio signal can never be better than the numbers that the mic pre provides.
The performance requirements of a microphone pre-amplifier are quite demanding. It must provide signal level gains of over a million while not adding any appreciable noise of its own, and it should (obviously) not add any coloration or distortion to the audio signal. Since the pre-amp will determine the best possible performance, proper design is critical and evident in the Avalon Design AD2022 discrete Class A dual mono microphone pre-amp.
Please note that 'Class A' is not some type of rating, but a description of how electrical amplification is achieved. This classification system was devised at the dawn of radio and used to describe he operation modes of both audio amplifiers and radio transmitters. Class C is not used in audio, it describes a non-linear mode of operation used for maximum efficiency in transmitter output stages. Class B is also quite efficient, one device (tube or transmitter) handles the positive half of the signal, while a complementary device handles the negative half. When no signal is present (during silence or at the zero crossing of a waveform), both devices are switched off. Although this is highly efficient, the transition between devices in the crossover region is never perfect. Class A-B was devised to overcome these nonlinearities, adding a small quiescent current at low signal levels to smooth out the crossover transition. Class B and Class A-B are often used in power amplifiers where efficiency is a factor.
Class A operation describes the situation where the devices never switch off. The amplifying devices are constantly operated in their linear region above cut-off and below saturation. This is very inefficient, but efficiency in low-power applications is not terribly important, and the benefits are quite desirable. There is no crossover glitch to content with, no need for massive amounts of negative feedback and the distortion components are even-order (the good kind). Whether made from tubes, transistors, or semiconductors, circuits operation in Class A mode proved the smoothest transfer functions and widest bandwidths and are undoubtedly the best choice for pre-amplifiers and other low-power audio circuitry.
Avalon Design has been building discrete Class A mic pre-amps for well over a decade, and the AD2022 is a fourth-generation design, joining the single-channel M5 and replacing the ten-year-old M2 and M2-V. A dual mono unit, the AD2022 shares the same basic construction and look of Avalon's AD2044 Compressor, notably the two large oval windows for the rear-illuminated VU meters. The heavy quarter-inch brushed aluminum front panel is beautifully machined, there are no sharp corners or edges, and the cutouts for the meter windows and the two front-panel instrument jacks are nicely beveled. The panel is secured to the formed steel chassis by large hex cap head bolts in each corner giving it a classy industrial look.
CAN'T SCRATCH THE SURFACE
The front-panel controls are arranged in an intelligent, symmetrical layout. There are four knobs and four switches per channel, with the input functions located to the left of the meters and output functions to the right. The knobs extend a full inch from the panel surface and are spaced widely apart, making them qu9te easy to operate. The input gain selector is a large knob to the left of the meter, and the 12-position rotary switch covers a range of 18 to 62dB in 4dB steps. An identical knob to the right of the meter operates a smooth conductive plastic rotary fader with a center detent, allowing the output level to be trimmed =3dB. There are two smaller knobs beneath these controls, a rotary switch on the lefts selects microphone input impedances of 50, 150, 600 and 1500W (labeled 'mic') with a fifth position to select the front-panel 100kW instrument jack. The ability to change the input impedance from the front panel is a wonderful feature, but the execution is a bit worrisome; unlike the other knobs, there is no front-panel mechanical support for this switch other than its connection to the pre-amp circuit board. The long knobs, while easy to handle, also provide a good amount of leverage; enough to flex the circuit board in the course of normal operation.
The small knob to the right of the meter (securely mounted to the chassis) selects the corner frequency of the 6dB/octve high-pass filter, continuously variable from 30 to 185Hz. Between the large and small knobs are two pushbutton switches. The ones on the left select the 20dB input pad and phantom power; on the right are polarity reverse and filter I/O. The clear plastic rectangular caps house orange LEDs to indicate status, and two additional LEDs add even more color to the front panel. Immediately to the left of the meter is the peak detector LED, which lights green at 0dB and red at +20dB. To the right is a blue LED labeled DC, presumably to indicate the power supply is operation properly. Finally, the large mechanical meters provide a no-nonsense indication of the output level. They are well illuminated and blessedly simple to re-lamp.
ON THE INSIDE
The internal layout is well designed and the construction is solid. The Class A circuitry and high voltages create a considerable amount of heat, and this has certainly been accommodated, undoubtedly one of the advantages of a fourth-generation design. The circuit boards are spaced a full inch from the bottom of the enclosure, and the slotted vents in the top and bottom covers will allow plenty of air to circulate provided that the unit is given some breathing room in the rack. A power supply regulator board is mounted in the back of the chassis with a large heatsink outside the enclosure. The torroidal power supply transformer is housed in a separate enclosure to eliminate any possibility of induced hum, and a ten-foot cable with four-pin XLR-type connectors joins the two units.
The mic pre-amp circuit boards are of heavy multi-layer construction with an entire layer dedicated to grounding. The majority of the circuitry is sealed in a metal enclosure in the center of the board and in addition to providing shielding, it undoubtedly protects proprietary information. It should be noted that the unit ships without manuals or schematics, but the supplied literature sheet reveals that the pre-amp is a two-stage design: The first operates in a cascaded-cascode FET configuration while the second stage amplifier is fully bipolar. You are at the mercy of the manufacturer for repairs, but my experience with Avalon has been quite good in this regard (one of our compressors fell victim to a coffee spill about a year ago, and they turned it around in record time--repaired, recalibrated and ready to go).
There is a considerable amount of hand-wiring in this device, and it is done quite well. The components are of extremely high quality, and the audio wiring to and from the circuit boards is insulated with a high silver content. Sealed relays with silver contacts are used in the input selector, input attenuator, phantom power, polarity reverse and filter inserts, which keeps the signal on the circuit board away from the front panel. Overall, the Avalon AD2022 has the appearance of being crafted, rather than simply assembled--this attention to detail has resulted in a device that performs as promised.
THE TRUE TEST
In some off-hour experimentation, our engineers put the unit up against a favorite vintage Class A pre-amp. Both sounded great, but they reported that the Avalon had a certain clarity and detail in the high end that was desirable.
A better test opportunity arose when engineer Chuck Ainlay was in Sound Stage's Backstage studio in Nashville. Tennessee to work on a live two-hour Richard Marx concert. Featuring a full band and an audience packed with old friends, the concert was shot for video and recorded to a 48-track digital machine, but the emphasis was on the live two mix for a subsequent webcast. Chuck immediately put the Avalon on Richard's vocals, one channel at his guitar-playing position and the other at the piano. Chuck liked the sound well enough for the Avalon to stay in place.
The Avalon Design AD2022 is admittedly attractive for vocals, but there probably isn't a thing that it wouldn't sound good on. This dual mono microphone pre-amplifier is very easy to use, and it sets a fine standard of performance for the rest of the recording chain. The clarity and range this mic pre-amp provides give one a great deal to work with down the line.
The AD2022 performed admirably on the bench, well within its stated specification. Barely down 1dB at 30kHz, the high end exhibits a smooth roll off, and the low end is virtually flat down to 10Hz. The response graphs shows the left and right channels, separated by 1dB for clarity. The upper traces sow the unit set flat, the bottom traces shows the resultant output when the high-pass filters are inserted.
The stated THD and IMD distortion figure of 'nominal 0.05 percent' was also verified. The graph shows THD + N for outputs of 0dBu (signal presence indicator threshold) and +20dBu (clip indicator threshold), and within these operating parameters distortion was certainly within spec, with the right channel performing somewhat better than the left. Distortion is primarily 2nd harmonic typical of Class A amplifiers.
The unit has and ample amount of headroom, capable of outputs up to 36dBu. The upper traces show the THD at +30dBu, where there is a slight increase in high frequency distortion.
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