Dual Mono Microphone Preamplifier

By Russ Long for Pro Audio Review - All Rights Reserved 3/1/01

I have long respected Avalon's product line. The company's no-compromise design approach, as well as its commitment to discrete, Class A circuits, is admirable. I could not resist the chance to put the new Avalon AD2022 Dual Mono, Pure Class A Preamplifier to work.


The AD2022 ($3,000) is a fully discrete, symmetrical, Class A microphone preamplifier that includes all the sonic strengths of the renowned M2, M22 and M5 preamplifiers. It also has several additional features, including an improved high-voltage Class A regulated power supply, selectable impedance, Hi-Z instrument inputs, silver wiring, double-plated circuit boards and a variable-passive high-pass filter - all packed into a 15-pound, 12-inch deep, 2 RU box - and is designed to provide the utmost in audio quality.

The rear panel of the AD2022 features a female XLR connector for each channel's microphone input. Each channel is equipped with two male XLR connectors for audio output. The first is balanced, pin-2 hot; the second is unbalanced, pin-2 hot, pin-3 ground.

A 4-pin male power input connector allows the 8-foot power cable to deliver the 90V AC that is needed to power the box. The front panel of the unit has identical controls for both channels.

The 7-pound remote toroidal power supply is built into an external 5-inch, by 7-inch, by 3.25-inch box. The supply is powered via a standard IEC connector and is 100V to 240V, 50-60 Hz selectable.

The 12-position stepped input knob allows the input gain to be adjusted from +18 dB to +62 dB in 4 dB increments. An identical knob provides output control allowing the signal to be trimmed +/-3 dB. The maximum output level is +36 dBu.
A small rotary switch provides the 2022’s input impedance selection. The ability to tailor the preamp’s impedance to a given microphone is a unique feature that makes it possible to achieve the utmost in performance from the microphone. This five-position switch selects input impedance as 50, 150, 600 or 1,500 ohms. The fifth position selects the front panel’s 100 kilohm 1/4-inch jack for Hi-Z instrument input.

Each channel features four switches that light when activated. The +48V switch activates 48V phantom power. The input attenuator switch activates a -20 dB passive attenuator. The Polarity switch reverses the phase of the signal. The Filter switch activates the variable high-pass filter. This 6 dB/octave passive filter is variable from 30 Hz to 185 Hz via a second small rotary switch.

The 2022 features the same visually striking oval VU meters found on other Avalon boxes.


In Use

I put the AD2022 to work under a variety of circumstances. It always yielded fantastic results. The box is built like a tank and it sounds outstanding.

I used the preamplifiers on a wide variety of acoustic instruments, including mandolin, violin, banjo and acoustic guitar. In each circumstance I was more than pleased. The selectable impedance is a godsend considering my love for ribbon microphones. I found the new Royer SF-1 ribbon mic the perfect match for the 2022.

I also had great results using the AD2022 on vocals. I recorded vocals through the preamp using a Sony C-800G, an AKG 414 and an Audio-Technica 4033 — always achieving killer tracks. The microphone preamplifier performed equally well with male and female vocals.

The high-pass filter sounded fantastic. I was amazed at how little, if any, equalization was required when it was adjusted properly. The filter leaves none of the phasey characteristics associated with cheap HP filters and even when used drastically, the sound retains its smooth, tight, well-defined characteristics.

I also had good results recording keyboards and bass directly through the front of the machine. These instruments' inputs always had a full, punchy sound. I used the box extensively for several days and I never encountered a situation where it didn’t perform tremendously.


The Avalon AD2022 is a no-compromise microphone preamplifier with every feature imaginable and a sound quality that cannot be touched.


As soon as I lifted the Avalon AD2022 out of its box, I saw that it was a classy piece of equipment. It looked solid as a rock. Peering through the perforated top plate, I was amazed at the number of components Avalon devotes to what seems to be a simple enough task. The AD2022 is just a two-channel microphone preamp; and the guts do not even include the power supply, which is packaged separately and connects to the main chassis with an umbilical.

Inside of AD2022

There are balanced microphone inputs on the back, along with balanced and unbalanced outputs — all on XLRs, interestingly enough — and phone jacks on the front that serve as Hi-Z unbalanced inputs. Each channel has a 12-position input-gain switch and an output trim pot (+/-3 dB with center detent). The input-gain switch is marked from 18 to 62 dB in increments of 4 dB to indicate the gain change from one setting to the next. In reality, the steps varied from 3.4 dB to 4.1 dB and the markings did not indicate gain. With the switch at 38 and the output control at 0 (the settings I adopted as standard), the gain from the microphone input to the balanced output measured 43 dB.

Except to check maximum output, gain and impedance at the unbalanced out, I made all measurements at the balanced jacks. Maximum output (in volts), voltage gain and source impedance to the unbalanced jacks are exactly half what they are to the balanced outputs.

Speaking of maximum output, the AD2022 can fry eggs! I measured +36.2 dBu at clipping at the balanced output; that is more than 50V! The impedance is nice and low too!

There is an attenuator button marked 20 dB; engaging it reduced the level by 17.8 dB, not 20. Not a big deal, but it is so simple to design a 20 dB pad that I was surprised to find any discrepancy. The inputs are virtually overload-proof. With the pad not engaged, the output clips before the input, even with minimum gain. With the pad engaged, I managed to clip the input circuit — but not until I fed +24.2 dBu (12.6V) into the microphone jack! I could not overload the Hi-Z input no matter how I tried; output always clipped first.

Avalon Design provides a choice of microphone loading via a switch marked Mic, 600, 150, 50 and Hi; the last position on the switch selects the front-panel Hi-Z input. Input impedance in the Mic position is a relatively high 5 kilohms, which means the microphone runs unloaded.

The impedance at the other positions was reasonably close to the markings: 625 ohms, 190 ohms and 80 ohms, respectively. The unbalanced input loads the Hi-Z source with 81 kilohms. For the record, frequency response is essentially independent of the load-switch setting.


Figure 1

Figure 1 (above) plots frequency response from the microphone input using maximum, minimum and standard gain, along with the response from the Hi-Z input using my standard setting of 38 on the switch. The curves don’t differ all that much. There is a bit more bass roll using the Hi-Z input, but it’s only 0.2 dB at 20 Hz. As usual, high-end response is best with minimum gain, but there is less change from high gain to low gain (-1.05 dB at 20 kHz with high gain, -0.72 dB at 20 kHz with low gain) than often is the case.

If you compare curves at 30 kHz, the lowest is for the Hi-Z input with a gain setting of 38. The next one up is for the Mic input with maximum gain. The solid one is for the Mic with standard gain of 38, and the top curve is for the Mic with a switch setting of 18 (minimum gain).


Figure 2

Figure 2 plots THD+N versus frequency at two output levels (+4 dBu and +24 dBu) using the microphone input and my standard gain setting of 38. The results are exemplary at +4 dBu where THD+N is below 0.010 percent from 75 Hz to 10 kHz on Channel 1 and from 50 Hz to 10 kHz on Channel 2. No complaints at +24 dBu considering that this is about as hard as you are ever likely to drive the preamp.

THD+N is less than 0.06 percent from 40 Hz up on Channel 1 and half that from 30 Hz up on Channel 2. From top to bottom, the curves are Channel 1 at +24 dBu, Channel 2 at +24 dBu, Channel 1 at +4 dBu and Channel 2 at +4 dBu.


Figure 3

Figure 3 plots THD+N vs. output at 20 Hz, 1 kHz and 20 kHz using the microphone input and standard gain. As you can see, +4 dBu corresponds closely to the point of minimum THD+N; at that point the bottom curve was taken at 20 kHz, the solid (middle) curve is for 1 kHz and the top curve is for 20 Hz. Although distortion rises in the bass, all three curves pass through 1 percent THD+N at the same level (+36.2 dBu). This indicates that maximum output level is independent of frequency.

The Avalon Designs AD2022 cuts the mustard where it counts — in noise performance, immunity to input overload, absence of distortion, maximum output level and frequency response — and I dare anyone to hear any crosstalk in this preamp. Input impedance, while not exactly on the mark, is close enough for all practical purposes. I would not pass up this microphone preamp.

Edward J. Foster


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