Your browser is out-of-date!

Update your browser to view this website correctly. Update my browser now


ADK AP-2 Microphone Preamp

Vancouver-Wash.-based ADK Microphones adds a unique twist to its AP-series microphone preamplifiers that sets them apart in the crowded and competitive preamp market: interchangeable components. Calling it "Pop and Swap" technology, ADK's preamp design features a daughter board-mounted input transformer and a socketed op amp package that conforms to the Jensen 990/API 2520 footprint.

Fast FactsApplications: Studio

Key Features: Dual-channel microphone preamplifier; DI inputs; two input impedance settings; swappable transformers and op amps; toroidal power supply; 1-year warranty; made in the USA.

Price: $2,399

Contact: ADK Microphones at 503-296-9400;

Review Setup

Lawson L47/L251, AKG 251, Shure KSM 44, SM 57, Blue Microphones Red microphones.Vancouver-Wash.-based ADK Microphones adds a unique twist to its AP-series microphone preamplifiers that sets them apart in the crowded and competitive preamp market: interchangeable components. Calling it “Pop and Swap” technology, ADK’s preamp design features a daughter board-mounted input transformer and a socketed op amp package that conforms to the Jensen 990/API 2520 footprint. Users can customize the preamps using a variety of preconfigured transformers (available from ADK) and third-party modern and vintage op amps.

(click thumbnail)

The ADK Custom Shop AP-2 ($2,399) incorporates two Class A/B discrete-component mic preamps and an internal toroidal power supply in a one-space rack mount chassis; a single-channel model is also available (AP-1 $1,199).

Each of the AP-2’s channels features XLR microphone input and line level output jacks on the rear panel and an unbalanced 1/4-inch high-impedance instrument input on the front.

The AP-2 preamp presents users with a clear and uncluttered front panel, thanks in part to a utilitarian feature set that does not include metering or clip indicators. A row of four pushbuttons provides control of 48V phantom power, microphone input impedance, an impedance-compensated 20 dB pad and signal phase reverse. When engaged, the impedance selector adds extra resistance to accommodate low-Z sources such as vintage ribbon microphones. The continuously variable gain knob adjusts input gain across a +25 dB to +65 dB range. Rounding out the front panel controls is a power switch with corresponding LED indicator.

Manufacturer’s specifications for the AP-series preamps state an input impedance of 1.5k ohms, -125 dB of equivalent input noise, THD of > .08 percent and a frequency range of 20 Hz to 20 kHz (no ±dB deviation provided).

As mentioned earlier, ADK provides a range of popular input transformers that come preconfigured and mounted on daughter boards for easy insertion into the AP preamp circuit. The input transformer included with the stock unit is from Crimson Audio, with Lundahl, Sowter and Jensen transformers optionally available from ADK. Although ADK does not sell alternate amp modules for the AP-series preamps, there are a number of new and vintage models available on the market with the same footprint (including the Jensen 990, John Hardy 990c, Avedis 1222, Forssell 992, Millennia Media MM99, Melcor 1731 and API 2520 models).

In Use

I first saw ADK’s prototype AP-1 preamp at the AES2004 convention and was immediately intrigued by its “Pop and Swap” concept. As a long-time DIY enthusiast, I am very familiar with the range of possibilities afforded by circuit modifications and component substitutions. I liked the fact that the ADK preamp provides audio engineers with some of the beneficial experience of DIY experimentation without the hassle and danger (primarily to the circuit itself) of wielding a soldering iron.

For this review, ADK provided the dual-channel ADK AP-2 preamp with stock amp modules and transformers, plus one each of the optional Lundahl (Sweden) LL 1538XL and Sowter (UK) 9820c transformers.

I used the AP-2 on a number of recording sessions over the course of a month, on a variety of sources. In general, the operation of the AP-2 is as simple as it comes: the buttons and connections are clearly labeled and the layout is completely functional and efficient. From a technical standpoint, the build quality, circuit design and implementation are top notch.

To get a feel for the preamp’s performance, I used the AP-2 in its stock configuration for the first week. Upon using the AP-2 for the first time, my initial observations were that it was impressively quiet (noise-wise) and very loud (gain-wise).

As always, describing how a preamp sounds – an inherently subjective judgment to begin with – using the written word is tricky business. But here goes…

In general, the stock AP-2 produced pleasing recordings, with plenty of mid and low-mid detail and a range that rounds off slightly in the extreme ranges. The preamp definitely has a character of its own, and a shape that leans a touch towards the dark side compared to other preamps I regularly use. In its stock configuration, the AP-2 produced some very good recordings, though I may not recommend it to someone as his/her only outboard preamps.

On the other hand, perhaps it is the perfect preamp to own, thanks to its interchangeable amplifier modules and input transformers. Changing the components is a reasonably simple affair: unscrew the top panel (5 screws), lift out the component in question and pop a new one into the socket. Of course, if the preamp is wired up and screwed into an equipment rack, swapping out parts is a bit more prohibitive. Speaking of racks, one nice touch is that plugging in to the front-panel DI jack automatically disables the rear panel mic input, so there is no problem having the preamp wired to a patch bay.

After a week of using the stock configuration, I swapped out the two Crimson transformers for the Lundahl and Sowter ones. While the initial difference in tonality between the transformers is probably not as dramatic as swapping out the amplifier module (or both components), the distinction was definitely noticeable. With more use, the distinctions became more obvious, and matching the various transformers to the mic, recording source and ultimate use/placement in the mix became a creative and fun process.

Several weeks and numerous sessions later, I established a clear preference for the optional transformers, though I found the Crimson transformer the most appropriate choice on several occasions (especially DI input of synths and acoustic guitar pickups, tambourine, certain rock vocalists, and cowbell). In general, I tended to prefer the Sowter on rock and roll elements (especially guitar and bass cabinets, vintage keyboards, snare drum, piano and vocals). I found the extended range of the Lundahl transformer worked best on solo and small ensemble recordings (especially good on strings, solo piano and drum overheads).

Of course, you may find that a completely different set of preferences emerge – that’s the beauty of being able to do your own component experimentation within the comfort of the AP-2 test bed.


In the AP-series preamps, ADK Microphones Custom Shop has produced a high-quality discrete-component mic preamplifier and instrument DI that falls easily in league with other outboard preamps in the $1,000-per-channel range. By swapping out the input transformer and op amp with commercially available and/or vintage parts, the ADK preamp can essentially reinvent itself, opening the door to wide range of tonal configurations.