Pendulum Audio MDP-1
Vacuum Tube Mic/DI Preamp
 by Michael Cooper - MIX June 2001

   Rather than trying to recreate vintage tube gear, Pendulum Audio designs tube processors that incorporate modern, extended-bandwidth, Class-A circuit designs. Both the MDP-1 tube preamp ($2,495) and the ES-8 variable Mu tube limiter ($3,495) are 2U, dual-channel units that feature transformerless outputs, low noise and a frequency response from below 20 Hz out to 65 kHz (75 kHz for the ES-8). Classy aesthetics, impeccable front panel screening, positive-action knobs and switches, gold-plated I/O connectors, and large, illuminated ANSI VU meters suggest the rigorous attention to quality that lurks below the hood. There you'll find gold-plated switch/relay contacts and tube sockets, polypropylene caps, metal film resistors and custom toroidal power transformers with hum-blocking shields. On both units, the chassis is ventilated through the top and side panels.

MDP-1 Tube Mic/DI Preamp

The MDP-1 can be ordered with either Jensen 13K7 transformers or custom transformers for the balanced audio inputs. (My review unit had the Jensen transformers installed.) Each channel's audio path passes through one 12AX7A and one 6922 tube. On the rear panel, an XLR connector for each channel
accommodates mic inputs and unbalanced 1/4-inch phone jacks service mic/line DI inputs. These DI jacks have a 1-megaohm input impedance when a DI input switch on the unit's front panel is set for instrument input. When this same switch is set for line input, the rear panel DI input jack's impedance becomes 100 kilohm, and a 20dB pad is inserted into the audio path. The circuit path is transformerless for all DI inputs.

Conveniently, the unit's front panel also features DI inputs on unbalanced phone jacks. When you insert a plug into a front panel jack, the rear panel DI jack for the same channel becomes disabled. The front panel DI jacks each have an input impedance of 10 megaohms for instrument inputs and, like the rear DI jacks, 100-kilohm impedance for line inputs. Depending on how much you want to load down your electric guitar or bass pickups, you can plug into a front or rear panel DI input and get a different sound (due to the differing impedances). I found that electric bass tended to have a slightly tighter bottom but less top end "air" when plugged into a rear DI input, as compared to plugging in through a front panel jack. It's great to have such flexibility to get sounds.

The MDP-1's rear panel outputs are unbalanced XLR and 1/4-inch phone jacks, wired in parallel. The transformerless, tube output stage can handle blazing output levels as hot as +35 dBu. A rear panel power switch feeds a soft-start circuit that mutes the audio output and preserves tube life. The three-prong AC cord is detachable. The MDP-1's front panel is replete with useful knobs and switches, yet remains very user-friendly. Separate toggle switches are provided for each channel to enable 48V phantom power, switch in a 20dB mic input pad, flip the mic signal's polarity, and switch between mic and DI inputs. An
additional DI input switch, mentioned earlier, optimizes the DI input for instrument or line sources.

Each channel sports a switched, rotary gain knob-fixed in 3dB steps-that controls the tube gain stage. The gain range spans 30 dB but varies for each type of input: +33 to +63 dB for mic input, +20 to +50 dB for DI instrument input, and 0 to +30 dB for DI line input. Additionally, continuously variable rotary knobs allow you to passively attenuate each channel's output level. Besides allowing you to smoothly bring levels down, these output knobs also serve a creative function. By cranking the gain knob that serves the tube stage and lowering the output gain level, you can overdrive the tubes and introduce subtle distortion. This feature makes the MDP-1 a very versatile beast, allowing you to dial in a variety of sounds from ultra-clean to slightly funky. I've heard cheaper units that offer tube overdrive topologies and many make your tracks sound like bumblebees trapped inside tin foil. The MDP-1, however, gives you that sweet splatter of harmonics you lust for.

Yet another rotary switch for each channel allows you to roll off the low end for mic signals at 10 different corner frequencies from 20 to 180 Hz, with an 11th setting for flat response. Finally, a three-way switch for each channel allows you to source the VU meters to show tube gain (pre-attenuator) or output level (post-attenuator) or to turn the meters off when you're pinning the meters. Sourcing tube gain allows you to gauge whether or not you're overdriving the tubes and by how much.

My first test of the MDP-1 was on male lead vocals. This singer had a very woolly voice, for which my AKG TL-II was the perfect mic. But I've never heard my TL-II sound as good as this. In short, the MDP-1 made my solid-state TL-II sound like an awesome tube mic. The vocals were beautifully articulate, sweetly scintillating, tonally balanced and clear as a bell, yet eminently warm. The overall sound was lush, brimming with nuance and possessing great depth. I got similarly great results using a Lawson L47MP tube condenser mic on another vocalist.

On acoustic guitar, recorded with a spaced pair of B&K 4011 mics, I was, again, completely blown away. The stereo imaging was hands-down the widest I've ever heard with any mic pre. Transient response was outstanding, offering copious detail but without a hint of stridency. For comparison purposes, the MDP-1 was more understated in the upper bass/low mids than a Millennia HV-3 mic preamp, but, nevertheless, was much warmer sounding (love that glowin' glass!).

Next, I A/B'd the MDP-1's DI path against that of my super-pristine Aguilar DB 900 tube direct box - a tough opponent. On electric bass, the DB 900 sounded cleaner and clearer, with a sweeter tone. The MDP-1 sounded slightly honky and veiled in comparison, but also bigger, lusher and more aggressive. Overdriving the MDP-1's tube stage and daisy chaining into an Empirical Labs Distressor yielded a bawdy, burpy bass, gushing with overtones. Next, I DI'd my '62 Strat. Again comparing to the DB 900, the MDP-1 at its cleanest settings produced a tone that had less air and nuance but fuller and creamier low mids, making for a beautifully mellow sound. Overdriving the MDP-1's tube stage to various degrees, I could get ultra-clean or subtly gritty tones. Interestingly, overdriving prerecorded ADAT tracks via the MDP-1's DI inputs (set to 'line') yielded far more subtle results. And, without overdriving the tubes, the line inputs sounded basically neutral - nothing added, nothing taken away.

The MDP-1 is an awesome tube mic pre that also does a great job on instrument DI duties. For the creative engineer, the topology of its tube gain stage and output level control bestow tones ranging from ultra-clean to slightly funky. Outstanding transient response, superior depth, superb spectral balance, unbelievably wide stereo imaging, extended frequency response, low noise and a lusciously warm sound make this fully featured and classy-looking mic preamp a great value at $2,495.

Pendulum Audio, PO Box 339, Gillette, NJ 07933; 908/665-9333;
info@pendulumaudio.com; www.pendulumaudio.com.

Michael Cooper is a Mix contributing editor and owner of Michael Cooper
Recording in beautiful Sisters, Ore.