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The Little Amp That Could

We drove the 1-watt impamp through unconscionable conditions beyond its design parameters, we’re evil
by Danny Kaey

At the 11th hour, Slimdevices could not deliver for review its Transporter, a wireless digital network hub and DAC aimed at the growing portion of the iPod generation that has begun to want more (musicality) from its portable devices. The Transporter review is scheduled for the future, but in the meantime, out of the blue, TIE’s USA Editor, Jerry Kindela, suggested that I review what one blogger called “The Boom Box That Apple Should Have Released.” I was all over it.

The iMPAMP, as it is called, is the brainchild of noted guitar-effects pedal wiz Zack Vex. After developing a following among guitar players with his funky-painted pedals, Zack turned his attention toward creating a purpose-built mini-amp for those who wanted more music from their portable players. For Zack, however, a mere solid state or similar design would not have been suitable—he’s a tube lover—and so he decided to create the world’s smallest vacuum tube stereo amplifier. Yes, you read that correctly. What Vex wanted to design was a tiny tube amp that would have enough juice to power your typical bookshelf speaker with a single input source. The result of Vex’s quest is the iMPAMP, a tiny 1-watt tubed amplifier that takes up very little real estate and can drive a pair of decently sensitive bookshelf speakers sited, preferably, either side of a computer monitor. 

To work the iMPAMP, the source must include some type of gain adjustment as the amplifier has no built-in attenuation of the incoming signal. This poses no problem for the intended user as the most likely source is the iPod, or any other digital player with output attenuation. 

Retailing for $700 (Vex notes that virtually all dealers will honour a street price of approximately $550), the iMPAMP is built to surprisingly fine standards. Its casing is cast aluminum, hand-punched and lacquered and it sports high-quality acrylic faces rising out of the amp’s front and rear edges. The front acrylic face carries a v-cut mechanically engraved Z.Vex logo that has been white silk screened, providing what Vex calls a “much more jewel-like look” than the laser cut approach he tried earlier on. The amplifier’s rear is as basic as one can get: a pair of RCA input jacks, screw-terminal strips for the speaker cable (very old-school but very efficient for this design) and an input plug for the 12VAC 1000 mA wall-wart power supply. The amp’s dimensions are 4.4" x 3.0" x 3.0". 

Two blue LEDs in front of each 6021W output tube indicate power status and power output. When the LEDs begin to flash too overtly, its time to turn down the source gain knob, as the flash indicates that you’ve entered the clipping zone. Incidentally, the LED glow refracts and highlights the Z.Vex logo in a soothing sort of way—beautiful to gaze at.

The specs are almost surreal: At 1watt output per channel, the iMPAMP claims a frequency response of 10Hz-20kHz +0/-2db; S/N 80db at 1khz and adjustable input level of -20/+4db (you can easily adjust sensitivity with the trimpots found on the underside of the amp). 

I decided to treat the iMPAMP unfairly, it as if were an audiophile-grade amp, which meant using it to drive highly efficient (101dB) Zu Druid loudspeakers. (See the sidebar for a review much more in keeping with its design orientation.) 

Set-up was a breeze. I connected my iPod to the iMPAMP and connected the speakers using a Home Depot power cord as a speaker cable (remember, the output terminals of the iMPAMP will not accommodate standard spades or banana plugs). To be honest, I knew not to expect miracles from this little one-hitter, 1-watt wonder, yet, my first reaction was rather positive. Playing back a mix of lossless and highest quality Lame encoded MP3 files through the iPod, the sound was rather filling, large in scale and seemed to offer a good sized image—that is, the iMPAMP offered the broad strokes of an aural landscape. 

Playing back a myriad of different material, from rock to jazz to pop to classical, the initial reaction was the same: this little thing is pretty darn cool for what it is. However, those expecting some sort of audio nirvana from the iMPAMP would be misled. While able to play at fair and moderate output levels, the amp’s design limitations are quickly reached when pushing it hard(er). With anything particularly bass heavy, the iMPAMP simply could not supply the requisite power to deliver resolution and character to bass notes. The midrange, however, is another matter. 

On his latest, and last release, American V, Johnny Cash is at the end of his phenomenal career. The death of his beloved wife is clearly dragging on him and yet his voice is still unmistakable. Played back on this system, the iMPAMP adds that certain lush tube quality we all love and cherish; masking vocal artifacts, the presentation is satisfying and pleasing. I have always said that I’d rather take the overly lush and bloomy midrange of a tube amp over a bad solid state design any day and this proved my point. Playing some of my favorite classical string selections, I couldn’t help but enjoy the sheer musicality produced by this tiny amplifier. Lush, sweet and sexy all came to mind as I listened. Never mind extreme definition or detail; one simply can’t expect these attributes with such an amplifier design. Sound staging and overall imaging are quite solid if not spectacular. Take for example Duke Ellington’s “Cong-Go” from Piano in the Foreground (Legacy Recordings 6759669), on which Ellington and crew take charge of a very intimate setting. A typical early stereo era hard left/hard right recording, bass and piano sit to the left, drums on the right. In my system, the sound stage is realistically sized with instruments having clear, well-defined locations. Here, the iMPAMP presents a more generic, smaller venue feel, with instruments seeming much closer together than the recording truly offers. Front to back density also suffers somewhat, presenting a sound stage that appears to be a good deal more crowded than I hear when I listen through my reference system. Obviously, testing the iMPAMP in this manner is not fair. The Druids note a minimum 8 ohm load (12 ohm nominal), while the amp’s claimed one watt is measured at 12 ohms. At 8 ohms, the output drops to 0.750 watts. Despite all these issues, performances like Duke Ellington’s have a pleasantness to them, an appeal, especially when one begins to grasp that all of this lush and sweet music—at least in the range where the amp is effective—is being driven by a one-watt (or less) wonder. 

Synopsis & Commentary
Overall, the iMPAMP is quite an interesting design, one that no doubt should find lots of takers, particularly for its intended use. I wish I could have tested it with the new Usher bookshelf speakers, which offer amazing value for the money; or some Paradigm or similar loudspeakers (you can find Zack Vex’s speaker recommendations on his website, [ ] On the other hand, running the iMPAMP with the Zu Druids and an iPod had a certain “cool” factor that many similarly priced products never could match. 

In the end, buying the iMPAMP will be a flip of the coin between features and the sheer coolness factor. If you intend on running it as intended, at the street price, I’ll take the iMPAMP’s coolness any day of the week! 

(Editor’s note: See the sidebar below for a different perspective.)


iMPAMP  Z.Vex Effects

Z.Vex Effects
$700 US  
Dimensions Weight


According to Zack Vex, “The input preamp section consists of two OP275 high slew-rate operational amplifiers, long revered by audiophiles as a very sweet and musical chip. These operate on 40V rails and drive the two push-pull triode output power amplifiers, which are independently fed by traditional linear power supplies using two torroidal power transformers inside the case. These provide step-up for the high voltage independent linear B+ (plate) supplies and auto-former step-up for the 40VDC rails on the op amps. “Bias is achieved by using the blue LEDs on the cathodes of the tubes.” It is the tube complement that is of particular interest. Each channel employs a JAN-PHILIPS 6021W subminiature military grade tube, with tube life rated at a rather spectacular 15,000 to 20,000 hours. The lifetime figures, however, need to be taken in context. Lacking a power on/off switch, the iMPAMP is designed to be plugged in virtually 24/7, which means the tubes will begin to lose their curves in about two years. However, Vex will readily replace the pair (he has hundreds of 6021Ws on hand) or provide you with replacement tubes and an insertion tool (for a fee, natch) if you’re a DIYer. Vex stresses that unplugging the iMPAMP when not in use will naturally extend tube life. Vex further notes that he “loves this tube. It just has a very sweet tone and remarkably high plate dissipation for such a small envelope.” Of note is the fact that the same tube is found in Audio Aero CD players.

Au Naturel  [note for helene, there should  be an accent over the 'e' in Naturel. Please refer to the magazine - pg 102] Asking the Z.Vex Effects iMPAMP to drive a tower speaker array like the Zu Druids, as is the case in the main body of this review, is not unlike asking Paris Hilton to stop emoting, “That’s hot!” Actually, that’s a bit of an unfair comparison, as the iMPAMP fared relatively well in presenting the essence of music—the all-important midrange—even though it simply couldn’t muster the power to clearly articulate the rest of the frequency extremes or deliver precisely the image outlines of which we’ve become so enamored when asked to drive the Z Druids. The Inner Ear is an audiophile publication, so putting the amp through that grueling test was worth the effort, but here let’s look at how the iMPAMP performed when used as its designer intended: driving a small pair of bookshelf speakers, flanking a computer display.

For this portion of the review, the iMPAMP was used to drive the B&W LM1 bookshelf speakers, a relatively easy load (91dB, 8 ohms) extending from 75Hz to 22kHz. Sources included a Sony Vaio computer and the 30G Apple iPod. 

While the iMPAMP generates 0.75 watts into an 8 ohm load, that was plenty to drive the LM1s to the point where writing this review became quite difficult. Over and over again the iMPAMP/LM1 combination drew me into the music, and I could sense Editor in Chief Ernie Fisher breathing down my neck to hurry up. Detail, wonderful extension at the high end, a lovely, even seductive midrange, and more than enough harmonic detail fleshed out the presentation, whether it was Frank Sinatra’s voice singing plaintively on Only the Lonely [Capitol73434 94756 2 5], the electronica/live orchestral density of The dZihan & Kamien Orchestra: Live in Vienna [Six Degrees 657036 1105-2] (reviewed in this issue), or Lee Ritenour’s dexterously light jazzy compositions on This Is Love [i.e. Music 314 557 290-2]. No matter what popped up from my iTunes playlist, the sound was never less than musical.

While some may say that 75Hz isn’t low enough to allow musical enjoyment, my answer is, it’s low enough to get an aural snapshot that delivers plenty of enjoyment. Likewise, while one won’t obtain too much depth and instrumental placement—though there is some sense of these qualities—more important is the sense of rhythm, pace, line and flow. In most situations, the speakers will be no more than two feet apart, so how much stage depth can be expected—truly? 

For a lark, I replaced the LM1s with a pair of Optimus Pro LX5s, the small Radio Shack speakers that caused a brief buzz among audiophiles some years ago. A bit on the warm, somewhat rich and dark side, that’s precisely the character that came through. The iMPAMP did not appear to impose any character of its own on these bookshelf speakers, other than, perhaps, its tube warmth. 
For my money, the iMPAMP works magic. Thanks to its tube architecture, this tiny monster reveals the essence of music, pure and simple, and for that reason, Z.Vex is getting a check from me. And Ernie, sorry about the delay, but after all, it was about the music.
—Jerry Kindela

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