With the increased reliance on digital recording media, the demand for vacuum-tube condenser microphones, those beloved tools capable of imparting to
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With the increased reliance on digital recording media, the demand for vacuum-tube condenser microphones, those beloved tools capable of imparting to

With the increased reliance on digital recording media, thedemand for vacuum-tube condenser microphones, those beloved toolscapable of imparting “warmth” to digital tracks, hasnever been greater. Unfortunately, vintage tube mic prices havesoared, thwarting the desires of many personal-studio buyers. Evenamong the spate of new tube mics, prices rarely fall below $1,500,and typically they're much higher.

The GT Electronics AM40 provides some relief at the relativelyreasonable price of $999. The deal is more attractive yet becausethe mic's design helps it cover a lot of ground. Rather thanqualify as a large-diaphragm mic (usually one with a diaphragm aninch or larger in diameter) or a small-diaphragm mic (whichtypically has a ½-inch or smaller diaphragm), the AM40 isdescribed as a medium-diaphragm condenser, thanks to its¾-inch diameter diaphragm. This, in addition to its tubecircuitry and top-address design, puts the AM40 somewhere betweenthe usual “instrument mic” and “vocal mic”categories. (The GT AM30, which employs Class A FET electronicsrather than tube circuitry but is otherwise nearly identical to theAM40, was included in the EM comparison review “To Tell theTruth” in the March 2000 issue.)

I tested a pair of AM40s for this review. Each mic arrived in afoam-filled aluminum flight case, sheathed in a drawstring pouch ofsilky black fabric. Also packed neatly inside each case were thePSM power supply and AC-power cord, a hard-mount mic clip, and a25-foot, 6-pin XLR cable.


The AM40 is a solid-feeling mic with a medium-diametercylindrical body and an attractive burnished silver finish. Twoswitches are located just below the mic's removable grille cap, onefor a 15 dB attenuation pad and the other to activate a 75 Hzhighpass filter.

The AM40 ships with the AMC1 cardioid capsule. I also tested theAMC2 supercardioid and AMC3 omnidirectional capsules ($129 each),which can be purchased separately. The capsules are easy to switchout; however, read the manual carefully before changing them. Whileattempting to unscrew the AMC1, I almost removed the top of the mic— a relatively easy way to mangle the AM40's electronicinnards. Fortunately, the instructions are specific and easy tofollow on this point.

The AM40's power supply is housed in a compact metal enclosurewith a solid heft to it. Unlike some tube mics' large and bulkypower supplies, the compact PSM is a cinch to put in a rack orconceal. The PSM's input is a special 6-pin female XLR lockingconnector and the output is a standard 3-pin male XLR connector.Because the PSM is strictly a power supply and not a preamp, youstill need to run the audio output through a mic preamp to getenough signal level for recording. But unlike some tube-micpower-supply boxes that can be damaged if phantom power isinadvertently supplied to the signal chain, the PSM rejects phantompower without damaging either the power supply or the mic — anice fail-safe feature.


I put the pair of AM40s to work in my personal studio for avariety of applications, using them to record tracks of my materialand on client's projects. I cut most of the tracks on an ADAT XT-20or direct to hard disk through a Digidesign Digi001. I alsoauditioned the mics in the analog realm using a Tascam 38 8-tracktape recorder. In addition, I ran the AM40s through a variety ofmic preamps, including those in my Yamaha 03D console and severaloutboard units including the dbx 1086 and A.R.T. Pro MPA.

Outside of my studio, I had significant assistance from JohnAnthony at the Maja Audio Group, a multiroom studio in the heart ofPhiladelphia's Society Hill Section. Anthony graciously lent a handwith A/B comparisons between the AM40 and comparable mics fromMaja's sizable collection. Extreme care was taken to duplicate thesignal path and microphone position when the AM40 was pittedagainst another mic. I also strove to ensure that the timbre anddynamics of performances recorded with different microphones wereas close as possible to those of the AM40's.

Anthony gave the AM40s some real-world workouts on recordingsessions with various artists booked at Maja. All tracks at Majawere recorded to Pro Tools through an Avalon VT-737sp for monooverdubs and custom-made mic preamps from Mill Creek Audio forstereo applications.


To test the AM40s as drum overheads in my studio, I flew themabout two to three feet above a great-sounding old Gretsch kit,positioned on separate boom stands roughly four feet apart with themic capsules angled slightly away from one another. I routed thesignals through the preamps on a Mackie 1202-VLZ mixer and loggedthe results to 1-inch tape at 15 ips on my Tascam 38.

In this analog application, the AM40 mics sounded, well,smashing. Compared to the Shure SM81s or Crown CM-700s (both aresmall-diaphragm condensers) that I normally employ, the AM40sexhibited a smoother overall response with warmer mids and plentyof air above 10 kHz. The AM40s also nabbed more of the low ringingtone (around 160 Hz) from the ride and crash cymbals, lending avisceral “body” to the crash cymbal. In addition, thetube mics minimized the toms' tendency to sound muddy in the lowermids from shell resonance — great news for the Keith Moonlurking in all of us.

I also tried close-miking the snare drum with the AM40, thistime running the signals to the ADAT at 20-bit resolution and tohard disk through the Digi001. For this application, I usually usethe ubiquitous Shure SM57, so I was curious to see how the AM40would fare. I was not disappointed. Overall, the AM40 sounded likea warmer version of an SM57, yet with more sparkle in the high endand cleaner, more detailed presence in the midrange. The mic alsocaptured plenty of snare-wire detail in the 800 Hz to 1.5 kHzrange, yet there was none of the tube distortion “fuzzfactor” you can get with certain older tube mics. If you needa microphone that handles the ruffs and ghost notes on a busy jazzsnare track without being overwhelmed, the AM40 is a formidableally.


For a song submitted by a client on an ADAT tape, I had tofabricate a world beat arrangement using various hand drums andsmall percussion instruments. The client's tape provided fourtracks: a fretless electric bass, an acoustic guitar, a referencevocal, and a relentless drum-machine-generated sidestick for theclick. On those tracks I stacked congas, bongos, a dumbek, an eggshaker, claves, a tambourine, and a triangle, stopping just shy ofthe proverbial kitchen sink. Most of the tracks were mono, but Imiked the bongos and congas in stereo using the AM40s and the SM81sin XY configurations.

It wasn't long before I opted to use the AM40 mics exclusively.With hand drums in particular, the AM40 seemed to sprinkle a bit of“fairy dust” on the tracks. The mics bathed congas andbongos in warm, woody hues and accurately rendered thoseinstruments' unique resonances. The dumbek had the same treatment;a single AM40 caught the clay-bodied instrument's hollow, earthenring and the sharp patter of finger rolls on the paper head,managing to sound bright and warm at the same time.

All other small percussion overdubs benefited from a touch ofthe AM40's sorcery. The shaker was appropriately“sandy” yet not scratchy; the clave was bright but notpiercing; the tambourine's jangle did not sound uncomfortablymetallic; and the triangle's diminutive tinkle seemed less small.Some of the mic's magic seemed traceable to a “warmspot,” a subtle boost between 375 Hz and 600 Hz, that lent abit of midrange presence to the small percussion instruments. TheSM81 also did an accurate job in each of these instances, but itsounded a tad more sterile.


Of the tracks recorded at Maja, the AM40 did its most impressivework when capturing room ambience, especially in the studio's bigroom. In one setup, I placed one AM40 at a distance behind the drumkit and the other across the room in front of the kit. Theresulting tracks provided a terrific bottom-of-the-giant-tankeffect and set the bossa nova beat swimming — but notdrowning — in woody ambience. A single AM40 also delivered ona track of backbeat hand claps: the claps were warm and realisticsounding and let the room sound “join the band.” Thesound quality of the room tracks recorded with the AM40 approachedthat of similar ambience tracks cut with Maja's vintage TelefunkenELAM 251 — a pretty amazing feat, given that legendary mic'sdown-payment-on-a-home price.


Most engineers use a large-diaphragm condenser mic whenrecording lead vocals to capture the voice's nuance and subtlequirks. But sometimes a smaller condenser or even a dynamic mic canhave just the right sound for a particular song, musical style, orvocalist. To test the AM40 as a vocal mic, I tracked my voice doinga lead part for a folk-rock tune I wrote.

First I cut the lead vocal with the AM40, and then I followed upby doubling the performance with the SM81 and CM-700 on subsequentpasses. Compared with the other two mics, the AM40 provided a bitmore low-mid detail (around 375 Hz). It also had what sounded likea 3 dB boost around 1.5 kHz, which added a smooth“hi-fi” presence to the track and helped it cut throughthe wash of guitars. In addition, the AM40 track had a subtlewarmth that I could only attribute to “tubevoodoo.”

I also used the AM40 to overdub some background harmonies on thesame song. The mic again delivered, imparting a warm “tubeflair” that helped the harmony lines cohere and made for anice blend at mixdown.

Typically, small-diaphragm cardioid condenser mics are moreprone to bass boosting from the proximity effect than arelarge-diaphragm cardioid condensers. However, when worked close,the AM40 exhibited less bass boosting than even the large-diaphragmmics I compared it with. I could lean in and work this mic in a waythat might have me living in downtown “Boomville” wereI using, for example, an AKG C 414 or a Neumann U 87. Of course,this is not to say that the AM40 is better than those studio vocalstalwarts — just that it let me record vocal tracks at veryclose proximity without building up too much bass.


For another client's demo, I used the AM40s on nylon- andsteel-stringed guitars. I positioned the microphones as a spacedpair on each instrument, with one AM40 aimed at the fretboard nearthe nut and the other close to the sound hole, angled slightlyoff-axis to mitigate any boominess.

AM40 Specifications

Element externally polarized (DC bias) capacitor (“true”condenser) Diaphragm ¾” diameter, 6-micron gold-evaporated Mylar Polar Patterns cardioid (standard AMC1 capsule); supercardioid andomnidirectional (with optional AMC2 and AMC3 interchangeablecapsules) Frequency Range 50 Hz-20 kHz (cardioid); 80 Hz-20 kHz (supercardioid); 30 Hz-20kHz (omnidirectional) Dynamic Range 122 dB Sensitivity @ 1 kHz 6 mV/Pa Signal-to-Noise Ratio 74 dB Self-Noise 20 dBA Maximum SPL (for <0.013% THD) 133 dB (148 dB with pad) Attenuation Pad 15 dB Highpass Filter 75 Hz, 12 dB/octave Dimensions 7.4” (L) × 1.2”(D) Weight 8 oz.

The classical guitar tracks sounded warm and full bodied, yetretained a bright, “spanky” quality. The steel-stringtracks were the same; the AM40s provided scads of clear detail inthe high frequencies, plenty of midrange girth, and overall asmooth, unhyped sound. In both cases the low mids — around125 to 375 Hz — were warmly represented but with a distinctlack of boominess. In contrast, a pair of SM81s I set up as abenchmark sounded thinner and less exciting.

While strumming an acoustic steel string at Maja, I heard how asingle AM40 stacked up to a Korby Audio CM3 large-diaphragm tubecondenser and to an AKG C 414 B-ULS. Impressively, the AM40, thoughnot nearly as robust in the midrange or bass as either of the othertwo mics, produced the most even-sounding and immediately usableresults of the three. The CM3 captured lots of midrange that wouldprobably need to be equalized out during mixdown, and the C 414,with its pronounced proximity effect, tended toward boominess.

I was also impressed by how easy mic placement was with theAM40s. It was a cinch to set them up in front of the guitars andquickly get a good sound, in part because they proved relativelyresistant to the proximity-effect bass overloading that oftenoccurs when positioning small-diaphragm condensers on acousticguitars. If you have to record a fidgety acoustic guitarist, thismic should be a good pick.


To record an electric guitar solo with the AM40s, I positionedone mic dead center against the center of the amp's grille clothand the other several feet back from the amp to take advantage ofthe AM40's strength as a room mic. The close mic was initiallyoverwhelmed by the amp's high output, so I turned down the mastervolume a bit and activated the microphone's 15 dB attenuation pad,which cleaned things up considerably.

The AM40's performance was impressive in both cases. The guitarsound through the close mic was full of lows without being toobassy, and the mids were rich and detailed. In addition, thereseemed to be a little extra energy from 1.5 to 3 kHz, which gavesome edge to the guitar solo. The distant AM40 provided a track ofjuicy room delay — nice!


I auditioned the AM40s on grand piano during a studio sessionthat required piano tracks to flesh out a jazzy, R&B-inflectednumber. I positioned one AM40 over the lower strings in the middleof the piano and the other over the treble registers closer to thehammers.

The resulting tracks were usable, but the overall timbre wasbright. The AM40s put a little too much emphasis on the high mids(between 1.5 and 4 kHz) and not enough on the lower frequencies(150 Hz and below). On this song, the piano needed brightness andedge to cut through the busy arrangement, so the AM40 tracks workedfine. However, based on the mics' brightness in this application, Isuspected they would sound brittle on a solo classical performanceor in a spare arrangement that exposed the piano more.

My suspicions were confirmed during a session at Maja in which Icompared the AM40 with a variety of mics while tracking grandpiano. It was here that the AM40 drew some of its sharpestcriticism. Not surprisingly, the large-diaphragm condensers (bothtube and solid-state varieties) captured the most midrange and lowend of the bunch; however, I was surprised at how thin the AM40ssounded, even against lower-priced small-diaphragm condensers. Somewho heard the playback said that the AM40s sounded“hollow” and “more modern than vintage,”and were “missing warmth.”

The AM40 also sounded harsh on flute and saxophone. I quicklyreplaced the AM40 with a warmer-sounding large-diaphragm condenserfor a flute overdub. The AM40 fared a little better on a tenorsaxophone overdub at my studio, but the track sounded shrill around1.5 to 3 kHz, and it required a lot of EQ thickening in the lowmids to mitigate an unpleasantly nasal timbre. Unless you want ahollowed-out or aggressively bright sound, steer clear of the AM40for recording sax.


I also tested the AM40 with the optional AMC2 (supercardioid)and AMC3 (omni) capsules. Not surprisingly, the AMC2's off-axisrejection was markedly better than the AMC1's, making it a goodchoice when increased separation is needed between competing soundsources. The AMC2 also proved the brightest and thinnest soundingof the three capsules — a fact I attributed to its low endrolling off at 80 Hz. This would make the AMC2 not a good choicefor recording bass instruments and other sources for which low endis critical. But the capsule worked well on higher-frequencypercussion sources (egg shaker, triangle, and bell tree), and italso did the trick on hi-hat, in which its tight polar patternhelped minimize snare drum bleed and low resonance from the rest ofthe kit.

Compared with the AMC1, the AMC3 (omni) capsule has a broaderfrequency response and sounds fuller in the low end around 120 Hz.This capsule also makes the AM40 sound more “tubey.”When positioned as a room mic on a drum kit, the AM40 with the omnicapsule colored the sound with an overall tube warmth — areally nice effect. When positioned, the AMC3 did a good job offattening up the sound of a bright acoustic steel-string guitar. Itdid a much better job on wind instruments than the AMC1 cardioidcapsule, rounding out the hollow sound nicely. The only concern wasSPLs, because the AMC3 proved easy to overload. For that reason, Iresorted frequently to the AM40's 15 dB pad when recording with theomni cap, especially on drums, guitar amps, and flute at closerange.


The AM40 combines features commonly found in small-diaphragm“instrument” mics (such as the top-address design andinterchangeable capsules) with tube circuitry and a medium-sizediaphragm, resulting in a good-sounding and quite versatile studiocondenser mic. For the most part, the AM40 treads the narrow pathbetween tube warmth and solid-state clarity, managing to soundbright and warm at the same time — at least in someapplications. Unlike many small-diaphragm condensers, it does notexhibit excessive bass boosting when used up close. This mic coversa range of duties, including those normally relegated tosmall-diaphragm condensers and certain applications typicallyreserved for large-diaphragm mics.

The AM40 is an excellent choice for hand drums and smallpercussion, and it also worked well on drum kit, both on the snaredrum and in a stereo pair as overheads. It was also anexceptionally natural, smooth-sounding choice on acoustic guitars(both steel string and nylon string). I also liked how it stood upto wailing electric-guitar overdubs. The AM40 even proved useful insome vocal applications — a role normally reserved forlarge-diaphragm condensers — particularly those in which asharp blade was needed to slice through a dense mix. Perhaps itsmost impressive performance was as a room mic: the AM40 lovinglycaptures a room's ambience.

Few mics work well on all instruments, of course. I was leastenthusiastic about the AM40 on flute, reeds, and acoustic piano— instruments for which it lacked the warmth that is normallyassociated with tube mics.

As for the interchangeable capsules, they worked well for 90percent of the applications I tested. However, if I purchased anAM40, the next items on my shopping list would definitely be thesupercardioid and omni capsules. The omni capsule would probably bemy first grab; it provides the most extended and accurate frequencyresponse and softens the excessive brightness that plagues the micin some applications.

John Ferenzik has toured and recorded with Todd Rundgren (amongothers). His current obsession is restoring an old Fender Rhodeselectric piano. He also would like to thank John Anthony and MajaAudio Group for their assistance with this review.


GT Electronics
tube condenser mic


PROS: Great at capturing room ambience. Exceptionally good forsmall percussion and as drum overheads. A natural-sounding choicefor acoustic guitar. Sturdy carrying case.

CONS: Thin and overly bright sounding in some applications.Changing capsules can potentially damage mic.


GT Electronics
(a division of Alesis Corporation)
tel. (310) 255-3400