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Earthworks QTC1 & Z30X

Omni-directional Microphones By Hugh Robjohns
Published July 1998

Earthworks QTC1 & Z30X

Still looking for the best mics/preamp money can buy? Hugh Robjohns unearths some real gems from the USA.

Earthworks is an American company that started out manufacturing specialist omnidirectional measurement microphones. Today, they have a small range of four very high quality audio microphones as well as a couple of very high quality microphone preamps designed to complement their products. Under review here are the omnidirectional QTC1 and the new directional Z30X, plus the 2‑channel LAB102 mic preamp.


Earthworks QTC1 & Z30X

The range‑leading omnidirectional QTC1 is actually supplied as a boxed set of two matched microphones — and the box is every bit as gorgeous as the microphones! Each microphone comes complete with a stand adapter (with a 5/8‑inch thread, but no 3/8‑inch adapter unfortunately) and a paper trace of its frequency response, which is flat (within a dB) between about 4Hz and 40kHz. And it really does go down that low — during part of my testing at a friend's home studio, the microphone picked up subsonic footfalls from the children upstairs that we couldn't even hear ourselves!

Other information contained in the box included an owner's manual and an application guide. The owner's manual offered some rather unusual advice, such as allowing at least one minute for the microphone amplifier to settle if connected to a desk with transformer‑coupled inputs (as opposed to capacitively‑coupled electronic inputs). The other amusing thing it said was that "...for some reason, one of the first things people do when they handle the microphone is to touch the tip". This is bad, because grease and dirt will clog the screen and thus adversely affect the sound quality. Strangely, everyone I showed the mic to touched the tip! If any psychologists in the SOS readership want to suggest the reasons for this bizarre behaviour, I'd like to know... and in case you were wondering, yes, I touched the tip first too!

The microphone has a stainless steel finish and measures 229mm long and 22mm diameter at the widest part. The business end tapers down to just 7mm, which makes the microphone look very special. At 225g the mic is quite heavy for its size , but that just adds to the professional impression it creates even before you have plugged it in. The serial number is engraved into the bottom of the main body.

Being a true capacitor microphone, it requires 48V phantom power from which it draws a fairly substantial 10mA of current — but that should not be a problem for any modern mixing desk. The internal microphone head amplifier puts out a relatively high level compared to many capacitor microphones and the sensitivity is quoted as 30mV/Pa or ‑30dBV/Pa. In terms of dynamic range, the microphone can cope with signals up to 142dB SPL and its self‑noise is a very reasonable (if not earth‑shattering) 22dB SPL (A‑weighted).

One of the key features of which Earthworks are keen to inform potential users of the QTC1, is that its impulse response is rather good. For those not familiar with the concept, an impulse is exactly that — a brief pulse or click — and the maths suggests that it contains all frequencies up to a certain upper limit defined by the duration of the pulse. Engineers like impulse responses, because they provide a very accurate way of analysing frequency and phase problems. Any filtering (mechanical or electrical) imposed on an impulse will alter its shape, typically with 'ringing' so that the pure 'spike' of the impulse will end up looking more like a mountain and range of foothills!

In the case of the QTC1, the impulse response does exhibit some ringing, simply because its frequency response is filtered (presumably as a result of the combination of the mechanics of the capsule and the electronics), but the ringing is well‑damped and occupies a remarkably brief period. Indeed, Earthworks claim that the impulse response of the QTC1 is the cleanest they have ever measured. Perhaps that should be taken with a pinch of salt, but the fact remains that it is a very clean microphone.

By now you are probably wondering what good a clean impulse response is anyway. Basically, what it means is that the waveform of the sound source will be captured faithfully — it does not impose a character of its own, it simply captures the actual sound that occurs around it. In some applications, you may want to use a microphone specifically because of the character it adds to the sound — such as many of the large diaphragm Neumanns, for example. However, that is not the raison d'être of the QTC1. The measurement microphone pedigree of its forebears is revealed in its extreme accuracy of rendition, and there are numerous applications where you really do want the microphone to tell it like it is!

The LAB102 mic preamp is certainly up there with the very best when it comes to passing on the detail and resolution of the finest microphones.

Since it is a pure omnidirectional microphone (a simple pressure transducer with no venting to the rear of the capsule, the QTC1 does not suffer from 'proximity effect' at all. Its tolerance of high sound pressure levels also means it can be positioned very close to sound sources. In practice, what this means is that you can use the QTC1 in situations where the natural instinct would be to employ a directional microphone. It is always worth remembering that there are two ways of achieving isolation with the microphone: polar directivity (ie. use a microphone which rejects sounds from particular directions), and proximity. The inverse square law of sound propagation means that halving the distance between the mic and sound source increases the ratio of wanted to unwanted sounds by four times. So using an omnidirectional microphone very close to a source can give just the same isolation as a directional microphone at a greater distance. The only problem is that by close‑miking, you only capture the sound from a small proportion of the instrument; the natural bloom in sound which the instrument generates when auditioned at a natural distance may be lost.

In my auditions, I found the QTC1 to be a very accurate, transparent microphone which conveyed even the most harmonically complex of instruments with perfect rendition. It has an extremely natural, extended and open top end (which was made all the more obvious when comparing the off‑tape DAT recording with the microphone's actual output!), and a phenomenal bass response which really does go down to a very small number of Hertz. This is an important point to be aware of — monitoring through small nearfield speakers is unlikely to reveal the mass of subsonic information the microphone is collecting — but it is there, trust me! The good thing is that the bass response, though extended, is not artificial or overblown in any way, and the microphone does not require an elaborate suspension system.

QTC1 Verdict

Earthworks QTC1 & Z30X

At £975.25 inc VAT, the QTC1 is certainly not a cheap microphone although its sound quality does justify the price tag. I would not suggest it as the first or only microphone to buy, but if you are looking for a top‑flight omnidirectional capacitor mic that looks as good as it sounds — and it sounds just great — the QTC1 is certainly worth auditioning.

Z30X Directional Mic

Earthworks QTC1 & Z30X

The Z30X is the latest addition to the Earthworks collection and the very first directional microphone they have produced. Supplied in a simple cardboard box rather than the hand‑crafted wooden one of the QTC1, the mic is packed with a plastic sleeve over the rearward ports to protect the diaphragm from LF shock waves in transit. Enclosed in the box are a stand adapter (identical to that supplied with the QTC1), and a neat foam windshield. The only information supplied with the mic was a MLSSA averaged frequency response chart from the QA test.

In appearance, the Z30X follows the house style of the QTC1 and indeed the main part of the body is identical. However, the new mic is slightly longer at 236mm and the business end is not as slim, narrowing down to 11mm. As you would expect of a directional microphone, the narrow end‑tube also differs from the omni with two rows of thin vertical slots (with tiny holes drilled between them) intended to allow sound to reach the rear of the capsule.

The nature of directional microphones is that they are inherently compromised, because of the physics involved. In the case of the Z30X, this means that the microphone's high‑end response starts to tail off at around 25kHz and the bottom‑end response is strongly affected by the proximity effect. Earthworks specifications quote the lower limit as 30Hz (+/‑ 1.5dB at 15cm).

The polar response is described as an 'enhanced cardioid' and if that doesn't mean much, I'm not surprised. During my listening tests I found the mic had a very broad frontal pickup, suggesting an almost sub‑cardioid response, but there was a hint of a hyper‑cardioid tail at the rear. I was none the wiser until I trawled the Earthworks website, which contains a polar response chart of the Z30X. This shows that the mic is indeed very wide at the front, not reaching the 3dB down points until at least 60 degrees (and more like 80 degrees at 8kHz). The rear of the microphone exhibits a very definite hyper‑cardioid shape which builds to a maximum as the frequency rises to 4kHz, with deep notches in the sensitivity at 135 and 225 degrees. Above 4kHz the rejection falls steeply to around 15‑20dB.

What this means in practice is that the selectivity of the microphone is not as strong as many other cardioids or hyper‑cardioids, but where it loses in directionality, it gains in a very smooth and natural off‑axis response — something which is often every bit as important, especially in crossed‑pair stereo applications (more about this shortly).

In terms of the remaining specifications of the microphone, it can accommodate peaks of 145dB SPL and its equivalent noise floor is the same as the QTC1 at 22dB SPL (A‑weighted). The output level, however, is considerably lower than the omnidirectional microphone at only 10mV/Pa (‑40dBV/Pa), and in practice I found an additional 15dB of gain was required to match levels between the two microphones. The capacitor microphone and head amplifier require 48V phantom and draw 10mA of current.

Listening to the Z30X, I was impressed with its overall quality and clarity. It managed to provide much of the openness of the QTC1 at the top end, but could add a useful degree of warmth at the bottom thanks to the (mis)use of the proximity effect. The mid and upper range of the microphone seemed extremely smooth and there was nothing by way of the (all too) common 'presence lift', which many manufacturers insist on imposing. The bottom end seemed a touch less uniform but was by no means 'lumpy' and the proximity effect, although quite strong, was manageable and usable. As I mentioned, the microphone's polar response is very wide at the front, which can be useful in many applications, and it tails off very smoothly and cleanly as sounds move around the side of the microphone. The rearward rejection is perfectly adequate, if not quite up with the best, but this was obviously a design decision in balancing the compromises and characteristics of the microphone.

Since I was given a pair of Z30X directional microphones to play with, the obvious thing to do was to rig them as a crossed pair for stereo. Starting with the standard mutual angle of 90 degrees, the Z30Xs produced sharp, well‑focused images but with a tendency to bunch at the centre and stretch at the edges. The problem is that the very broad frontal pickup means that the difference in sensitivity between the two microphones for angles around the centre of the stereo arena is rather smaller than normal. However, there is a simple solution: employ a greater mutual angle — with the mics set at 100 or even 110 degrees to each other, the centre of the image sounded much more evenly spaced, and the whole presentation seemed more uniform. The real strength of the Z30Xs in a stereo configuration is the very smooth and natural off‑axis sound which proved excellent in a test recording of a public performance by a small chamber orchestra. The audience applause, which can often sound very coloured on other crossed cardioid arrays (as it is essentially coming in on the back of the mics), sounded very smooth and natural. The Earthworks marketing brochures keep using the phrase "just like being there", but in this instance it really did sound like that!

Using the two QTC1s as a spaced pair, the stereo imaging was sublime... a velvety smooth watercolour of sound spread out before the loudspeakers

Z30X Verdict

Earthworks QTC1 & Z30X

The Z30X costs the same as the QTC1 (£975.25), and although it sounds very natural and detailed, and retains most of the transparency of the latter, overall I'm not quite as convinced of its blend of characteristics. There are many, many other good cardioid microphones around and a great many of those cost rather less than the Z30X.


Earthworks QTC1 & Z30X

I had not come across Earthworks products before this review, but I am extremely impressed with both their microphones and the preamplifier. The QTC1 is simply gorgeous — both from the sound and styling points of view. It is without doubt amongst the finest omnidirectional microphones I have ever used, and that includes the infamous B&K 4006s. The Z30X 'enhanced cardioid' is very good indeed and manages to approach the sound quality of the QTC1 in many ways, whilst adding the advantage of some front‑to‑back discrimination. However, I remain slightly less convinced of its value‑for‑money status compared with the equally priced QTC1.

The LAB102 microphone preamp is fantastic. A simple, no‑frills affair with all the money spent where it counts — on the circuitry rather than the industrial design of the box. Extremely transparent and faithful, very quiet, and with a very useful and well thought out blend of facilities.

None of the Earthworks range is obviously priced in home studio territory, and there are probably not that many professional studios that could happily go out and buy a crate‑load either, but if you aspire to the best, Earthworks products are certainly one point to aim for.

QTC1 Spaced Pair

Using the two QTC1s as a spaced pair, the stereo imaging was sublime — not the etched, pin‑point accuracy of coincident mics of course, but a velvety smooth watercolour of sound spread out before the loudspeakers. If you like the spaced pair presentation, you'll love what the QTC1s can do. And I don't know why, but I found that I tended to prefer the mics with a narrower spacing than I would have employed with, say, B&K 4006s, although the stereo imaging always seemed more expansive!

LAB102 Dual Channel Microphone Preamp

If you are a microphone manufacturer, the obvious thing to extend your product range, and to allow people to extract the very best from your microphones, is to design a preamplifier. And guess what? That is exactly what Earthworks have done with the single channel LAB101 and dual channel LAB102 (supplied for review).

The LAB102 preamplifier is a no‑frills device with two identical sets of controls and is housed in a hard‑wearing, black‑painted unit occupying 1U of rack space. The rear of the unit carries four connectors for each channel plus a 3‑pin low voltage (18VAC) socket which mates with the lead from a supplied external mains transformer (the transformer housing employs an IEC mains socket for the power input). The microphone input and both line level outputs use XLR connectors wired to the pin‑2 positive convention. One of the two output sockets is a fixed level feed taken directly from the switched gain control but the second XLR, and an adjacent TRS quarter‑inch jack socket, take their feeds from the variable output control.

The front panel is simplicity itself. Both channels on the dual‑channel version are identical, so starting from the left the first control is a horizontally mounted toggle switch to reverse the signal polarity. This clicks and thumps when operated, but not badly. Next is a phantom power toggle with yellow LED, followed by an On/Standby switch which mutes the channel output — handy when plugging or moving mics. The large rotary knob is a stepped gain control calibrated in 6dB steps from 0 to 66dB, with a nice firm feel to its operation. A 'Clip' overload LED is mounted between the gain control and the variable output knob which is scaled from 0dB down to ‑20dB, with ‑6dB at its centre position. This is very handy for applying a fine trim to recording levels on digital recorders if you are keen to extract the maximum dynamic range from a recording. All switch contacts are gold‑plated, which should ensure a long and trouble‑free life.

The LAB102 specifications are very impressive. For example, it can deliver its maximum output signal of 10 Volts at less than 0.02% THD, and its frequency response is flat between 2Hz and 100kHz (+/‑0.1dB) — even at 200kHz it is only half a dB down! The noise specifications are equally impressive with a quoted effective input noise of ‑134dBu at 60dB of gain (measured with a lowly 50 Ohms load). In fact, the noise figures actually get better with increasing gain, which means that although the machine is perfectly quiet enough at low gain settings, it improves when the going gets tough! Quoting the EIN with a 50 Ohm load is a bit naughty as most mics present 150 or 200 Ohms, and higher load impedances will increase the noise figures accordingly, but it is still a very quiet preamp by any standards.

The technology used for the input stages is the familiar Analog Devices SSM2017 chip, which is socketed. The output driver chips are also socketed and the three outputs are driven independently, so that even if one output was damaged by connecting it to something silly, the others should remain functional. A great deal of attention has been paid to designing the circuitry to ensure that every nuance of the sound which Earthworks' own microphones can capture are raised in level as cleanly and purely as possible, with nothing added or taken away. In my auditioning, I found the LAB102 preamplifier to be a very fine piece of equipment indeed and one which lives up to its claims. It was remarkably quiet, even at very high gain settings, and it appeared to match the performance of a Focusrite Red preamp I was comparing it with.

It was not hard to overload the LAB102 mic preamp at high gain settings with the QTC1 omnidirectional microphone, simply because of its enormous output and fine ability to capture the fastest of transients, but turning down the gain always cured the problem. With other microphones from the AKG and Neumann ranges, the LAB102 fared every bit as well as with its own family of mics. Quiet, clean, open, and very accurate — this mic preamp is certainly up there with the very best when it comes to passing on the detail and resolution of the finest microphones. At £1468.75 including VAT it's expensive, but justified once again.


  • Well‑designed for the job in hand.
  • Stunningly transparent.
  • Extraordinary frequency range on the QTC1 mic.


  • Expensive.
  • Unusual polar response on the Z30X mic.


Earthworks manufacture top‑flight microphones and preamplifiers, and these are three of the best. The QTC1 omni is easily one of the best mics available at any price and the LAB102 mic preamp is the ideal partner, with a performance which perfectly matches that of the microphone. The newest addition, the Z30X directional mic, is a very credible product which comes surprisingly close to matching the qualities of the omni.