CAD VSM1

Large-Diaphragm Valve Condenser MIc

Published in SOS August 1999
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Reviews : Microphone

Extending their popular range of condenser microphones, Conneaut Audio Devices have introduced the impressive retro-styled VSM1 tube microphone near the top of their range. Hugh Robjohns plugs it in.

Based in Ohio in America, Conneaut Technologies produce a popular and largely affordable range of studio condenser microphones - most with a vintage look about them. The Conneaut Audio Devices (CAD) range starts with the distinctive Equitek series, which includes the E100 and E200 (see SOS June '97), and ends up with a range-topping valve microphone, the VX2. The latest addition to this fleet is another valve microphone, the VSM1, although this uses hybrid technology and is more conservatively priced. The new mic shares many of the generic family characteristics of the other CAD microphones, both in design and sound, and features a cost-effective combination of technologies derived from both the flagship VX2 valve microphone and the solid-state Equitek series.

The VSM1 microphone is a true large-diaphragm capacitor microphone, as opposed to the more 'fashionable' fixed-charge back-electret designs which are so prevalent amongst manufacturers' recent offerings. A pretty substantial device, it weighs about 900g and measures roughly 174mm in height, 73mm across and 58mm deep. It does not have the traditional circular body, but a slightly squashed and elliptical one, finished in a textured matt grey with a nickel-plated wire mesh grille and black metal fixings. In fact, with a few scratches and dents it would probably fool most performing artists into thinking it dated from the '50s! The microphone features a fixed cardioid polar pattern with the usual bass filter and attenuator facilities. A remote power supply module is included, as is a rugged shockmount, and the whole kit is contained in a strong black ABS carrying case.

The shockmount is, to all intents and purposes, an integral part of the microphone, because they are semi-permanently fixed to each other with three thumb screws through rubber grommets, one on the middle of each side and one at the bottom of the back. The inner bracket fixed to the microphone is suspended from a semi-circular outer bracket by a pair of rubber O-rings splayed out to form opposing 'V's. This arrangment gives remarkably resilient support in both the vertical and horizontal planes. A conventional stand adaptor at the rear of the outer bracket completes the shockmount, and is supplied with a 3/8-inch thread adaptor to convert the 5/8-inch standard mount. An oversized knob affords a high degree of purchase to secure the angle of the stand adaptor in relation to the bracket.

On the front of the mic under the CAD badge, an elliptical recess houses two miniature plastic toggle switches. The one on the left introduces a high-pass filter (-3dB at 80Hz) and the one on the right inserts a pre-attenuator in the head amplifier. Unusually, this has a three-stage function providing 0, 8 or 16dB pads (confusingly, with the zero point being in the centre position). The microphone's serial number is etched on the rear of the case along with the model number, just above five milled ventilation slots. The common 12AX7 valve can be seen suspended upside down and glowing through this slot when the mic is in use (although you would have to stand behind the mic to appreciate it!). A 7-pin gold-plated XLR connector at the base of the microphone provides the interface to the external power supply unit using the supplied 30-foot cable.

As with most valve microphones, the VSM1 requires an external power supply to generate the high anode plate voltages required by the valve, as well as the various other biasing voltages needed for the valve, capsule, and solid-state electronics. In this case the PSU (VSM 1PS) is a simple no-frills affair with voltage selector, fuse holder and IEC mains inlet on the rear panel. Another gold-plated 7-pin mic XLR, 3-pin output XLR and a power switch with green LED illumination are the only facilities on the front panel. The unit measures a compact 48x142x152mm (height, weight, depth) and weighs a surprisingly solid 1.2kg. Although the unit emitted a faint harmonic buzz from the internal mains transformer, this is very unlikely to cause a problem during normal use.

CAD VSM1 £993 pros Characteristic 'large diaphragm with valve amplifier' sound. Chunky retro styling. Well defined polar pattern. Good general technical specifications. cons Needs care over the cable dressing and shockmount assembly. Hypercardioid tail at high frequencies could be troublesome. Slightly peaky on-axis frequency response. summary The VSM1 features the distinctive CAD retro styling and employs hybrid valve/solid state electronics with a hand-built genuine capacitor capsule. In keeping with its appearance, the microphone provides the kind of detailed but warm character you might expect, with a well-defined polar pattern and good noise performance.

Connecting the VSM1 and its power supply is perfectly intuitive and straightforward. After connecting the microphone to the power supply with the supplied 7-pin cable, the output of the PSU is linked to the console with a normal microphone cable, and an IEC mains lead plugged into the back of the power supply. After switching on, the microphone requires only a few seconds to function, but definitely benefits from being allowed to warm up and stabilise over a period of 10 to 15 minutes.

A typical working life is not given for the VSM1's valve, but replacing it is certainly not an onerous task. In fact, it merely involves removing the mic from its shockmount cradle, undoing four screws to release the lower cover and switch bezel, and slipping the valve from its holder and out through a metal frame. The whole replacement proceedure could be completed in under five minutes without any problem at all, and there are no realignments to worry about. The valve base exerted a firm grip on the valve pins in the review model but there was no other means of supporting the valve at all.

The microphone's technical specifications are very good, although perhaps not quite the best of its class. On-axis frequency response is nominally flat up to 20kHz, although there are a couple of small peaks above 5kHz and a very distinct presence peak around 12kHz. The trace supplied with the mic showed a mild lift at the bottom end too, although the extent of this would be obviously be dependent on the distance of the sound source, since this is a cardioid mic which inevitably suffers from the proximity effect.

Sensitivity is a high 22.4mV/Pascal and dynamic range is quoted as 118dB from the noise floor to its maximum sound pressure level. Typical close-miked voices and instruments generally provided a signal output at around the -40dBu mark, although the mic is apparently capable of generating a stunning output level of around +10dBu when subjected to its maximum (unattenuated) peak level of 134dB SPL! The signal-to-noise ratio is given as 79dB for an input signal at 94dB SPL and, for the record, the noise floor is specified as a fairly respectable 15dB SPL (equivalent level). The maximum sound pressure level which the mic can tolerate (for 0.5percent distortion) with its 16dB pad inserted is 149dB SPL (133dB SPL without either pad) but distortion rises rapidly above this point with 5percent being reached only 1.5dB higher. The pad switch appears to alter the whole gain structure of the internal amplifier, rather than simply attenuate the output from the capsule, as it takes several seconds, with a few bumps and pops along the way, before the output settles to the new level.

The VSM1 is a lovely microphone to use because it has that flattering, slightly warm sound quality which is so characteristic of a valve amplifier combined with a large capsule. Performing well on a wide range of sources, it gave a fast, detailed and involving sound with a lovely warmth and presence in most cases. I particularly liked the VSM1 on close-miked voices and more distant stringed instruments, and the presence peak seemed to complement male voices particularly well. In some cases, however, the same peakiness could sound a little edgy on some female voices, and when used close on various woodwind instruments I felt it tended to over-emphasise some of the breath and keyboard noises. A little extra distance (when possible) and careful angling to take advantage of the defined polar pattern usually allowed a good impression of the relevant instrument to be captured. With vocals, the mic benefited from a decent external pop screen, although I would not suggest it was overly prone to blasting or popping.

Technology     The heart of this new microphone is CAD's Optema Series AS110 capsule, manufactured in the company's Ohio factory. The 1.1-inch hand-built condenser capsule certainly qualifies as a 'large-diaphragm' capsule, and possesses all the attributes normally associated with the breed. It is constructed with highly tensioned polymer film diaphragms front and back, just three microns thick, each sputtered (coated) with 24-carat gold to provide the necessary conductive surfaces. The other mechanical components of the capsule (back plate and surrounds) are machined from brass and stainless steel. Following construction, CAD calibrate each capsule, again by hand, for damping and to ensure a consistent frequency response between capsules. The internal head amplifier in the VSM1 is a hybrid configuration employing a selected 12AX7 valve (a Russian Sovtek bottle in the review model). CAD claim that the circuit topology is optimised for both maximum headroom and low noise, and the output stage appears to be based around a Burr Brown OPA2107 high-speed dual operational amplifier. This is used in a differential arrangement, and is said to be capable of driving long balanced cables without them having any detrimental effect on the performance of the microphone.

The polar response is actually quite a narrow cardioid, especially at the high end - something which is again fairly typical for a large-diaphragm microphone. All frequencies up to 1kHz provide a polar pattern which is about 5dB down by 90 degrees to either side. The rear null is well defined, although with frequencies above a couple of kHz it acquires a significant rear tail and becomes much more like a hypercardioid pattern. Nulls form at 135 and 225 degrees, and the tail centre at 180 degrees becomes only around 10dB less sensitive than the 0 degree front axis. Above 5kHz the sensitivity to sound from the sides declines steeply, becoming 5dB less sensitive than to mid and low frequencies at only 45 degrees to each side.

In practice this means that if the sound source moves off-axis the drop in level and change in quality is very noticeable indeed, so the vocalist will need to be fairly well bolted down! However, this property can also be extremely useful in providing a high degree of separation when used in a multi-mic situation, although care is needed in aiming that high-frequency hypercardioid tail.

In all applications I found the mic to be sufficiently quiet, and its high output paid dividends with budget mixers of limited front-end capability! Although it has an enormous headroom, it paid to make use of the pre-attenuators, as the onset of severe distortion was quite sudden with very loud close-miked sources.

The shockmount proved slightly disappointing - but not in its ability to reduce mechanical noise from the microphone stand, which was perfectly acceptable. No, the problem was actually in its tendency to suddenly and unexpectedly relinquish its support of the microphone, allowing it to droop! A corrugated washer between the metal blades of the standmount and suspension bracket provided good grip when the mechanism was tightened, but a very small downward pressure on the microphone was all that was needed to rotate the stand clamp bracket slightly. This was usually enough to reduce the clamp tension sufficiently for it to suddenly release its grip entirely (although the microphone never came to any harm when this happened, and it could not fall out of the suspension). When it occurred during a take, though, everyone recording was certainly aware of it happening! I found a simple solution was to reassemble the entire shockmount with the microphone suspended on the opposite side of the bracket, so that the weight of the microphone tended to tighten the clamp rather than cause it to release. From then on, the whole construction proved much more stable and reliable. Perhaps my review unit was assembled incorrectly and this problem will not beset other users.

The 'Gotham Audio Cable' lead which connected the mic to its power supply was quite thick and stiff - rather more so than I would have preferred - and I found it paid to take care in dressing the cable to the stand, as mechanical noise could easily pass up the cable and straight into the microphone, bypassing the shockmount completely. This is a common problem, though, especially with remote-controlled, stereo or valve microphones which require substantial multi-core cables.

Overall, I found the VSM1 to be an impressive microphone which sounded great and performed very well when used appropriately. Its styling and size give it a kind of visual credibility which is matched by its sound quality. I would be more than happy to have one or two of these mics in my cupboard.

information

£992.88 including VAT. Unity Audio +44 (0)1440 785843. +44 (0)1440 785845. Click here to email www.unityaudio.co.uk