Microphone PreampReviews : Microphone
Schoeps make some of the most revered mics on the planet, so when they release a commercial version of the mic preamp they use for testing, you have to take it seriously...
Schoeps are one of the worlds leading manufacturers of ultra high-quality, small-diaphragm microphones, which are very highly regarded for studio and location sound-recording applications. The company was founded by Dr Karl Schoeps in 1948 in Karlsruhe, Germany, with the intention of being involved “in all forms of audio technology.” Dr Wilhelm Küsters — described as a remarkable technician — was taken on as the companys first employee and he worked on a range of products, including valve microphones, tape recorders, and various cinema sound installation equipment. However, it was really when Jörg Wuttke joined the company in 1970 as a young graduate seeking work experience that Schoeps started to evolve into the company we know today. Sadly, Dr Küsters died the year after, with Wuttke inheriting his position. That change led directly to the launch of the Collette series of microphones in 1973 — a range which is still going strong today, nearly 40 years later. The Schoeps company remains family owned, with Ulrich Schoeps (son of Dr Karl) becoming involved in 1980, and now running the company, along with Helmut Wittek.
Although Schoeps have been well known for manufacturing an extensive range of mounting hardware and various plug-in filters and pads to support their microphone range, its only very recently that theyve branched out into producing other electronic equipment and software. For example, an impressively versatile (and free) Double-MS surround-sound decoder plug-in was launched a few years ago, and this year the company have released their first microphone preamp, the VSR5.
The VSR5 is intended to serve as a dual-channel, high-end studio mic preamp. It boasts a wide dynamic range, extremely low distortion and superb radio frequency interference immunity — which is something Schoeps have developed to a high level of competence in their microphone designs too.
However, although the VSR5 is Schoeps first commercial rackmounting mic preamp, the electronic design of the preamp is far from new. In fact, it is basically a repackaging of the companys long-standing in-house reference mic-preamp design, which theyve used for many years in the testing and development of all Schoeps microphones. This is an entirely bespoke preamp design, which Schoeps refer to as a Core SVM amplifier, but the circuitry is apparently inspired by what is now considered a classic integrated preamp-module design from a company called Valley People, which dates back to the late 1970s. The TransAmp LZ was a potted preamp module, valued for its exceptional (for the time) low-noise performance, and it used a fully differential current-feedback topology, rather than the more common voltage feedback approach, so presumably the Core SVM is a broadly similar design.
RF immunity is not something that is often discussed or even mentioned in the spec sheets, but it is an increasingly important aspect of professional audio design, simply because of the huge proliferation of mobile phones and Wi-Fi systems, as well as the general RF noise created by so many digital products generally. Its a situation that can only get worse in the future, too, and with the large amounts of gain required of a typical preamp, and the extended bandwidth people now expect, careful attention to every aspect of electromagnetic integrity is crucial.
Construction & Controls
The VSR5 occupies a very tightly constructed 2U rackmount case that extends 195mm behind the rack ears, and at 4.7kg it is considerably heavier than it looks. The two preamp sections are clearly separated on the front panel, and the same is true internally. The power supply, mounted between the two preamp sections, is a linear design, incorporating no fewer than three torroidal transformers, and is switchable for nominal 115 or 230V AC operation. Internal DC supply-rail regulation can accommodate voltage variations of the mains supply by as much as -20 and +8 percent. The power consumption is a modest 10VA, and the IEC socket on the rear panel incorporates an integral power on-off switch. The protection fuses are all internal.
Besides the IEC power inlet, the rear panel carries a pair of female XLRs (with gold-plated sockets) placed towards the outer edges to accept the microphone inputs, with two pairs of male XLRs (again, with gold-plated pins) to provide separately buffered dual outputs for each channel. The inputs and outputs are all electronically balanced, the latter using a floating servo-balanced circuit (with a low 50Ω output impedance), so that when driving an unbalanced destination the signal level remains the same as for a balanced destination. There is also a small three-way toggle switch, which can be used to directly couple or totally isolate the audio and chassis grounds if required, although the default Hi-Z position provides an RC filter network to keep the two usefully tied down while still preventing audible ground-loop currents.
The VSR5s manual goes to some lengths to warn against using XLR cables in which the connector shell is bonded internally to pin 1, as these will seriously compromise the RFI performance of the preamp. It also explains what a ground loop is, how it can occur and how to avoid it, which is very educational.
The front panel is laid out very cleanly and is typically Germanic in its quiet efficiency. Each preamp section is styled as a separate brushed stainless-steel plug-in module within the overall grey Nextel front panel. The operational controls are restricted to an enormous 21-step rotary gain switch (adjusting the gain in 3dB steps from 0 to +60dB), and six illuminated buttons. The gain accuracy is specified as ±0.3dB per step, permitting easy stereo matching without the need for a separate continuous level-trim control. All of the buttons are permanently lit at a dim level while the preamp is powered — which takes a little getting used to — but glow much more brightly when the relevant functions are activated.
Arranged horizontally across the top of the panel is a large 20-LED bar-graph meter showing output signal level, spanning -48 to +9dB in 3dB increments and scaled as a DIN PPM — which is the prevailing analogue meter format in Germany and much of the rest of continental Europe. The meters nominal zero is factory-calibrated for +6dBu (but is adjustable internally), and the separate Clip LED is set at the factory to come on at +21dBu. This is around 8dB before the outputs clip at +28.5dBu. The meter LEDs are coloured green for the range below -12dB, yellow from -12 to -3dB, and red for everything above. This automatically encourages the user to set the gain for a pragmatic headroom margin (would that more metering schemes made it that obvious!). The meter ballistics conform to a full-wave analogue peak meter, with the DIN-standard specification of a 10ms integration time and a decay of 0.5s per 20dB.
The three buttons to the left of the gain control activate phantom power, invert the signal polarity and mute the output. To prevent audible thumps or bangs when phantom power is turned on or off, the output is automatically muted (via sealed relays) for about 2.5 seconds, to allow the DC-blocking capacitors on the input to charge or discharge fully, with the Mute buttons red illumination indicating this status. The status of the phantom power is remembered when the unit is powered off, and will be reactivated automatically if necessary when the unit is next powered up. A unique feature of the VSR5 is that if phantom power is activated but current isnt flowing, the button glows a bright yellow, turning green only when a phantom current of more than 1.7mA is being drawn: a nice and surprisingly useful feature! The status of the Mute switch is also remembered and reinstated when the power is cycled, while the signal polarity switch does exactly as you would expect, the function being provided by means of sealed relays rather than active circuitry.
The remaining three buttons to the right of the gain control provide various high-pass filtering modes: none, 40, 80 or 120Hz, with 12dB/octave slopes. The filters cannot be combined, and selecting one will automatically cancel any other selected, the active filter being indicated by white illumination of the corresponding button. The selected filter mode is also remembered when the unit is powered off and reinstated automatically at the next power up. With no filters selected, the response is flat to 12Hz (-0.1dB).
There is no switchable pad option, but the maximum input level is +25.4dBu (with the gain control set to zero), which should accommodate the loudest of microphone inputs fairly comfortably.
The VSR5s well-presented rear panel hints at the engineering quality inside the case.
The VSR5s well-presented rear panel hints at the engineering quality inside the case.
The technical specifications for the VSR5 make impressive, and at times unusual, reading. The mic input impedance, for example, is unusually high, at almost 10kΩ. Theres a growing trend for higher input impedances on mic preamps: many classic Rupert Neve designs used inputs of more than 5kΩ, and lots of preamps now have at least one higher impedance option. It doesnt make a huge difference to modern transformerless capacitor microphones, but for moving coils, ribbons and transformer-coupled mics, it can be quite beneficial.
Common mode rejection with maximum gain is well above 80dB across the audio band, which is excellent, and below 120dB for RFI all the way up to 1GHz.
Distortion is specified very impressively at less than 0.0006 percent for 30dB of gain, although my own bench tests showed that figure varies a little with frequency (as youd expect) and rises for both higher and lower gain settings — as youd also expect. But the performance remains extremely good, as the graphs on our web site (www.soundonsound.com/sos/nov10/articles/schoepsVSR5media.htm
) show, and even with the clip light flashing away merrily and the output sitting at +25dBu (higher than most A-D converters can cope with), distortion only rose to 0.005 percent. I measured a broadband THD figure of less than 0.0008 percent for any output level below +18dBu, which is very impressive.
Equivalent input noise is also a very commendable -128dBu measured with a flat response, and an excellent -130.5dB when measured with an A-weighted response. Both figures were obtained using an Audio Precision test set with 60dB of gain dialled into the VSR5 and a 200Ω source. (Most European manufacturers work with a 200Ω nominal source, whereas American manufacturers prefer a 150Ω source, the latter providing an EIN figure about 1dB lower).
The VSR5s frequency response extends between 12Hz and 90kHz within a ±0.1dB window, and a claimed 2.5Hz to 400kHz with a ±3dB window. The group delay introduced by the high-pass filters is moderate and tidy, although I am slightly mystified as to why the 40Hz filter should have a different trace shape to the other filters. Its not a problem in any way, as the sound isnt noticeably different, but it is odd, nonetheless.
The phantom power passed all my usual tests, with a nominal voltage of 47.1V and no problems delivering the maximum 10mA on both channels at the same time. Not surprisingly, the VSR5 also sailed through the Windt Hummer tests, so there shouldnt be any ground-loop problems here.
In terms of sound quality and character, the VSR5 is a very quiet and clean-sounding preamp with a neutral and transparent personality, especially through the more commonly used mid-range gain settings around 30-40dB. There is a massive headroom margin, thanks in large part to the nice ballistics and factory calibration of the metering, with wonderfully clean and open transients and a sound lacking any hint of dynamic compression or hardness, even when pushing into the red. The potentially massive proximity effect of closely worked AEA and Coles ribbon mics on vocals and electric guitar amps caused no headroom problems at all, and while the VSR5 didnt enhance the warm character inherent in this technique, it didnt take anything away or add anything nasty either.
I much prefer this kind of clean and neutral preamp generally, as they suit the orchestral, choral and jazz material that I enjoy recording on location particularly well. There are very subtle character differences between the VSR5 and my own ultra-clean GML 8304 and AEA TRP preamps, but all three are very much in the same tell it like it is vein, even when working with ribbon microphones. Its also worth noting that all three preamps are transformerless. The higher than usual input impedance of the VSR5 is a positive benefit for passive ribbons, and the Schoeps preamp responds similarly to the AEA TRP in that respect, although the TRP does have a slightly more pronounced larger than life element to it than the VSR5. The switchable high-pass filters are very well judged, both for turnover-frequency options and slope, allowing unwanted LF rumbles to be tamed firmly but discreetly.
Talking of recording on location, Schoeps meticulous attention to RF immunity makes the VSR5 an excellent choice, removing that concern from the long list of potential pitfalls that can plague unfamiliar recording venues. While the 60dB maximum gain of the VSR5 is arguably a little on the low side for distant ribbon mic techniques, it is perfectly adequate for close ribbon applications, and more than comfortable when working with Schoeps own capacitor mics (and most other manufacturers too, of course), which are generally blessed with very healthy output levels anyway.
The sound character of many preamps changes with the gain setting, the majority being sweetest-sounding in the mid-gain range. As the aforementioned measurement plots on the SOS web site show, this is technically true of the VSR5 too, but the sound is so sweet all the time that everything just sounds fantastic, whether youve set the gain to maximum or backed it right off. As long as the output level is sensible, the overriding character remains the same: clean, clear, neutral, detailed and strain-free.
With matched gains and settings across the two preamps, stereo imaging was flawless and stable, with immensely wide and deep sound stages using ORTF and coincident pairs, and massive scale with spaced pairs — all undoubtedly thanks to the completely individual preamp construction and over-specified power supplies with independent transformers. I measured inter-channel crosstalk at 10kHz and maximum gain to be better than -103, and better than -113dB at minimum gain. Again, those are impressive numbers.
My overall impression of the VSR5 is of top-flight, ultra-clean, ultra-revealing performance. There is nothing to fault and only praise to lavish. Although clean and neutral, the VSR5 never sounds cold or hard, and still portrays that larger-than-life character through the solid, extended bottom end that is such a signature of good, high-end mic preamps. The VSR5 doesnt flatter or cosset poor mics. If theres a hint of harshness or a slight resonance, this preamp will let you know about it — but not by over-emphasising it. It just presents the information if you care to listen — and thats what a really, really good preamp is supposed to do. Straight wire with gain. Nice. 0
There are several very highly regarded, ultra-neutral, top-flight, dual-channel mic preamps on the market — all of which I covet on a daily basis. I would suggest comparing the VSR5 with the likes of the AEA RPQ, the Benchmark MPA1, Crane Songs Flamingo, GMLs 8302, the Grace Design M201, and the Millennia HV3C. Of those, Id find it hardest to choose between the M201 and the VSR5, especially given the very similar pricing.