CLM Dynamics DB400s

Two channels good, four channels better — but 4‑channel preamps are thin on the ground. Hugh Robjohns has suceeded in sniffing one out.

CLM Dynamics are a Scottish company based in Dundee who manufacture a range of specialist audio equipment. They describe the DB400S reviewed here as a no‑compromise, extremely high quality, 4‑channel microphone pre‑amplifier. The unit has all the normal bells and whistles, plus built‑in MS (Middle and Side) decoders and adjustable peak limiters. The decoders use a design called the 'WHIP matrix' (Wide Horizontal Image Plane) and may be configured for converting the outputs of an MS pair into conventional L‑R stereo, or — with a little lateral thinking and some bodge‑leads — can be used as a stereo‑width control for normal stereo signals using a combination of all four channels.

Hardware

The DB400S is a relatively shallow 2U rackmount unit constructed in a steel case. The front panel uses clear white text on a green background to mark the control and switch functions; the rear panel carries all the connectors, starting with an IEC mains socket (with integral fuse holder) on the left‑hand side. There is no facility to change mains voltage, but a small grey push button just above the mains socket separates the chassis and signal earths to avoid hum loops.

Each of the four amplifier channels is equipped with three connectors. A 3‑pin female XLR at the top of the chassis accepts a balanced microphone signal, and the amplified line‑level output is made available through a 3‑pin male XLR directly below it. Between the two is a standard 3‑pole quarter‑inch jack socket and a small grey push button. The socket provides an unbalanced insert point which can also be used as a line‑level input (to take advantage of the MS decoders). It is wired so that the ring carries the send signal and the tip is return. The push button introduces a 14dB pad in the output signal to convert from professional +4dBu levels to the semi‑pro standard of ‑10dBV.

Internally, the DB400S is very well constructed, with each microphone channel housed entirely on its own PCB card. These are mounted vertically and run the full depth of the unit from front to back. At the right‑hand end, another vertical PCB carries the power supply components; a large torroidal transformer is bolted to the side panel. Although I'm sure it is perfectly adequate, the mains power switch gave me cause for concern on a couple of counts. Firstly, the mechanical alignment of the PSU board was such that the button jammed in its panel cut‑out and didn't operate reliably; secondly, the mains switch connections on the PCB seemed alarmingly close together. The panel has sufficient space for a decent rocker‑style mains switch, and this would, I feel, be a significant improvement.

A ribbon cable daisy‑chains across the tops of the audio cards carrying the power supplies and stereo cross‑linking for the MS decoder functions. There are a number of handbag links on the circuit cards but these are concerned with configuring the cards as stereo pairs within the carcass.

The electronic components appear to be of very high quality and most of the ICs are socketed but, because the circuit boards are mounted vertically (which no doubt enhances the crosstalk figures), even minor servicing of the unit would probably require almost complete disassembly.

Controls

The front panel is very clearly laid out, with four distinct sets of operational controls. Towards the right‑hand end of the unit is the mains power switch — a small grey push button with associated red LED. As already mentioned, on the review model this jammed against its cut‑out and did not instil confidence at all.

The amplifier controls are virtually identical for all four channels, the only difference being that channels 1 and 3 have an extra button to engage the MS‑decoder circuitry. To the left of each section is a column of six (or seven) grey push buttons, with three knobs arranged vertically in the middle of each section and a bargraph LED meter on the right.

The push buttons are used to select phantom power, a 20dB input pad, polarity reversal, a high‑pass filter (24dB/octave at 80Hz), a fast attack‑time constant for the limiter (10ms as opposed to 1ms), an output mute, and the MS decode (on the odd channels only). All are self‑explanatory to use and the only observation I would make is that, as is frequently the case with this type of push button, it is not always obvious whether a function has been selected. None of the buttons are illuminated and only the phantom power and mute buttons have associated LED indicators — although these are at least the two most critical functions.

The three rotary controls determine the input gain (25dB up to 70dB), the limiter threshold (+21dBu down to ‑6dBu with an off position), and an output fader with 10dB of gain above the nominal operating point. A single LED above the bargraph meter illuminates when the limiter is working, but there is no gain‑reduction display as such.

The peak‑reading bargraph meter is calibrated from ‑18dB up to +9dB, but this figure is relative to the output level that you've selected with the rear‑panel pad. In 'pro' mode, the zero LED corresponds to +4dBu, and the top four LEDs have an automatic peak‑hold facility. The zero point is shown on a yellow LED; higher readings are illuminated in red and lower ones in green.

In Use

On the review model there was a minor typo on the rear panel, where three of the insert sockets are labelled 'Insert 2'. Perhaps, when this is corrected, a wiring diagram for the insert sockets could also be added. Another strange detail on the rear panel that would benefit from a re‑think is that, if the unit is used to decode MS originated microphone signals, the left output comes out of the right‑hand channel (2 or 4) and the right output from the left channel (1 or 3). This is most unusual, to say the least; it goes against normal convention, and will undoubtedly cause confusion. On every other device I have ever come across that provides MS decoding, the sum signal is output from channel 1 (the left) and the difference from channel 2 (right). While this policy has been correctly adopted on the DB400S for its inputs, it is incorrect on its outputs — oops!

The DB400S is very straightforward to use and the unit works extremely well. The electronically balanced inputs reject interference well — I experienced no problems with buzzes or hums, even in quite hostile locations. The input circuitry is commendably quiet, giving rise to noticeable noise and hiss only at maximum gains; it also has masses of headroom.

Although I'm not a big fan of limiters, they do have their uses; the DB400S's circuitry incorporates a chip made by US company THAT to implement the dynamic control functions. The attack time can be switched between two settings to determine the trade‑off between tight control and dynamic distortion, but the release time is automatically determined by the nature of the programme in relation to the manually set threshold. Release times vary between 0.3 and 3 seconds and can be observed on the illumination of the associated LED, which seems to display the limiter control voltage directly.

In practice, the limiter system works well enough if the gain reduction requirements are modest and restricted to the occasional unexpected peak. Heavy gain reduction is not the forte of this design, but it never disgraces itself, even under severe provocation.

The high‑pass filter is well chosen, and its steep slope makes it very useful for removing rumble or wind noise without affecting the programme quality too badly. The mute button seems redundant when there is also an output fader with which to kill the channel signal. The fader is well calibrated so that its position could be easily and accurately re‑set.

The MS decoders perform as expected (not withstanding the left‑right output reversal) and their inclusion can only be praised, as they should encourage more people to experiment with this remarkably useful and flexible stereo technique. The handbook provides some sensible advice about setting levels on the M and S channels too, which is to be applauded.

Using the DB400S to provide a stereo‑width control might appear rather complicated at first, but the system works and could prove surprisingly useful — for example, a normal stereo sub‑mix of sources could be connected to channels 1 and 2 via the insert points (as a line‑level input). The fader positions of channels 1 and 2 must be accurately matched to ensure that central sources are equal on the two channels, and the MS decoder button engaged so that the original L‑R stereo signal is converted to MS. Next, the outputs from channels 1 and 2 are connected to inserts 4 and 3 respectively (because of the reversed MS decoding matrix configuration) using XLR to A‑type jack leads. The MS decoder on channel 3 is engaged to convert the MS signal from channels 1 and 2 back to conventional left‑right stereo. Channel 3's fader is set to unity; channel 4's is used to control the stereo width. With the fader at zero, the output is pure mono; increasing the fader gradually widens the image until, at unity gain, the stereo image is the same as the original source signal. Increasing channel 4's fader beyond the unity position increases the stereo width further, eventually producing a 'hole‑in‑the‑middle' effect. Confusing at first, possibly, but this is a powerful and creative technique that can be very useful indeed.

Overall

The DB400S is generally well built and performs to very high standards. I remain concerned about the mains power switch and the non‑standard implementation of the MS decoder outputs, but these points are relatively minor, and could be quickly and easily remedied.

As a mic preamp, the DB400S is quiet, provides a useful gain range and good interfacing options, is well equipped with both the expected and more unusual facilities, and is certainly worthy of inclusion on your shortlist if you are considering the purchase of a 4‑channel mic preamp.