CLM Dynamics DB8000S

The DB8000S from CLM Dynamics provides eight channels of high-quality mic preamplification, with limiting, M&S matrixing and no less than three separate outputs per channel.

Multi-channel outboard mic preamps have always been popular with mobile recording facilities and high-end live sound rigs. However, with the prevalence of budget digital consoles and hard disk recording systems, interest in them has risen even further. To satisfy this demand, CLM Dynamics have now produced the DB8000S — a 3U rackmounting unit providing eight versatile mic preamps with some unusual facilities and a plethora of outputs. However, to cram all these in a rackmounting chassis, the box has ended up being quite substantial, measuring 205mm deep and weighing 7kg.

The front panel reflects typical CLM styling, with each channel boasting three silver Dalek-style knobs, each with a deep vertical groove to indicate position. Eight grey buttons — each with an associated LED — allow the various channel facilities to be selected. A vertical peak-reading bar-graph meter allows levels to be established with precision.

Front-panel Controls

Running down from the top of each channel strip, a pair of buttons activate a 20dB input attenuator and 48V phantom power, with yellow and red LEDs respectively. The first silver knob below these buttons switches the mic preamp's coarse gain in 4dB steps from +22dB to +66dB. The handbook recommends using the pad switch if the gain control cannot be advanced past the 42dB position without the meter peaking in the red region (above +3dBu), as this optimises the signal-noise ratio.

Another pair of grey buttons introduce polarity reversal and a high-pass filter (12dB/octave from 80Hz), and these are followed on the panel by a continuously variable fine level Trim knob, spanning the range from -14dB to +6dB. Unity gain is marked at the two-o'clock position. The third silver knob determines the threshold level for the Soft Stop adjustable peak limiter. The knob is scaled from +24dBu to -6dBu, so setting a specific maximum output level is simple. Turning the control clockwise reduces the threshold, exerting greater restraint on the output level.

The Soft Stop system is intended as a protective peak limiter rather than for artistic effect, although it does have a soft-knee transition so that peak signals just nudging the threshold are gently compressed at a 2:1 ratio before the 10:1 limiting characteristic is imposed. The release time is programme dependent between 0.3S and 3S, while the attack time is preset to 1mS to avoid transient distortion. If required, a faster 10µS attack time can be engaged for more stringent limiting — for example with large and fast percussive transients.

The LED bar-graph meter is scaled from -24dBu to +15dBu in 3dB increments, with the top five LEDs (+3dBu to+15dBu) coloured red and featuring a peak-hold function. At the very top of the lamp array, a blue Stop light illuminates when the limiter is triggered — there is no indication of the amount of gain reduction.

Four final buttons complete the strip. On the odd-numbered channels the first provides a stereo link facility, ensuring that the Soft Stop limiter tracks accurately across adjacent odd/even stereo channels. When stereo-linked, the threshold controls of both channels must be set identically and both Soft Stop limiters switched on. On the even-numbered channels this button introduces a matrix circuit to decode a stereo M&S microphone array connected to the adjacent channels.

Another button provides an output mute (leaving the meter active so the channel's signal level can still be observed), and the last two control the Soft Stop limiter — the first to activate it and the second to implement its fast attack mode. All of the channel controls are clearly labelled and their operation is entirely intuitive and logical.

Round The Back

The rear panel of the DB8000S comes as something of a surprise, as there is rather more functionality here than might have been assumed. Each channel strip is provided with a balanced XLR microphone input and an unbalanced high-impedance (1MΩ) instrument input using a quarter-inch socket. This bypasses the main microphone preamplifier stage and expects a nominal 0dBu signal level. The front-panel Trim control allows -14 to +6dB of level adjustment, and although the high-pass filter remains available, the phantom power, coarse gain, pad and phase facilities are redundant.

Each channel is equipped with three independent outputs, the first of which is available on a Tascam-compatible multi-channel output (25-pin D-Sub connector) and, simultaneously, on eight XLRs. It is possible to reconfigure individual channels via internal links so that the D-Sub and XLR outputs are derived immediately following the preamp stage but before the Soft Stop limiter, mute and matrix facilities.

The second and third outputs are via TRS quarter-inch sockets and are balanced electronically, and their output level can be switched jointly to operate at -10dBV rather than the normal +4dBu, via a small button just above the XLR connector. This is slightly confusing, as the layout of the panel suggests this switch affects all three outputs, whereas it actually only affects the two TRS sockets.

Another grey button on the rear panel allows the chassis earth to be lifted from pin one of all eight output XLRs via a 10Ω resistor, the idea being to prevent ground loop problems. It is also possible to completely isolate the earth pins of individual output XLRs by moving more internal jumpers.

The IEC mains inlet is of a type which incorporates a pair of fuses (protecting the live and neutral lines separately), while the mains power is turned on via a front-panel button with associated LED. This has to be pushed slightly below the front panel surface to operate or release the latch — a safety feature which ought to prevent accidental powering down should someone carelessly lean against the unit.

Performance

The only CLM Dynamics product I have had any personal experience of is the DB500S Expounder (see SOS November 1999), which I found very impressive indeed. In fact, my only real complaint of this unusual device was the lack of indicators associated with some control buttons. Fortunately the same criticism cannot be levelled at the DB8000S, since every switch on the front panel has an adjacent LED.

The specification figures for the DB8000S are certainly impressive, and precisely what you would expect from this conscientious Scottish company. For example, the equivalent input noise is -128dBu unweighted — which is extremely good indeed — and distortion is better than 0.001 percent. The frequency response extends between 15Hz and 60kHz (-3dB points) and the input stage can accommodate +7dBu before clipping (+27dBu with the pad) with +22dBu of internal headroom.

The mic amp circuitry is apparently based on the popular SSM2017 chip, although the circuit board has also been designed to allow a more traditional transistor/NE5534 configuration to be constructed instead, if required. However, the SSM2017 chip is now obsolescent and CLM Dynamics are waiting to try the soon-to-be-released THAT1510 — a drop-in replacement for the SSM2017. This new IC is claimed to offer a 6dB quieter noise floor too, but CLM Dynamics say owners of the original design will be able to upgrade their units easily should these impressive claims turn out to be justified.

The present SSM2017-based mic amps sound very quiet, with a smooth noise floor and an entirely neutral and spectrally well-balanced characteristic. They also proved perfectly capable of recovering the output of a weedy ribbon mic on the end of my full complement of microphone cables without excessive hiss, as well as accommodating the superabundance of a high-output capacitor mic placed above a drum kit without running out of headroom. What more could you ask of a mic amp?

It would have been nice had the Instrument inputs been accommodated on the front panel, but space is limited and I doubt it will prove a frustration in practice. This input worked well with a variety of guitars and keyboards, providing a useful range of level adjustment. Musicians with multiple instruments will find the DB8000S makes a very nice active DI box, complete with multiple record/PA outputs and internal protective limiting — the Soft Stop limiter can even be employed very effectively to add sustain to electric guitars! However, it is important to turn the coarse gain control to its minimum setting to ensure any residual noise from the unused mic stage doesn't break through.

And talking of breakthrough, I discovered that very high signal levels in one channel (sufficient to light the entire bar-graph meter) also caused the -24dBu LED of all the other meters to illuminate. Despite this, I could not detect any significant audio crosstalk between channels, so it appears to be a metering issue rather than a channel crosstalk problem.

As an enthusiastic proponent of the M&S stereo recording technique, the inclusion of matrix decoders for adjacent channels is a major plus to me — and I'm sure to many other potential owners. It is even possible to use four channels (in two pairs) as a stereo width controller by using the matrix of the first pair to convert from conventional stereo to M&S. This M&S signal can then be linked across to the Instrument inputs of the second pair of channels (bypassing their mic input stages) and decoded back to conventional stereo by the second matrix. This arrangement allows the Trim control of the fourth channel to adjust the level of the Sides component, altering the stereo width from near-mono to an extra-wide stereo. It may sound complex, but it is easy to set up and use, and extremely effective. If you want, you can even patch an equaliser into the Sides signal and use it to implement a degree of frequency-dependent width enhancement.

The Soft Stop limiter is useful in providing a comfort zone when recording to digital media, although I found it could sound a little abrupt if pushed too hard. There is no substitute for allowing adequate headroom when recording digitally, and I feel that using high-quality dynamics processors in the post-production environment always produces better results than compression during recording. However, if the Soft Stop system is only 'tickled' by the highest unexpected peaks, it does provide a very worthwhile safety net and is audibly transparent below the front-panel threshold setting.

The stereo linking worked effectively, although it would have been easier had the thresholds of both channels been determined by only the odd-numbered channel's control knob. And for users who require the utmost fidelity, CLM have thoughtfully even provided the option of bypassing the limiting circuitry's VCA stage.

The multiple output capability will help the DB8000S appeal to users in the live sound, location-recording and broadcast fraternities, where the unit may be employed as an active mic splitter or distribution amplifier. Moreover, the ability to change the operating level of the two TRS outputs will also be very handy when working with semi-pro systems.

There are relatively few multi-channel mic amps around, and most of the ones that do exist are frighteningly expensive. By comparison, the CLM Dynamics DB8000S is extremely cost-effective at under £200 per channel in the UK. While this design cannot be controlled remotely (many similar units intended for the live sound market can be), it also doesn't suffer unwanted control signal crosstalk, and is a highly competent and desirable machine.