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CLM Dynamics DB500S Expounder

Dynamic Equaliser By Hugh Robjohns
Published November 1999

CLM Dynamics DB500S Expounder

Nice 4‑band equalisers are not uncommon, but this one includes dynamic filtering and equalisation in a bid to lift it above the competition. Hugh Robjohns finds out what 'expounding' can do for your mixes.

Scottish company CLM Dynamics have added yet another noteworthy product to their growing portfolio. Turning their attention to equalisation, they've produced a distinctive‑looking and fabulous‑sounding 4‑band equaliser called the Expounder. The name's similarity to 'expander' is quite deliberate, because although most audio manufacturers might be happy just to design a nice‑sounding equaliser, CLM have gone much further with the Expounder, providing the extra sophistication of dynamic filtering and dynamic equalisation (see the 'Dynamic Discussion' box on page 44 for some background on why dynamic filtering and EQ can be useful).

The Tour

The Expounder is a well‑specified dual‑channel or stereo equaliser possessing four overlapping frequency bands, plus high and low‑pass filters and an overall gain control for each channel. Occupying a 3U steel rackmount case and weighing around five kilograms, the front panel is painted in white and grey and features distinctive golden dalek‑like knobs. The rear panel seems positively sparse compared to the relatively crowded front, as it hosts just two pairs of XLR connectors and an IEC mains input socket. The latter has an integral fuse holder and is accompanied by a voltage selector and an earth‑lift switch separating signal and chassis grounds.

The audio connections are electronically balanced, each pair having an associated switch to select the nominal operating level (in and out) between +4dBu or ‑10dBV. The audio specifications are to CLM's usual high standard, with 19dB of headroom above the nominal operating level and a noise floor below ‑100dBu with the controls flat (rising to ‑80dBu with full boost). Distortion is better than 0.001 percent and the frequency bandwidth extends between 15Hz and 60kHz at the ‑3dB points.

Returning to the front panel, the two channels are equipped identically with a generous collection of 15 knobs, 26 buttons and a bar‑graph meter! These controls are divided into six distinct sections: the high‑ and low‑pass filters, the four separate equaliser bands, and the master section.


The two filter sections are each equipped with a knob to determine the required turnover frequency, plus a Resonance control. Each also has its own Enable button allowing the section to be switched in or out of circuit as necessary. Normally the filters operate as simple second‑order (12dB/octave) high‑ or low‑pass designs, but small grey push‑buttons allow the cutoff slope to be increased to 24dB/octave. A second button associated with each filter section engages the Resonance control and allows a resonant peak to be introduced at the turnover frequency. At its maximum setting, of around 12dB, the resonance is just below the point at which it would self‑oscillate. The remaining button in each filter section is labelled 'Track', and this dynamically links the filter turnover frequency to the amplitude of the appropriate band of frequencies in the audio signal, moment by moment.

The idea behind this tracking feature is that when there is, for example, little HF content in the programme, the turnover frequency of the low‑pass filter is automatically lowered, with the aim of reducing the audibility of any high‑frequency hiss in the audio signal. The same is true of the high‑pass filter section — but operating in the opposite direction, of course. The dynamic filtering relies on the psychoacoustic phenomena of auditory masking — the process by which a loud signal masks a quieter one in a similar part of the frequency spectrum. In the example given above, it is assumed that hiss is inaudible (or, at least, not annoying) in the presence of loud high‑frequency signals, but becomes audible (or annoying) when the signals have little HF content.


Moving along the front panel, there are four separate and fairly conventional equaliser bands. The top and bottom sections both offer a choice of shelf or bell characteristics, while the two middle bands provide a choice of either bell or notch settings with adjustable Q (bandwidth). As with the filters, every band has an individual Enable button to allow the section to be switched in or out of circuit — useful for checking the benefit of a particular EQ setting.

The LF section is equipped with a boost/cut control normally providing a +/‑20dB gain swing, although an adjacent button labelled '/3' reduces this range to +/‑6dB, allowing much finer adjustment when required. The turnover frequency can be adjusted between 40Hz and 300Hz with a second rotary control, and another button alters the equalisation characteristic from a shelf to a bell response. The bandwidth of the bell can also be altered with the Slope button, which increases the steepness of the bell curve from 6db to 12dB/octave. This function has no effect when in the shelf mode but makes the equaliser more selective in the bell mode.

The last button in this section is marked 'Dynamic', and it dynamically links the boost control to the amount of LF energy in the signal, moment by moment. The result is that this portion of the frequency spectrum can have its dynamic range expanded upwards, increasing the warmth and impact of low‑frequency instruments. The process only works with the gain control set to boost, so it's no help in taming an over‑enthusiastic bass player! However, moderate boost settings result in a gentle dynamic enhancement, generally boosting the perceived warmth of the programme material. Advancing the gain control intensifies the dynamic enhancement correspondingly, increasing the apparent percussive attack of bass instruments, for example. The dynamics side‑chain has fixed attack and release time constants which are both pretty fast, to ensure that the amplitude envelope of the signal remains relatively intact once the initial transient has passed.

It doesn't take a brain the size of a planet to realise that any kind of upward expansion is going to eat significantly into the available headroom of the circuitry. CLM have tried to mitigate this by fitting a Peak LED to warn when headroom is getting a bit thin. The indicator illuminates roughly 12dB below clipping, and it is surprisingly easy to trigger it! The only solution is to reduce the input level to the Expounder, which might not be very easy to do if the unit is connected to a mixing console insert point, for example. Fortunately, CLM have provided the ideal solution with a dedicated input level control, allowing the headroom to be optimised whatever equalisation and dynamic processing is being applied.

The HF section is equipped identically to the LF section, but with the frequency range control spanning 1.5kHz to 20kHz. All other controls function in the same way. Using this section to increase the dynamic range of the high‑frequency region tends to enhance the detail in the audio programme, often with benefits in perceived stereo imaging, and a greater sense of 'air'. Large amounts of boost result in a lot of extra snap and an almost painful percussive impact to the sound.

Middle Of The Road

The two mid sections differ from the LF and HF bands, and not just in their respective frequency ranges. The lower‑mid covers the 200Hz‑2kHz region, whereas the high‑mid runs between 1.0kHz and 8kHz — both bands providing a sensible overlap with the outer sections as well as each other.

Aside from the knob to set the centre frequency, each band has two further rotary controls: one to determine the Q (bandwidth) of the bell response; and the other to set the gain. This last control normally ranges over +/‑15dB but a push‑button labelled '/3' reduces the gain window to just +/‑5dB, allowing greater precision if required. Another button engages a Notch mode in which the gain control is reconfigured to provide up to 30dB of cut (fully anticlockwise) with a unity gain position (0dB) when fully clockwise — there being no boost available in this mode. Notch mode is very handy for surgical removal of a resonance or other unwanted narrow‑band sound. Again, peak LEDs illuminate when only 12dB of headroom remains for each section, and individual Enable buttons are provided. Neither of the mid sections have any kind of dynamic equalisation facilities.

The final operational control section is to the right of each channel's filter and equaliser controls. A vertical peak‑reading bar‑graph meter indicates the output signal (relative to the selected operating level of +4dBu or ‑10dBV), and covers a range between ‑17 and +9dB with five red and five green LEDs. Below this, a rotary control determines the input signal level over a +/‑15dB range, which, as already mentioned, allows the headroom of the processor to be optimised for any equaliser or dynamics control settings.

One of the two buttons in this final section allows processing to be disabled, allowing a comparison to be made between the original signal and the (dynamically) equalised version — the input gain control also helping to match overall signal levels in this regard. The remaining button cross‑links the dynamics side‑chain with the other channel. Unusually, a second linking button is provided on the second channel and, apparently, both must be selected to ensure accurate stereo operation and prevent the image from wandering around when expansion and dynamic filtering is applied. It is also important that all the operational controls on both channels are set alike to achieve identical responses.

In Use

The Expounder can be used in several different ways: as a straight equaliser, as a dynamic filter, and as a frequency‑conscious upwards expander — or as a combination of these things. Consequently, I started by assessing the unit as a basic 4‑band equaliser with high and low‑pass filters.

It hardly needs to be said, given the track record established by other CLM products, that this is a fine‑sounding equaliser. There is a good overlap of the frequency bands and plenty of range on the gain controls, allowing both creative and corrective equalisation curves to be set up quickly and easily. The provision of the '/3' gain switches is particularly helpful when working with subtle, creative curves, since the gain knob becomes far more responsive. Equally, the Notch mode on the two middle bands is great for reducing the audibility of annoying resonances and the like, without damaging too much of the surrounding frequency spectrum. Perhaps a slightly higher Q (narrower bandwidth) would have been worthwhile in this mode, but it works well enough most of the time just the way it is.

The high‑ and low‑pass filter sections are excellent for removing the subsonic and supersonic detritis from tracks before applying other, more creative, equalisation. I found they were particularly effective in the 24dB/octave mode, to remove mic thumps with negligable affect on male voices, for example. The filter Resonance control had a detrimental effect in normal filter applications, although it became rather more useful when the dynamic tracking function was engaged. The output level meter and input gain control permitted effective optimisation of gain structure and headroom, with the individual bands' peak LEDs quickly identifying any problem areas. The manual recommends driving the unit reasonably hard (peaking around +6dBu) to achieve the best noise performance, although it is actually very quiet and clean‑sounding even when moderately underdriven.

It hardly needs to be said, given the track record established by other CLM products, that this is a fine‑sounding equaliser.

My only real complaint about using the Expounder as an equaliser is that only the section Enable buttons have associated LEDs. It is difficult to see which of the numerous small grey buttons have been depressed, particularly those on the grey backgound, and a few additional indicators would have been very helpful — especially for the Notch and Dynamic buttons, which have a dramatic affect on the way the machine operates! With the space available on a 3U panel I should have thought finding room for a few more LEDs wouldn't have been too hard. Self‑illuminating buttons could have been used, come to that; the improved ergonomics would have more than compensated for the modest additional production expense.

Although capable as a fully specified 4‑band equaliser, the raison d'etre of the Expounder is as a dynamic equaliser with both amplitude and frequency‑conscious tracking. The key to using the machine effectively in this mode is not to attempt too much — dynamic equalisation can often improve the perceived quality of a noisy track, for example, but it won't make it perfect. A compromise must be reached between the amount of unwanted noise allowed to remain and the degree of high frequencies retained. In general, using the steeper filter slope makes the trade‑off more acceptable, and increasing the resonance slightly can often fool the brain into thinking the bandwidth is wider than it really is — another of those strange psychoacoustic phenomena! Winding in a lot of resonance results in some peculiar synth‑like effects — interesting certainly, useful occasionally, but peculiar always!

The dynamic equaliser sections certainly work effectively, but again must be treated with care — it is very easy to overdo the processing. Any amount of upward expansion at the frequency extremes creates an exciting sound, but often all you end up doing is making things louder rather than better, and the mid‑band signals suffer by comparison in the resulting balance. However, adding some gentle expansion at the high end certainly increases detail and sparkle — not unlike an enhancer in many ways, but without the characteristic artificiality of many such devices. I found that higher boost levels rarely worked, other than as a wacky effect. On the other hand, the LF dynamic equalisation was more usable. Small amounts of boost enhanced the fullness and warmth of recordings quite effectively and subtly, while greater expansion added a punchy impact to the bassline on appropriate tracks.


Most of my auditioning was done on finished mixes, which is not ideal — the Expounder is far more effective if you mix through it, thereby taking account of the dynamic alterations to the mix as they happen, just as you would with a broad‑band or multi‑band compressor. Perhaps the Expounder is at its most powerful when processing individual instruments, though — I discovered some wonderful effects when processing solo voices and string basses, which stayed with the instruments right through the mixing stage and contributed a lot to the end result.

The Expounder won't be everyone's cup of tea, but when used carefully and in the right circumstances, it can make an enormous difference. If you're looking for a decent outboard equaliser with something special thrown in, this is a nice box to have in the rack. It's a luxury item for many, but it could also be just the tool to bring a mix alive.

Dynamic Discussion

Dynamic filtering is nothing new, but dynamic filter units remain relatively scarce. The idea is to vary either the turnover frequency of a filter, or the amount of boost/cut of an equaliser, or both, in order to increase the dynamic range of a specific frequency region. For example, the corner frequency of a low‑pass filter might be reduced when there is less energy at high frequencies — a very effective way to clean up the sound of a noisy keyboard or guitar amplifier. Dynamically enhancing both ends of the frequency spectrum can also help to add punch and detail to a complete mix without becoming overpowering and fatiguing in the way a fixed EQ setting might.


  • Well‑specified equaliser.
  • Versatile filter operation.
  • Useful dynamic filtering and EQ facilities.
  • Sounds great.


  • Many of the more important buttons need LED indicators.
  • Downward expansion would be useful.


A classy equaliser in its own right, but with the added bonus of dynamic filtering and equalisation. The Expounder can be used to clean up noisy tracks in a more subtle way than noise gates, as well as enhancing the detail and warmth of a mix without the usual side‑effects of straight equalisation or expansion. A luxury rather than an essential, but worthwhile if you have the space and budget.