RSPM Reference Standard Professional Monitor


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Reference Standard Professional Monitor Mk IV.

Drivers reference

Super tweeter : Coles STC-4001G; mid-range: unknown, bass: KEF B139.


The IMF Reference Standard Professional Monitor Mk. IV loudspeakers are supplied in matched 1 mirror-image' pairs. The drive units are clustered inwards to avoid reflections from wall boundaries and to maintain phase symmetry on the listening axes. To ensure that they are correctly orientated, the facia panels are also mirrorimage and each loudspeaker should be positioned with the 'IMF' logo inwards.

The loudspeakers must be supported on the stands supplied. The speakers are designed for optimum results when supported off the floor so as to provide air space under the enclosures. (Such an arrangement also inhibits structural acoustic feedback.) The overall height of the loudspeakers when on the stands is arranged so that the drive units are at the correct level when the listener is seated. The stands also tilt the speakers back; an integral part of the overall design concept to maintain optimum phase at high frequencies with evenness of response both axially and hemispherically.

Positioning of the loudspeakers in the room is less critical for transmission line loading than other systems, but it is still advisable to arrange the room for best siting where possible. Small loudspeakers give increased bass output, at the expense of colouration, by the additional room gain of a wall or corner position. The IMF Monitors are little affected by this and do not rely on a corner to maintain bass response. Therefore moving the speaker forward, away from a corner, even by a small distance can provide a marked reduction in room colouration components.

Spacing of the speakers should subtend an angle from the listening position of between 60 ° and 90° The distance of the listener from the speakers should ideally be not less than the distance that the speakers are spaced apart. Whether to angle the speakers inward will be dependant upon this spacing to listener ratio. However, by virtue of the skewed polar distribution of these Monitors, very little angular rotation should be necessary, and probably no more than 15° even when the listening position is close relative to their distance apart.

The loudspeakers should be connected to the amplifier, in phase with each other, using highcurrent carrying wire of at least 5 amp rating. The longer the connecting cable, the heavier should be the gauge, to avoid loss of power and amplifier, damping. The use of screened wire is neither necessary nor recommended.

A fuse can be fitted in series with the positive of each lead to provide a degree of protection against accidental sustained overload, particularly applicable where very high power amplifiers are employed. A fuse value of 3 amp (fast blow) is suggested. Unfortunately this provides no guarantee that the loudspeakers may not be damaged by transient overload before the f use blows. Experience shows that under normal conditions of use for speech and music (not discotheque or public address), damage is usually caused by intermittent fault conditions developing in the amplifier, rather than normal overload.

The basic crossover configuration is shown in Figure One, although more actual components are employed. Three controls are incorporated i n the circuit: a low frequency 'FlLTER', and a frequency response 'TILT' control with 'SLOPE' adjustment. These are accessible, after removing the facia plate, retained by four finger nuts, situated at the bottom of the loudspeaker grill. Beneath this is the control panel illustrated i n Figure Two. The threeposition pre-sets may be adjusted using a wide blade screwdriver.

The 'TILT' and 'SLOPE' controls act in conjunction with each other to determine the energy response between 300 Hz and 15 kHz. The Tilt control in the 'RISE' position causes a lift across these frequencies of about 1 dB whilst the'FALL'setting results in the opposite. (See Figure Three.) The Slope control affects the response above 2 kHz approximately + 1 dB by use of the'AD D'and 'CUT'settings. (See Figure Four.) The Slope control functions at all settings of the Tilt control and therefore a wide range of combinations are available. (See Figure Five). By operating the switches on low level random noise (from an off-station tuner), these functions can be quickly assessed. Each control setting is deliberately made fine and no configuration will impair the inherent smoothness and phase coherence of the loudspeakers.

The speakers are supplied with these controls in their 'FLAT' settings which nominally provides the mostuniform response under anechoic test conditions. This may not be optimum where room characteristics, positioning, the response of ancilliary equipment and even personal preference dictate different settings. For example, speakers situated near reflective surfaces may benefit from lowering the Slope control 'CUT', whilst for dry listening acoustics the preferred setting may be 'ADD'. Use of the loudspeakers under conditions of high ambient noise, or in a bass heavy environment, may benefitfrom lifting the Tilt control 'RISE', then making final adjustments with the Slope control. Possible settings for quiet conditions would be to turn the Tilt control to 'FALL' and perhaps restore the treble energy by switching the Slope control 'ADD'. Such combinations would produce a marginally receded midrange- a loudness contourthat may be preferable at relatively lower listening levels.

There is no basic reason whythe response of one speaker may not be adjusted differently from that of the other, to compensate for their different locations in the room. However this may not always be a totally satisfactory subterfuge since it impairs matching between the loudspeakers.

The low 'FILTER' controls energy at the bass, near and below sub-sonic frequencies. (See Figure Six.) Switched to 'IN' it provides some protection from DC pulses, such as rapid FM tuning, and from excessive'cone weave'on warped discs that can cause unwanted intermodulation components. It has negligible effect within the audible bandwidth. The 'CUT' position increases this protection and rolisthe bass off below about 50 Hz. This is used where full reproduction of the lowest frequencies is undesirable - such as when rumble causes feedback through floorboards and under high level playback conditions where the possibility of damage is increased. The'OUT'setting removes the Filter and the full range of the loudspeaker is directly connected to the amplifier. This position is onlyfor use with the finest ancilliary equipmentunconditionally stable and highly damped amplifiers, silent turntables with low mass arms isolated from acoustic feedback. The Filter should only be left 'OUT' when this results in an definite improvement, otherwisethe FiIter 'IN' position should be considered normal.

It is importantthat noneof thesethree pre-set controls,'TILT' 'SLOPE' and 'FILTER', are used as 'tone controis'. They provide only subtle forms of adjustment between laboratory settings and those ideal forthe loudspeakers when at their location of use and connected to the equipment eventually driving them. Switching should be carried out at low volume settings. As soon as the optimum settings have been established, by determining the most 'neutral' combination over a wide range of differing source material, the facia plates should be replaced. It is then necessary to re-adjust the preset controls only if either the location of the loudspeakers, or the equipment driving them, is significantly altered. Regular adjustments of balance that may be required between different programme material should be carried out with the amplifier controls.

Nominal specifications

Reference Standard Professional Monitor Mk IV


41" x 16 3/4" x 19" wide

104 x 43 x 50 cm

Allow 463 (116 cm) to high on stand

Drive Units

Bass unit 11 3/4" x 8 1/4" flat polystyrene diaphragm loaded by transmission line

mid range unit 6" plastic cone containing in separate line

tweeter 1 3/4 " diaphragm

3/4" chemical dome super-tweeter


Electrical four way at 350 Hz , 3 kHz and 13 Khz

Frequency Range

17 Hz to beyong audibility

Matching Impedence

4 to 8 ohms

Efficiency Measured via Pink Noise at 1 metre on axis for 40 watts

96dB (dependant of control setting)

Driving Power Requirements

50 - 150 watts


Filter, Tilt and Slope Controls

Net Weight (each)

46 Kg plus 2.1 Kgs for stand

Gross Weight

122.5 kgs (pair with stands)