The Bus Speaker

Extension Loudspeakers are often converted to CAIRO simply by installing a 3-pin DIN plug on the original two-core cable (at pins 1 & 2) and this is a perfectly legitimate practice. However, when the item is plugged-in, for example, at the Auxiliary socket at the rear of a Dual Operator Box, it occupies that socket to the exclusion of all the other purposes which that DOB socket might otherwise support; e.g. a shared Trample-to-Talk footswitch or a fist-mike for occasional use.

To avoid this "loss" of a sometimes significant socket in a configuration, an extension loudspeaker may be completely rewired to have a full CAIRO tail (four-core, individually-screened cable) with a 5-pin DIN plug on its free end. This is then wired in the usual pin-mapped "bus" convention, through to a DIN-7 chassis socket which is set in an unsightly corner at the rear of the loudspeaker housing. The loudspeaker itself is connected between ground and the incoming speaker core as it passes through to pins 1 and 6 of the auxilliary socket.

Now, all the basic CAIRO signals which were present at any socket into which this item is plugged, continue to be available from the rear-corner socket of this extension speaker, for other accessories. By this means it will always maintain the count of outlets in any CAIRO configuration where the item is used. It also means that a simple, single-operator termination can comprise of this extension speaker, on a desk say, with just a fist-mike plugged into it - somewhat like operating an actual transceiver !

For a low-traffic single-operator Outstation, this speaker might well be used with a telephone handset to provide the necessary "intimacy" when traffic is specifically for this station, while the speaker supports the general "monitoring" requirements inbetween times. Trials have confirmed that this is a very satisfactory facility for a lightly-loaded solo operator.

Bus-Speaker Controls

Once this "bus" scheme has been adopted for a particular extension loudspeaker, there are several refinements which can be considered for additional versatility. These are based on two controls which, like the cable-entry and outlet socket in the lower rear corners of the housing, occupy the upper corner voids that invariably occur inside square or rectangular cases when the loudspeaker itself is of circular or oval construction. The first refinement (Fig.1- left) includes a 100 ohms, 2W cermet potentiometer as a volume control for the speaker itself, though not the onward speaker line coupling to the DIN-7 socket pins 1 and 6.

Figure 1

Figure 2

Rear View

Volume Control & Listen-Loud Switch

Bus-Speaker Rear-Panel Layout

The second refinement (Fig. 1 - right) includes a "listen-loud" feature to override the volume control. This uses a momentary-action, single-pole, double-throw switch to supply the loudspeaker from the potentiometer wiper, through the normally-closed (NC) pair of contacts. The normally-open (NO) contacts are wired to supply the speaker from the incoming line, at full level.

If the switch is a push-button type (e.g. [320-657]), mounted on the front of the housing, say at the top-left corner, the listener may press it to override the present setting and hear received traffic at the maximum available level. At some times, in some configurations, a speaker wired like this may have been muted by having had its volume control turned down to OFF. If so, this switch gives instant recovery to full level for anyone who needs to hear an exceptional item of Net traffic. Equally, if the volume control has been adjusted for a non-zero, comfortable working level, the use of this switch, perhaps by a Controller, leaves the Operator's preferred setting unaltered afterwards. (The top left-hand corner is suggested because it favours the right-handed user who may well be required to write messages while using this facility.)

When there is insufficient space on the front of the housing for a push-button switch, the "listen-loud" facility can be achieved by installing a biased-action toggle switch (e.g. [317-162]) on the rear-panel, which users can 'find' with their fingers, more easily than a push-button, as they reach over the housing from the front. (See HOIME illustration.)

When a toggle switch is used and it is installed at the top-right corner of the rear panel, for left-handed "reach-over" use, the volume control can be set in a mirror postion on the top-left of that panel. Again assuming that "reach-over" adjustments will be made, this volume control can be wired unconventionally, for its output to increase with what is a counter-clockwise rotation of the potentiometer itself, so that it requires a natural clockwise rotation when approached from the front. To ensure maximum control convenience under these circumstances, the larger diameter knob (e.g. [498-839]) is recommended.

Finally, the volume control may be wired to include a "pedestal" resistor if it is felt that a loudspeaker should not be capable of being turned fully OFF, when in use in a CAIRO configuration. The general "pedestal" resistor is 6R8, but an optimum value may be selected on test because it is specific to the loudspeaker in question and may be chosen to give a satisfactory minimum "Off" level according to the acoustic efficiency of the device itself and the housing in which it is mounted.
The lower value of 2R2 was found to be most satisfactory in the author's evaluation prototype.

Many of the above features may be incorporated into the solo operator's Desk-Top Console accessory.