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PROJECTS (04)

EXTENSION SPEAKER - High efficiency extension speaker

Extension
                          Speaker by M0MTJ
Extension Speaker by MØMTJ


Here's a nice simple little construction project that will make pleasant use of hour or two and produce a very useful communications speaker. My finished loudspeaker has a pleasant, clear sound and is very efficient, making the most of any transceiver's audio output power.

This speaker that I used is very efficient so that it makes the most of the audio output of the radio. For example the action of the volume control on my Yaesu FT-7900 (oddly) stops having any effect on increasing volume after the one o'clock position. This is an effect common to all FT-7900's that has been reported elsewhere. With most stations this is not a problem, but with some stations with very quiet audio there is not enough adjustment to increase the volume level when using a typical, fairly low efficiency extension speaker. However, now that I have this efficient loudspeaker connected I find that there is plenty of volume available for quiet, low deviation, stations.  


I have found that most of the small communications extension speakers that I have used have a harsh, resonant and 'tinny' sound that does assist in making speech especially intelligible. Another problem is that many speakers are also quite inefficient - that is to say that they need a good degree of audio power to drive which can mean that the audio amplifier in the transceiver may be run towards its power limitations resulting in undesirable harmonic distortion or even worse, clipping distortion.

To overcome the harsh, tinny audio reproduction requires the use of a larger loudspeaker drive unit. To maximise the available audio output of the transceiver's audio amplifier requires the use of a highly efficient loudspeaker driver unit.

Loudspeaker efficiency can vary quite a lot. For example, a typical hi-fi loudspeaker for a good quality stereo system may have an efficiency of between 84dB to 89dB.  Many operators use a hi-fi speaker with their transceiver and they will certainly work. I have done so with satisfactory audio results, but when turning up the audio gain control (volume), distortion can result due to there not being enough power to drive what may be a relatively inefficient loudspeaker system. The speaker I used had a quoted efficiency of 86dB/ Watt / 1 metre - so not especially efficient, but fine when used with a 50 watt hi-fi amplifier. 2 or 3 watts from a rig is a little different.

If the loudspeaker efficiency is increased by 3dB, that is equivalent to doubling the  power of the audio amplifier. Likewise a speaker with a 3dB lower efficiency is equivalent to halving the audio amplifier's power. So loudspeaker efficiency is very important when the typical transceiver 'only' produces a power of around 2 or 3 watts audio output.   


I found a number of high efficiency drive units available. Maplin supply a couple of quite large and very efficient loudspeaker drive units that are perfect for communications speakers. Sound quality from both of these units is good when mounted in a good enclosure and efficiency is very high at between 96 dB and 99 dB / Watt:

The GL12 is a medium sized 120 mm (4.5 inch approx) diameter unit. Specifications: Round; 6 watts power handling; 8 Ohms impedance; Frequency Range 120Hz to 9.2kHz; Efficiency 96 dB (+/- 3dB) / Watt.

The GL13 is a larger 165 mm (6.5 inch approx) diameter unit. Specifications: Round; 10 watts power handling; 8 Ohms impedance; Frequency Range 80Hz to 9kHz; Efficiency 99 dB (+/- 3dB) / Watt.


However, for this project I wanted something a little smaller. After some research I found a couple of interesting loudspeaker drive units at ESR in Newcastle:

The ESR 203-023 measures 38 x 105 x 156mm.  Specifications: Elliptical; 4 watts power handling; 8 Ohms impedance; Frequency Range 135Hz to 10kHz; Efficiency 95 dB (+/- 3dB) / Watt.

The ESR 203-025 (Eagle L001B) measures 36 x 77 x 125mm. Specifications: Elliptical; 6 watts power handling; 8 Ohms impedance; Frequency Range 130Hz to 9kHz; Efficiency 92 dB (+/- 3dB) / Watt. (Eagle reference number L001B)


The Eagle L001B (ESR part number 203-025) was chosen as it conveniently fits into an MB5 ABS enclosure. The MB5 box is available from many suppliers and is a good size for a speaker cabinet measuring 150 x 100 x 60mm - perfect for mobile use and also for placing on the shelf in the 'shack'.


Update 2015: For another loudspeaker project (shown below here) I needed a smaller, round, 3 inch (76mm) diameter loudspeaker. I found one at Bowood Electronics (part number SPK005). Once the project was complete and I had connected it up, I found the audio quality to be at least as good as the the Eagle L001 shown in this project.

I would therefore recommend that you also consider the use of the Bowood Electronics SPK005 3 inch loudspeaker for this project. It would also allow you to use a slightly smaller enclosure if you wish.

Bowood_Electronics_SPK005
The Bowood SPK005 loudspeaker


Bowood SPK005
                                loudspeaker unit
An SPK005 installed in a different project shown below here


In the final version of my loudspeaker, I decided to mount the drive unit inside the MB5 box with 4 nylon screws and nuts. The grill was drilled out using the pattern shown in the photograph below.

Five or more additional vent holes also need to be drilled in the lid of the enclosure (on the opposite side to the speaker), otherwise the sound will be too restricted. I also lightly stuffed the enclosure with some felt wadding, also shown in the photographs. This felt wadding was cut from an old ironing board pad that I had saved - just in case. I am glad I did, it's very useful material!

The speaker cable exits through a hole in the rear panel (lid) - smoothed off with a file and countersunk either side, the minimize wear on the cable. The cable is also tied into a knot on the inside of the case, as a strain relief, to prevent the soldered connections being pulled off the loudspeaker terminals.

I found that the lid of the MB5 box vibrated at medium to loud volumes, even when the screws were tightened down. To prevent this I removed the lid, stuck black insulation tape around the edges of the box, then replaced the lid. The vibrations were then absent.

If mobile mounting is required, it would be a simple matter to form a U bracket from an piece of metal and fix it to the sides of the case with short bolts or screws.

 
Extension Speaker
                          by M0MTJ
The Eagle L001B (ESR 203-025) loudspeaker unit with MB5 ABS enclosure, marked up for drilling the front grille.



Extension
                            Speaker by M0MTJ
The Eagle L001B (ESR 203-025) loudspeaker unit should be attached inside the MB5 box with 4 nylon screws and nuts.
The lid of the MB5 box is used as the rear panel, with some vent holes drilled into it.

Extension_Speaker_M0MTJ

Extension_Speaker_M0MTJ
The enclosure is lightly stuffed with some felt wadding to help minimize resonances.
The felt wadding was cut from an old ironing board pad.

I found that the lid of the MB5 box vibrated at medium to loud volumes, even when the screws
were tightened down. To prevent this I removed the lid, stuck black insulation tape around the
edges of the box, then replaced the lid. The vibrations were then absent. 


Extension
                            Speaker by M0MTJ
Photograph showing the vent holes drilled in the rear panel of the finished loudspeaker cabinet.

The internal wadding can also be seen through the vent holes. The speaker cable exits through
a hole in the rear panel - smoothed off the minimize wear on the cable. The cable is also tied into
a knot on the inside of the case, as a strain relief, to prevent the connections being pulled off the
loudspeaker terminals.


Extension Speaker by M0MTJ
The front view of the finished loudspeaker


Parts List

Eagle L001B elliptical loudspeaker
(ESR part number 203-025 - http://www.esr.co.uk )
or
Bowood Electronics SPK005 - www.bowood-electronics.co.uk

MB5 ABS Enclosure - or other enclosure of choice

4 Nylon or zinc plated steel Screws and Nuts
or
 4 steel machine screws and nuts and small terminal tags to fit the SPK005 (or use contact adhesive)

2 Metres of 2 Conductor Cable

3.5mm Jack Plug

Felt Wadding

A suitable piece of sheet metal and two small nuts and
bolts if a mobile mounting bracket is required

Extension Speaker by M0MTJ
Rear view of the finished speaker





Modifying a Pure S-1 DAB radio extension loudspeaker

Some time ago I picked up a used extension speaker designed to add stereo sound to a mono Pure DAB portable radio. It looks very nice in its real wood veneer cabinet and I thought that it might make a nice, quality, external speaker for an amateur radio.

I was disappointed to find that while it looked great, it lacked treble response and was also very inefficient, needing a lot of audio power to drive it to good volume. It therefore sat on the shelf for a couple of years without being used. Then, I thought, I must do something about this!

I dismantled the speaker and removed the original full range drive unit. It was fixed in place onto the front baffle with four small screws and some glue around the edge. Once the screws were removed it was possible to ease the speaker away from the baffle.

I found it to be a 3 inch (76mm) diameter device, so I checked the Bowood Electronics website and found a 3 inch speaker being offered for sale, part number SPK005.

Unlike the original unit, the SPK005 speaker did not have four holes around its circumference to secure it. Therefore it could have been simply glued into place using a small amount of impact adhesive (Evo-stick) or, as I opted to to, secure it into place using the four screws and four small clips. (see photo below)

I did not have any specific purpose designed speaker clips, so I searched my parts store and found some small washers and some small electrical terminals that seemed about the right size and were reasonably sturdy. Once screwed down with the washers and terminals, the speaker was firmly held in place.

On testing with an FT-7900, I found the sound quality to be very good 'communications quality audio', very crisp and clear. Due to the high efficiency of the SPK005, it was able to be driven to more than adequate volume levels.

Please refer to the photographs below for more detail:


M0MTJ
The Pure S-1 extension speaker cabinet (left) with the original drive unit (right) removed.


M0MTJ
The 'full range' 3 inch drive unit originally fitted inside the Pure S-1 extension loudspeaker.

 
Bowood SPK005
                              loudspeaker unit
The Bowood Electronics SPK005 loudspeaker


Bowood SPK005 loudspeaker unit
The Bowood SPK005 installed onto the front baffle using the original screws, some washers and small electrical terminals.


I definitely recommend the Bowood Electronics SPK005 and would not hesitate to use it for any other project.


Suppliers of
                            Electronic Components - Bowood Electronics
www.bowood-electronics.co.uk





Hiss 'Filter': This low pass filter is entirely optional and can be omitted if not required. If the filter is required I found that by simply switching in a 100uF (non polarised) electrolytic capacitor across the loudspeaker terminals worked quite well. This has the effect of reducing hiss, by rolling off the treble audio, and can make listening to weak signals a little less unpleasant.

* Important Note: Admittedly this is not a very technically elegant solution, but it seems to work well enough in this application.The impedance of a 100 uF capacitor at 3000Hz will be very low (maybe less than 1 Ohm), so the audio output stage of the rig will be driving into a very low load impedance at these frequencies - this is what causes the high frequency roll-off. This arrangement is may result in higher distortion levels and might overload the audio stage. However, there are several commercial extension speakers that use this method of 'filtering' and I have used this method on various radios without any noticeable distortion or other effects. Use at your own discretion.

Part Required:

SPST (on-off) switch
100uF 63 volt Non Polarised Electrolytic Capacitor

Extension Speaker by M0MTJ

Extension Speaker with
                                            Low Pass Noise Filter by
                                            M0MTJ
The completed speaker with the 'filter' switch fitted
Extension Speaker by
                                            M0MTJ




Suppliers of
                            Electronic Components - Bowood Electronics

Icom


Links to Electronic Component Suppliers >>





DISCLAIMER
: If you attempt any of these projects proceed with due caution with regard your
own safety and the safety of the equipment that you are working with!

 
I cannot be held responsible for any accidents, injuries or damage
caused to any equipment that may result.








Mike Smith - MDS975.co.uk © 2003 - 2016





M0MTJ