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Super portable Twin Dipole Guitar/PA Speaker by Allan-Linton Smith Features & Sp Drivers: Weight: Efficiency: Total cost: ecifications Two 12-inch Ce lestion loudspeakers (recommended) about 17kg 100dB/1W<at>1m ~$350 with the recommended dr ivers Drive options: top only, botto m only or both Simple construc tion Easy to fold up and transport 24 Silicon Chip Australia’s electronics magazine siliconchip.com.au This guitar/PA speaker is dead easy to build and it’s really portable. It’s also LOUD and it sounds great. When on stage, you can project plenty of sound to the crowd while still being able to hear your own playing (no foldbacks needed). If you build it using our recommended drivers (as used by Jimi Hendrix and Slash), it has a mellow, old-fashioned tone. But you can also use different drivers for a more modern, harder sound. B ecause of the way this speaker is designed, it can be folded flat for easy transport and can be built by anyone with only rudimentary carpentry skills and tools. You can easily build it in a day, with a perfect finish and a professional appearance. You can even choose from a range of colours to suit your band! provide a tone to suit an electric guitar. And three, there are 22 different drivers in the series to choose from with their own unique sound profiles, power levels and efficiencies. This includes four from Celestion’s Heritage series, three from the Alnico series, one from the Signature series (the Eddie Van Halen), 14 from the Classic Series and two from the Originals series. Choice of drivers You can see a list of suitable drivers at: https://celestion.com/product/26/ One of the great things about this heritage_series_g1265/ design, besides the advantages laid For many of the drivers, audio samout above, is that you can choose ples of guitar playing are provided, from a range of drivers which have allowing you to get an idea of what it their own unique sound. sounds like before purchase. And because it’s a twin-driver rig, Note that this list includes some you can use two different ones (as drivers which are not suitable, ie, those we have done), giving you the opwhich are not 12 inches (305mm) in tion of three different tones: upper diameter or those which are not availdriver only, lower driver only or Guitarist Marcus Child, (from the band able with a 15-ohm or 16-ohm nomiboth together. “Country Members”), putting the dipole nal impedance. The drivers we are recommending speaker through its paces. The maximum power level of the come from a series with a long history. They have been used by some people you may have suitable Celestion drivers ranges from 15W up to 100W. Our unit uses the G12M Greenback and Vintage 30 drivheard of including Angus Young, Jimi Hendrix, Slash, Eric ers, which are rated at 96dB <at> 1W, 20W maximum and May, Brian Gibbons and Eddie Van Halen. While you can use just about any two 12-inch (30cm) 100dB <at> 1W, 60W maximum respectively. Since these are from the Heritage and Classic series, they drivers, we are recommending Celestion units in this progive a laid-back sound with plenty of mid bass and highject for several reasons. One, they are good quality. Two, they are designed to end. If that isn’t your bag, see below for some other options. Fig.1: the distortion level is not particularly low but sometimes, that’s what a guitar player actually wants! siliconchip.com.au Fig.2: the usable frequency response is from 80Hz to about 8kHz – more than enough for guitar and even PA use. Australia’s electronics magazine September 2018  25 The G12M was a favourite of Jimi Hendrix; he used a whole raft of them to handle his powerful riffs and was also used extensively by Eddie Van Halen. On the other hand, the Vintage 30 has been used by Slash, Steve Stevens and Peter Frampton. They’re both available through Australian distributors; see the parts list for details. These drivers are designed to be rugged and use paper cones with a small amount of doping. They are also very sensitive and are generally rated at 96-100dB/watt at one metre because the lead guitar has to be LOUD! Not your grandpa’s guitar speaker Traditionally, guitar speakers consist of a small wedgeshaped box with one to four drivers arranged along the front. These generally have no acoustic filling, bracing or damping and often have a resonant or “boxy” sound which accentuates notes played at the resonant frequency of the box. This gets boring pretty quickly. Our aim for this project was to provide the guitarist with a sound which specifically suits their instrument. That’s quite a personal thing but since the enclosure is customisable, that’s no problem! We settled on a “dipole” arrangement of two 12-inch drivers which can be used individually or together, depending on whether you’re practising or performing. If you have multiple electric guitars, you may find that some work better with one driver while others sound best with the second driver. Vive la différence! The speaker’s “angle of attack” can also be easily adjusted to suit different venues. The dipole completely removes cabinet colouration so that the resulting sound is quite neutral across the entire audio spectrum. Basically, you’re just hearing the characteristics of the driver, which is determined by the manufacturers. And since the drivers we’re using are specifically intended for lead or rhythm guitars, that’s the ideal situation. The sound from the dipole setup is not terribly directional; that is to say, it doesn’t matter terribly where you point it because the sound is much the same at the back, front or sides. So not only is it its own foldback speaker, the crowd will enjoy your gig regardless of whether they are right in front of the speaker or off to the side. Testing it out We had our resident guitar player, Bao Smith, test the speaker and he confirms that it has the most important property that any guitar speaker needs: it can definitely make a lot of noise and sounds good doing so! Allan also had his guitar-crazy mate Marcus test it out in an extended jam session, using a Gibson guitar and 30W valve amplifier. Afterwards, he commented: “It’s great for a lead guitar and has plenty of volume which will rise above the other instruments in my band.” (We had to wear hearing protection while he was playing!) I made an efficiency measurement with both speakers connected in parallel and got a result of 105dB/watt at one metre – that’s pretty amazing! We have made some performance measurements but remember that this speaker is not intended for hifi use, so we aren’t looking for ultra-low distortion. In fact, many guitar players like having plenty of distortion! A plot of distortion against frequency for the two drivers in parallel is shown in Fig.1 and you will notice that distortion is very high below 80Hz and above 7.5kHz. That’s because these frequencies are outside the response of the drivers and so the sound level is dropping off quite significantly. Anything below about 60Hz has too great a dropoff to be audible. There are some spikes at intermediate frequencies (eg, around 320Hz) but these could be measurement artefacts as they do not seem to be audible. Fig.2 shows the frequency response of the two individual drivers plus the combined drivers. These are very “noisy” measurements due to the fact that the microphone has been placed some distance away from the speaker in order to provide a realistic result. But room resonances and interference then affect the readings. The “near-field” responses, taken with the microphone right in front of the two drivers, do not suffer from this. In any case, you can see that all of the responses start to fall off below about 100Hz and above 6-7kHz. That’s a pretty wide range for a single driver. The lowest string on a six-string guitar is normally tuned to 82Hz (E) while the highest fret on the highest string is normally tuned to 1047Hz (see fret table). The speaker response covers this entire range of frequencies with plenty of room on top for harmonics. Of course some of the more popular alternative tunings (dropD) have lower frequencies (73Hz). Speaker impedance Fig.3: the combined impedance from both drivers barely dips below 8 ohms, so should not be a problem for the vast majority of amplifiers (even “hifi” amps!). We even succeeded driving it from the headphone output of a guitar amp. 26 Silicon Chip Australia’s electronics magazine We’re using the 16Ω version for both speakers, so when driven in parallel, they present a modest 8Ω siliconchip.com.au The two 12-inch drivers we’re recommending, both from Celestion. At left is the G12 M Greenback; at right is the Vintage 30. You can substitute other drivers but we can’t guarantee that they will perform as well as these do. load to the amplifier. The impedance of each combination of drivers is shown in Fig.3 and this shows that when driving both in parallel, it barely dips below 8Ω. So any amplifier should be able to easily drive them both. The resonance peaks (clipped off) are 127Ω at 80Hz for the G12M and 159Ω at 63Hz for the Vintage 30. You could use 8Ω drivers instead, indeed, there are 8Ω versions of both drivers specified; the Altronics driver mentioned below is 8Ω only. In this case, make sure that your amplifier will handle a 4Ω load; most will and they will typically deliver more power into a 4Ω load than an 8Ω load. So if you’re building the speaker with high-power drivers, that will be your best option. Note though that if you do this and then you add more speakers in parallel (eg, monitors), the combined impedance may fall below 4Ω and overload your amplifier. Many guitar amps are valve-based and since they require output transformers, they usually have several output impedance taps, which may influence your selection. Choosing an amplifier The reason guitarists tend to prefer valve amplifiers is for the way they sound when they’re overdriven, as a valve amplifier will typically sound better than a solid-state one when driven to its limit. This speaker works really well with valve amplifiers because of the fact that it has a relatively high impedance and because of its high efficiency, which suits the relatively low power output of a typical valve amplifier. But you certainly can use it with a solid state amplifier and it doesn’t necessarily have to be a guitar amp. As long as you have appropriate signal conditioning (ie, some form of guitar preamp), you could use a hifi or PA amplifier too. You don’t even need to use it with a guitar! Fig.4: cutting diagram for both the front and rear of the dipole speaker panels, which are joined at the top with butt hinges to form an A-frame. We used Kaboodle door panels from Bunnings but chipboard, MDF etc would be fine. siliconchip.com.au Australia’s electronics magazine September 2018  27 Fig.5: the circuit is very simple indeed, basically it is just the four 6.35mm sockets and some wiring between them and to the speakers. You could use it with a different type of electric instrument like a synthesiser, bass, harpejji or you could connect a microphone and a suitable preamp/amplifier and use it as a portable PA system. Sourcing the timber frame The two halves of the “sandwich board” are made from Bunnings Kaboodle kitchen cabinet doors which measure 720x450x18mm. We purchased these in gloss white but there are many other colours to choose from. Why not try a piano black finish, or be daring and go for “seduction red” in full gloss, or a more conservative “Myrtle gloss”? Many Bunnings outlets have samples of their finishes on display so you can look at and touch them before deciding. Note that gloss white is usually in stock at most Bunnings stores but other colours may need to be ordered and will take about two weeks to arrive. We used Kaboodle cabinets to build the Majestic (June & September 2014; siliconchip.com.au/Series/275) and Senator (September & October 2015; siliconchip.com.au/Series/291) speakers. They have very good acoustic properties. You could also use plywood, MDF or any other material instead as long as it is at least 18mm thick. The rear support is not critical and 16mm thick material (eg, melaminecoated fibreboard) is suitable. The two halves are attached at the top with two 85mm stainless steel hinges. Junction box operation The circuit for the junction box is quite simple, as shown in Fig.5. It uses just four DPDT switched stereo jack sock28 Silicon Chip Fig.6: and here’s a pictorial view of that wiring. The labelling of the four input sockets coincides with the panel artwork shown in Fig. 7. ets and some wiring to perform all the necessary functions. The sleeves of all four sockets are joined together to form a common ground connection, which is wired directly to the negative end of both drivers. All the signal routing is done to the positive side of the drivers. Fig.7: same-size front panel artwork for the Jiffy box mounted alongside the speaker drivers. You can also photocopy this (or download it from siliconchip.com.au to use as a drilling template for the four input sockets. Australia’s electronics magazine siliconchip.com.au Each socket consists of the three usual contacts for a stereo socket – tip, ring and sleeve – plus two insulated double-throw switches which are actuated when a plug is pushed past the tip and ring contacts respectively. These switches are used to route the connections to provide the required functions. CON2 is an output socket to go to a monitor amp (eg, driving headphones worn by the player) and the switch at pins 5, 6 and 7 of the other three sockets are connected such that when you insert a plug in any of those other sockets, its tip connection (at pin 4) is routed to the tip of the monitor socket, via the normally open contacts of the switches (pin 5). This means it receives the input signal regardless of which driver(s) are being driven. By the way, you should never plug your headphones directly into CON2 if you value either your headphones or your hearing! The normally-closed half of the three double-throw switches is used to apply the signal to the correct loudspeaker, based on which socket you have inserted the plug into. If you plug into CON1, the tip connection is wired directly to the positive terminal of SPEAKER1 so signal flows to that driver. The NC terminal of that socket’s switch at pin 7 is no longer connected to pin 6 so the signal does not flow to the other driver. Similarly, if you plug into CON4, the tip connection feeds the signal to SPEAKER2 but the switch is disconnected from anything but the monitor socket and so the signal does not go to SPEAKER1. But if you plug into CON3, the signal from the tip is instead fed to the NC terminals of the switches in both CON1 and CON4 and since nothing is plugged into those sockets, the signal then flows to the positive end of both SPEAKER1 and SPEAKER2 via pin 6 on those sockets. Construction The assembly process is straightforward and assuming you have the right tools on hand, you should be able to go from an assortment of parts to a finished speaker in a few hours. You need to make two circular cut-outs in the front panel and one large rounded-rectangle shape cut-out in the rear panel. The details are shown in Fig.4. Start by marking the 283mm driver cut-outs in the front panel using a compass, then carefully cut out the circles using a jigsaw. Note that if you are using Kaboodle cabinets or similar coated timber then you should cut them from the reverse side using a good quality, fine tooth jigsaw blade (preferably a new one!). Use a similar process to cut the large hole on the rear panel. You can mark the two circles in the same manner, then join them using a long straightedge before making the cutout. Lay the two panels end-to-end and mark out the drilling locations for the hinge attachment screws. Drill small pilot holes (again using masking tape to protect the front finish), then attach the hinges to both panels using 15mm countersunk wood screws and check that the two boards fold correctly. They should fold flat against each other. The next step is to attach the drivers. This is easiest to do if you fold the assembly together and support it horizontally between two stable benches or other supports. Drop the drivers into the holes and rotate them so that the siliconchip.com.au Parts list – A-frame Guitar/PA Loudspeaker 1 16-ohm Celestion Classic Series G12M Greenback 12-inch driver [Electric Factory (T1221) or Scarlett Music] and 1 16-ohm Celestion Classic Series Vintage 30 12-inch driver [Electric Factory (T3904)] or 2 8-ohm 12-inch 100W polypropylene woofers [Altronics C3070] (see text) 2 12-inch metal speaker protection grilles with mounting brackets [Altronics C3712] 2 Kaboodle 720x450x18mm kitchen cabinet doors [Bunnings] or plywood/MDF sheets (see text) 2 85mm stainless steel butt hinges [eg, Bunnings 4160027] 1 slim carry handle [Altronics C3660, Jaycar HS8022 or Bunnings 4230073] 8 No.3 x 10mm countersunk head wood screws (for mounting drivers) 4 No.3 x 20mm pan head wood screws (for grilles) 12 No.4 x 15mm countersunk head wood screws (for hinges) 2 No.3 x 40mm pan head or countersunk head wood screws (to suit handle) 2 No.4 x 10mm pan head wood screws (for mounting Jiffy box) 2 19mm cup hooks [eg, Bunnings 3930140] 1 350mm long cloth or rubber strap with loops at each end or 1 400mm length of small diameter rope/blind cord 4 DPDT mono or stereo 6.35mm jack sockets, chassismounting [Jaycar PS0182, Altronics P0072] 6 heavy-duty adhesive 20 x 50mm felt strips (or larger strips cut to size) 1 UB3 bulkhead Jiffy box (130 x 67 x 44mm [not including flanges]) 1 adhesive panel label for Jiffy box lid 1 1m length of speaker cable (or two 1m lengths of red & black heavy duty hookup wire) various lengths of heavy-duty hookup wire (see Fig.6 for suggested colours) labels on the back will be right-side-up, then mark out the mounting holes. Remove the drivers and drill these with a pilot drill bit, then reinstate the drivers and attach them with 10mm countersunk wood screws through the front of each surround and into the front panel timber. Now place the grilles on top of the driver surrounds, arrange the supplied mounting brackets around the edge (equally spaced), as shown in our photos, and mark the required hole positions for these brackets, then remove the brackets and drill pilot holes in those locations. Fix the brackets to the front panel using 20mm wood screws, ensuring that the grilles are held firmly in place. You can now attach the handle to the top edge of the front panel, again by drilling pilot holes and then attach it with 40mm wood screws. Now is also a good time to stick the adhesive felt strips on the bottom of both panels, one at each end and one in the middle. Next, drill a couple of pilot holes at corresponding points on the inside of the front and rear panels, so that you can Australia’s electronics magazine September 2018  29 Looking through the rear panel, showing the two speaker drivers and input box secured to the front panel. (Its position is not important – just make sure the rear panel cutout is large enough to accommodate it when closed). You can also just see the cord which stops the front and back panels opening too far. (Yeah, we know we could have made the rear panel cutout a bit straighter . . .) And when you’ve finished your gig, simply unplug the amplifer, fold the two halves together and carry the speaker away. Mind you, at about 17kg (most of which is the two 12-inch drivers), we hope you don’t have to walk too far! This photo also shows why such a large cutout is required on the rear panel (otherwise you would not be able to fold the front and back flat). lid. For more information, see our website at siliconchip. com.au/Help/FrontPanels for details. With the label in place, attach the sockets using the supplied nuts and then solder various short lengths of heavyduty hookup wire between the sockets, as shown in Fig.6. This makes the connections as shown in the circuit diaJunction box assembly gram, Fig.5. Note that several amps can flow through this The first step is to drill four holes down the centre of wiring if you’re driving the speaker hard, hence our recthe Jiffy box lid and then enlarge them (using a stepped ommendation to use heavy-duty wire. drill bit or tapered reamer) until the sockets are a good fit, You can use the same wire, twisted together, to connect without being too loose. to the drivers. That’s how the prototype was built. Or you The panel label shown in Fig.7 can be used as a template could use figure-8 speaker wire, which would be a little to space these holes. It can be photocopied or, if you pre- neater. Solder the four speaker wires to the socket termifer, downloaded from siliconchip.com.au/Shop/11/4688 nals as shown in Fig.6, then drill a small hole in the side Stick on the panel label and cut out the socket holes with of the Jiffy box and feed the speaker wires out through this a sharp hobby knife. You can print it and laminate it, then hole, then solder them to the tabs on the drivers. attach it using contact adhesive or silicone sealant. Or you Make sure that the wires for SPEAKER1 go to the top could print it mirrored on transparent film and glue it on driver and the wires for SPEAKER2 go to the bottom driver with the ink towards the lid, using a thin smear of clear and don’t get the positive and negative wires mixed up or silicone sealant. you will get sound cancellation when using both speakYou can also get adhesive-backed paper for inkjet and la- ers at the same time. ser printers which you can simply cut out and stick on to the Next, drill a couple of holes in the Jiffy box base and two corresponding pilot holes in the back side of the front panel. You can then feed a couple of short wood screws through the inside of the Jiffy box and into the holes on the panel, then screw the lid onto the box and the whole assembly should be firmly attached to the speaker. Refer to the photo above to figure out the best location for mounting this box. That’s it – your speaker is finished. Now all you have to do is connect a lead from your guitar amplifier’s external output socket to one of the three input sockets Fig. 8: we show this more for interest sake than anything else – it’s the on the Jiffy box and you’re ready frequencies of each note when either a rhythm or bass guitar is tuned correctly, to jam! The dipole speaker as described here will handle notes down to about 80Hz. SC screw in the cup hooks and then tie the cloth strap or cord between them, to limit how far the assembly will open. This prevents it from falling over when in use. Adjust the length of the strap until you are happy with the angle that the panels sit at when opened up. 30 Silicon Chip Australia’s electronics magazine siliconchip.com.au