Introduction: Gourd Amplifier (for Acoustic Bass Guitar)
To develop a spherical shaped combo guitar amplifier for use by acoustic bass guitarist as portable sound reinforcement. The amplifier should ideally weigh less than 8 Kilos for easy portability, be able to run on mains or battery power and should give high quality sound and good bass reproduction characteristics despite its small size. Necessity as always provided the motivation, I wanted to improve the depth and quality of tone of my acoustic bass guitar, just enough to bring the level in line with un-amplified acoustic fiddles, whistles, small pipes etc. playing together in a typical folk music session setting.
Note: My acoustic bass guitar is well made but the body being quite heavily built to take the stress of those big bass strings it lacks acoustic resonance just where you need it most, at the bottom end. Unamplified the instrument has a thin tone and rattles because you have to really hit it hard to get the volume. When I purchased the instrument, a Tanglewood TAB2CE-ST0 I considered it to have the best bass resonance of the different models I had tried (all priced at under £400) Perhaps the more expensive offerings perform better but I would imagine any acoustic bass guitar would benefit from a little boost to the lowest frequencies.
In the same way as the loudness contour switch on a stereo amp allows us to hear music in more detail when the volume setting is low, my little amplifier boosted the bass and treble frequencies just the right amount to make the bass a significant presence in a traditional folk acoustic instrumental session in the snug bar. Session goers often frown upon anyone bringing in an amplifier, but nobody seemed to feel intimidated by my gourd, in fact most people loved the idea of it and the cuddly appearance. It became affectionately known as Colin's organic pumpkin amplifier. People still ask me about it though since the re-build it has not been used much in public.
LINE ARRAY EFFECT
The line array principle was very much on my mind when designing the layout of the small speakers in the gourd amp, having experienced playing gigs using the Bose L1 system. If you haven't heard one I urge you to do so or at least read the excellent description given on the Bose web-site:
It was the line array phenomenon that got me all excited, the line of small speakers will carry the sound of your band to the back of the auditorium with far less fall off in power so nobody in the room has to get their internal organs rearranged because of excessive sound pressure levels.
A pre-amp is needed to boost a passive electric guitar's small output to match the gourd amp's line-level inputs.
The photo above showing the gourd with Tanglewood acoustic bass guitar plugged directly without any preamp does work as the on-board piezo-bridge preamp has a relatively high output. Any guitar effects pedal will most likely serve as a preamp, I would recommend using a graphic equaliser pedal also offers flexible EQ and this is what I used most often to obtain a nice rounded bass tone from the gourd.
iPod or similar devices can be connected direct without pre-amp. The benefits of the line array effect became apparent when I used it this way, I noticed with music playing at about half volume in the lounge I would be able to hear it perfectly clearly in the kitchen next door even while washing dishes. Music playing through a conventional stereo speaker system would hardly be audible in the next room and the sound lacking detail.
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My Electronic hardware was taken from an Altec Lansing 30w desktop computer sound system, with 20w subwoofer cabinet and a left and right hi to mid range speaker each housing twin 1.5 inch drivers 6w per channel.
The exact model from which I sourced my parts seems a rarity now, I cannot find any reference to it on the web, I did however find a 5:1 variant of the system, here's a video review of the Altec Lansing VS3151 5.1 Speaker system.
It has a 20W subwoofer cabinet just like the one I took apart, this box houses the main amplifier circuits and power supply transformer. If you can get hold of this model the 10 small speakers would likely make for a more robust high and mid range sound reproduction. The decision about how to arrange them around the spherical enclosure is where your creativity comes in. I recommend assembling them close together, in two horizontal rows of 5, centre pair at the centre. In this model the centre speaker module is rated at 10W while the other four are rated at 5W each.
Now back to the Gourd amp I made.
Steps in the making process
1. A Gourd measuring approximately 355mm diameter was used for the enclosure.
2. The tapered stem of the gourd was cut off and at the point where diameter is roughly 8 inches. The cut surface was filed flat and some wooden screw-blocks glued around the inside edge so that the base board could be screwed onto that surface. The inside skin of the gourd is softer than the outside, I tried PVA wood glue but it did not work well. I found two part epoxy filler (Car body filler) gave the best result and I used this material extensively to plug holes I made in the gourd as the design developed.
3. Make the base board - about 9 inch diameter circular panel of 1/2 inch thick birch plywood. Into this the preamplifier module and subwoofer driver were mounted. Take care to mark out their relative positions before cutting anything.
4. Offset to one side - Cut a circular hole in the base board for interior-mounting the 5 inch subwoofer speaker. The original perforated steel grill cover can be screwed over the under surface of this panel to protect the cone.
5. Cut rectangular hole in the base board to accept the preamplifier module as a push fit.
6. 4 rubber door stops were screwed to this board as feet to give about an inch of clearance off the floor. This amount of space seems to enhance the effectiveness of the down-firing subwoofer and works equally well on hard or carpeted floors.
6. 5 Holes were cut into the gourd in a vertical and equatorial line to mount the 4 high frequency speakers in two pairs separated by a reflex vent tube. NOTE: The inner end of the reflex tube is best angled towards the skin of the gourd, a spherical enclosure is sure to suffer from standing waves, at certain frequencies the dead centre of the sphere would be most likely a node or dead spot with very little pressure fluctuation.
7. The electronic hardware of the amplifier including power transformer can be accommodated within the gourd. Since Gourds do not come in standard sizes and tend to be irregular in shape I see no point in specifying exact locations and measurements, you'll have to figure it all out.
Electronics Parts list:
• Taken from disassembled Altec Lansing amplifier
main circuit board with attached aluminium heat sink,
Separate control box with Volume, bass, treble controls and headphone socket.
Four 40mm dia drivers
One 5" subwoofer speaker.
• To this was added -
1 Sealed lead acid battery pack, 12volt
battery / mains selector switch - 2gang 2 way. one side is 240v make / break, the other is 18v changeover.
Stereo 1/4" Jack socket for line level input. ( A slider switch allows a mono or stereo input to the same socket )
Two RCA line input sockets mounted either directly into the skin of the gourd or a metal plate could be recessed in to give a more professional finish.
A length of PVC Tube to create a bass reflex enclosure. Approx 1.5 inch diameter, 8 inches long.
Speaker drivers: Eight full range speakers 4 inch, 25 watts each,
one downward facing subwoofer 5 inch 20 watts
How does it sound? listen
Recorded in living room with carpeted with two sofas, so... not much room reverb natural
Left channel - Close mic: Audix OM2 near the sub port
Right channel - Ambient mic: sE USB2200A about 6 Metres away
MkII design concept -
A MORE POWERFUL TONE WOULD MAKE THIS A USEFUL PRACTICE AMPLIFIER FOR ANY KIND OF BASS GUITAR.
Eight 100mm speakers in an equatorial ring and a horn arrangement on the downward facing bass port.
The MK II model is going through a lengthy development phase, here are the steps it has been through:
1. Sockets and switches: A Speakon socket with crossover wired into the lower rear side of gourd so speaker array can easily to connect to an external 50W stereo amplifier. This facilitates A/B comparison of loudness, distortion and frequency response. A SUB WOOFER polarity crossover switch allows further comparisons, having three positions it offers thefollowing options: Subwoofer in phase, subwoofer off, subwoofer out of phase. This makes quite a difference and reveals how much interaction there is between the full range speakers and the subwoofer due to air coupling around the outside of the spherical enclosure (I think).
2. The Subwoofer is isolated from the upper speaker area by enclosing the driver with a length of plastic drainpipe 170mm internal diameter, capped with a disk of 15mm thick plywood, bolted to the base board with 6mm studding. this plywood disk also provides an anchor for a length of 6mm studding bolted through the top of the gourd to firmly fix the subwoofer assembly to the gourd. (MK I had the sub baffle screwed to blocks of hardwood glued inside the edge of the gourd, prone to splitting)
3. The eight equatorial speakers wired in two rings of four, i.e. alternating L & R channel so one speaker at each 90 degree compass point. this arrangement destroys any stereo image but I wanted to experiment, what happens when one set is switched to reverse phase ? (with the subwoofer switched off, all lower frequencies cancel out, only a thin trebly sound is heard) this has no practical use except to demonstrate this cancellation effect. I don't do well with purely theoretical designing, I need to put theory into practice and train the ears.
4. The gourd sounds like it is pumping too much pressure and there is no vent, except the odd ¼ inch socket hole. Driven by external amplifier (no subwoofer) the output is clearly much louder but the tone is coloured, i.e. not natural transparent sound that hifi designers aim for.
NEXT PHASE OF DEVELOPMENT - Active / passive speaker array
5. Rewired the 8 full-range 4 Ohm speakers. Four front facing speakers are series wired in pairs, so each channel drives two speakers, series wired to give 8 Ohm load. This wiring arrangement eases the loading on the two 6watt channels, this in theory lowers the power output, however having two speaker cones moving in sync gives the line array effect and the apparent loudness seems not much diminished. (subjective observation, some measurements would be useful here)
6. The remaining four speakers facing backwards are not wired to anything so they act as passive radiators, thus the gourd behaves somewhat like an open baffle loudspeaker • , i.e. A flat mounting board without any enclosing sides or back. The gourd clearly is an enclosure but its speakers radiate sound in a similar way to the open baffle, as the front speakers push out the rear ones pull in. I think this gives a better tone and radiates the sound just as much around the room as with all 8 speakers working in unison.
This is all theoretical, a sound studio equipped with microphones and spectrum analysers would be needed to develop the speaker design in a scientific way, I don't have those things so I'm reading all I can about HiFi speaker design and being guided by intuition and observation, i.e. using my ears.
I will post more as the project develops.
• Here is a link to an article about open baffle speaker design