Intro: Keyboard Amp/Desk
A portable worktop for keyboard & Sound modules, with built in stereo amplification*.
- Why build it?
Do you wish your keyboard had speakers ?
This desk gives full range stereo sound near-field, 20W amplification.
A stage piano or keyboard-synth needs an amplifier, A quality keyboard combo amp is bulky and heavy.
20W on-board amp is loud enough for rehearsal or performance alongside unamplified instruments. Secure platform for keyboard, outboard effects / MIDI sound modules, Semi-concealed wiring.
Some woodworking skills are needed, but no complicated joinery, it is a relatively simple project for the confident craft woodworker. Skills: hole saw drilling, high speed routing, hand sawing, planing, gluing, clamping.
- WHY DID I BUILD IT ?
Working with iPad instruments and synthesisers. (Where to put the iPad ?)
Music apps for iPad now include a growing range of performance grade synthesisers and sampled instruments. To incorporate the iPad into a performance rig it needs a secure mounting and audio interface. Focusrite iTrack dock does the job very well. The dock is really aimed at the home studio market, hardly robust enough for regular gigging but it has good connectivity, well designed controls, pro audio quality .
I started out with iTrack Dock velcro strapped to a board screw mounted on a photographers tripod. I did a couple of gigs with this, it worked well but took up precious rehearsal time setting up keyboard & stand, combo amplifier, dock on a tripod, and a tangled mess of cables. I started taking an old wooden board to rehearsals with a crude raised plinth at the rear edge on which the iTrack Dock and several effects pedals were balanced. After several months of modification and re-building the self contained desk/amplifier took shape partly out of necessity, and partly to satisfy my urge to build an adaptable multi-purpose electronic music work-bench.
- WHAT WILL IT DO ?
The prototype shown is intended to sit on an X frame keyboard stand making it suitable for home or stage use.
At the rear of the desk a hinged long box has an angled shelf recess for a half-rack size sound module, built in amplifier and speakers. Power adaptors are hidden away beneath the box, reducing cable clutter, a single power cable connects to the wall socket. The prototype desk includes the following features:
• iPad (4 or later) mounted on iTrack Dock,
• Midi controller keyboard. (Novation 49SL MkII)
• Yamaha MU80 tone generator module.
• Stereo amplifier and speakers.
• Hinged box for access to wiring.
- WORK IN PROGRESS
The design presented here should be considered work-in-progress, it has evolved according to the direction I have taken experimenting with electronic sound sources. The desk could easily be adapted to suit your needs. If you are a confident woodworker the long box could be re-styled to accommodate a range of other gear, some suggestions listed below.
1. A lap-top computer up to 33cm wide in place of Dock (Dycem* pads prevent sliding.
2. Mac mini computer beneath the box, small monitor on top, running LOGIC, MAINSTAGE etc
3. Audio/Digital interface in the synth slot.
4. Korg Volca Keys modules or other MIDI controllable hardware synths
- NOTES / REFERENCES
Balanced Mode Radiator (BMR) Speakers,
Tectonic Elements Part No. TEBM65C20F-4
A flat panel speaker driver giving full range HiFi quality sound.
The desk has limited space for speaker enclosures, so the driver
was chosen for it's 30W power handling, good bass response,
and being suited to either sealed or vented enclosure.
Learn more about the BMR
Dycem - high friction matting, comes in rolls easily cut to size.
Step 1: MAKING THE DESKTOP
Measuring tape, steel rule, panel saw, electric drill, 108MM Hole saw and arbour (optional), Jack or smoothing plane, Medium and fine abrasive papers, cork block, floor paint. Also a fine tooth saw, marking gauge and wide chisel for fitting butt hinges.
Plywood or blockboard sheet 12, 15 or 22mm thick.
Tips: Cheap light weight blockboard is prone to hidden voids, spaces between battens or knot holes. these can cause rattling or buzzing, remember the desk forms part of two speaker cabinets, bass vibrations transmit through the board.
Birch ply is the best quality I know of, quite dense but very hard wearing. Marine ply is hard wearing but tends to be heavier than other plywood types.
HOW TO MAKE IT
Cut to size, 500mm deep. 1010mm long. or longer if it suits your keyboard.
PLANE the long edges straight,
BEVEL the sharp corners, 2 or 3mm in from edge. (you'll be glad you did this when carrying the desk)
SAND all round with medium grade abrasive paper and block as preparation for varnish or paint finish
PAINT : I used Ronseal floor paint, at least two coats recommended, no need for separate primer.
Note: I chose floor paint for its hard wearing matt finish, dark colour provides visually distraction free back ground tone and of course a non slippery surface.
Brass butt hinges can be screwed directly to the desk edge, or recessed as shown (woodworking skills needed for this).
1. Lay hinge onto the desktop edge, folded so it lies square on.
2. With a sharp Stanley knife cut the cross-grain lines.
3. Set marking gauge to thickness of hinge knuckle and score depth lines both sides of the desktop
4. Carefully make a series of closely spaced saw cuts, just to the depth mark.
5. Carefully remove waste by paring action with a 1" wide chisel or similar.
Try the hinges in place. When satisfied with the fit, screw the hinges in place, lay the desktop aside and start with the long box.
*THE HOLE... is optional: 108mm dia, position 390mm from front edge, 430mm from left hand end
A sub-woofer experiment that did not work out left a hole 108mm dia. If you choose to install a mini computer or other gear under the box you won't want this hole, but having made it I find it a useful feature. It provides
1. A comfortable and well balanced hand hold for carrying, most of the weight is in the long box
2. A cable routing option for mains power and sustain pedal.
After cutting with 108mm hole saw (see saftey guidelines in section soften the edges with abrasive paper, work progressively with course, medium and fine.
45 Degree bevel edges and rounded corners make the thing easier to handle, especially if lugging it around to gigs.
PROTOTYPE HAS A HOLLOW DESKTOP, not recommended (22mm thick,)
A recycled 1950s wardrobe door was chosen for its light-weight hollow sandwich construction and it appeared well made and in sound condition. After sawing to length, a filler piece was glued into the open end. 12mm MDF 75mm wide, which gives something solid to screw into for the box clamp-down mechanism. Cutting a hole for a subwoofer* meant a series of carefully made radial strengthening segments had to be glued into the void along with copious amounts of Isopon woodfiller. I will probably need to inject builder's foam into the areas forming part of the speaker enclosures to damp unwanted resonance. Please be aware you will need to solve similar problems if you choose to recycle a hollow door for your desktop. A solid ply or blockboard desk is easier to work with. TIP: If you choose 12mm plywood for the desk, a long piano hinge for long box will add rigidity to the structure. Some light weight cross bracing underneath the desk board may also help.
Maker suggestions and "I did it this way" comments are most welcome.
Step 2: CONSTRUCTING THE LONG BOX
(Drawing needed to show construction, hope to upload soon)
TOP: pine or redwood plank, 935 x 165 x 19mm (36¾" x 6½ x ¾")
BACK: plywood or MDF 127 x 12mm (5" x ½")
Angled Edge Trim 20 x 30mm cross section, 935mm long (¾ x 1¼ x 36¾")
Template for end pieces - mounting Card
Template for speaker enclosure panels - mounting Card
Steel angle brackets
High strength sealant/glue (EverBuild-Stixall-Extreme power)
Hand Plane or electric hand planer
Sandpaper & block
2 speed Battery Drill
Hole saws, large 108mm, small 83mm ( L 4¼" S 3¼" )
High speed router / 45degree cutter with guide bearing / straight cutter
Cut all parts as per drawing (See note above)
DRILLING THE SPEAKER HOLES do this before box assembly
! SAFETY TIP: Use a two speed battery drill. HIGH SPEED / SCREW / SLIP CLUTCH set to maximum torque.
The slipping clutch saves you from injury or damage to workpiece if the saw teeth grab.
Hold the drill with two handed grip and stand firm.
Drill down a little, check for even depth of cut.
Continue with light pressure, allow the teeth to cut smoothly. High speed is essential for clean cut
1. Measure speaker centre + equidistant from the plank edges, in 90mm from end, mark with a fine X
2. drill a 4mm pilot hole, without this guide it is difficult to align the hole-saw centre drill correctly.
3. Clamp the plank securely to bench or table top overhanging to avoid damage to bench.
4. Attach 108mm hole saw to the arbour, and drill down slowly no more than 12mm deep.
5. Remove saw and attach the smaller 83mm hole saw to the arbour,
6. Drill down slowly until it penetrates right through the plank. keep it vertical (perpendicular) Drill down with light pressure, the lower face will not be damaged if the saw rotates at high speed and breaks through slowly.
7. Insert straight cutter in router collet chuck. set to 12mm cutting depth and mill out the rebate in two steps 6mm deep then 12 mm. This can be done freehand, the large hole-saw cut provides a safety margin and a precise circular edge.
8. Turn the plank over and clamp as before, change to 45o router cutter.
9. Set depth of cut to create a flared opening at the rear face of the speaker driver, be aware the speaker flange screws need some thickness of wood to grip securely.
! UNPLUG ROUTER from power supply: when changing cutters
Plane edges to get them straight and square.
------------------- Construction details needed here ----------------------
After assembling the long box
place Hinges into the recesses previously cut in edge of desktop
1. Mark position of fixing screws with pencil,
2. Remove hinges and drill pilot holes. (Drill size = inner solid core of screw thread).
3. Secure hinges to desk with screws 15mm long
4. Measure along the length of desktop, with a pencil mark the centre line
5. Measure along the length of long box, with a pencil mark the centre line
6. Present the box to the desktop, square to the rear edge with centre marks aligned.
7. Fold hinges up to the box and mark the holes with pencil (run fine pencil point around inside the hole to draw a small circle)
8. Pilot drill as before, Insert screws to secure the parts together
9. Fit the restrainer (Made from Aluminium strip 15mm x 2mm according to instructions on photos)
10. Make sure the box will hinge back easily and that it sits flat on the desktop when closed. Some planing may be necessary if there are gaps.
11. Glue in place the foam window sealant strips. Make sure to do this AFTER levelling the underside of box.
BOX ENDS: 19mm Plywood offcuts,
ENCLOSURE PARTS: 9mm ply offcuts
Wood Screws: 50mm long, for fixing top plank to box ends, and rear panel to box ends
Restrainer: Aluminium strip 15mm x 2mm.
Clamp-down system: 2 Steel Angle brackets, 25mm dia steel washers, 2 plastic cupboard fixings,
Hinges: 2 or more butt hinges (steel or brass), or piano hinge to fit
Window draught excluder self-adhesive foam strip,
Step 3: Amplifier & Speakers
AMPLIFIER AND SPEAKERS
On-board amplification: 10 + 10Watt stereo amplifier Class-D, 12VDC, Stereo pair BMR speakers.
Amplifier board: DROK® Mini 2.0 Hi-fi, Amplifier (Amazon.co.uk)
Plastic instrument case, 105 x 58mm Internal Depth = 20mm *
6mm switched jack socket x 4
Speaker driver x 2: TEBM65C20F-4 30W power handling, Bass response down to 60Hz, 10mm coil excursion, 25.4mm voice coil diameter, Suitable for sealed or ported enclosure.
1. Choose your amplifier box carefully, mine turned out to be 3mm too shallow, a cardboard spacer had to be made to ensure a snug fitting lid.
2. Speakers can handle 30W, alternative amplifier modules could be substituted and built into a larger case. Standard 6mm jack connectors have been used to facilitate experimentation with alternative amplifiers, Step 6 shows car radio as amplifier, to drive the speakers up to their full power handling.
The 11mm Dia reflex ports improve low bass response a little, which is noticeable when playing low piano notes however the TEBM65C20F-4 speaker is capable of delivering descent amount of bass response when housed in larger enclosure.
Ideal volume for a single driver would be as follows:
Sealed enclosure: 2.3 Litres. (Bass rolls off dramatically below 100Hz)
Vented enclosure: 20.8 Litres. With a port 38mm internal dia and 140mm long ( low bass extended to 45Hz)
If realistic bass sound is more important to you than space, the long box could be fixed to the desk, and with the addition of a central divider, would give two enclosures closer to ideal volume.
For my prototype long box the built in enclosure size is compromised but this was not without good reason.
I wanted plenty of space beneath the box to allow scope for experimenting with different hardware, and I intended to use a 2 + 1 amplifier. as mentioned before the 108mm dia hole in the desktop was for a 5inch subwoofer salvaged from a previous project. This worked pretty well but the 2 + 1 amplifier took up more space and I really wanted the box space for other things.
The addition of a separate stand-alone subwoofer would of course give even better sound, although it would be a large heavy item.
Step 4: ITrack Dock Mounting
iTrack Dock is designed to sit on a table, not moved around or used for live performance.
By attaching two strips of self adhesive velcro and drilling recesses in the box top plank the dock can be mounted securely to the desk.
PLEASE NOTE: Remove iPad before tilting the long box, retaining clip is needed to make it safe. Work in progress.
Read notes on photos for further details.
Step 5: Half-rack Module Mounting
Rack mounting system
You may be familiar with the standard 19inch equipment rack system.
Two half-rack modules should fit in a 19" rack with a special shelf to hold them in place.
In this case a wooden shelf is provided and the equipment is not mechanically secured.
Work in progress: I am considering a more secure fixing for the module. In practice I don't find this necessary but clearly if the desk is set up on stage and needs to be moved in a hurry the module and iPad would be at risk of accidental damage. Current practice, MU80 and iTrack dock are removed and boxed before transporting the desk.
Exploring the Possibility of a 12V DC system.
Total load of the desk as it stands would be about 5 Amps. If you live in a static caravan running direct from the 12V system is quite appealing and more efficient than converting to AC mains then back to 12V.
iTrack Dock 12V 1.5A,
MU80 module 12V 1.5A
DROK® Mini 2.0 amplifier 12V 2A
Many MIDI controller keyboards will happily run off USB power iTrack Dock is designed to accommodate this*.
NOTE: * According to Focusriite The Novation SL MkII Keyboards need a 9V DC power adapter to run with iTrack Dock. a separate 12 to 9V DC converter would be needed if running on a battery powered system.
Car Radio Experiment
The Clarion Car radio will drive four speakers, 2 front, 2 rear at 25W per channel RMS. I enlarged the module slot to accommodate it as a trial. To keep the sound module in place the radio could be slung beneath the desk in a centrally positioned cradle, my X frame stand top bars leave a space between of about 40cm.
If considering this option please be aware this type of radio consumes far more power than a Class D amplifier of similar power output. The manual says each of the four channels deliver 25W, speakers should ideally handle 50W each to handle transient peaks. The user manual states a 15A fuse is adequate. A mains power supply to deliver 15A at 12V is a very costly item. Two channel operation is possible on a transformer supply rated at 3A provided the amplifier is not driven hard.
Step 6: ASSEMBLED DESKTOP
I made my long box a little shorter than the desk, allowing space for the clamp-down fixings and restrainer at either end. When connecting the two don't forget to bear this in mind.
When drilling screw holes for hinges drill size should be equal to or less than the inner solid core of screw thread.
Measure along rear edge of desktop, with a pencil mark the centre line
Measure along rear panel of long box, with a pencil mark the centre line at the lowest edge.
Place the box on the desktop and adjust position until the two centre marks align.
Fold hinges up to the box and mark the holes with pencil (run fine pencil point around inside the hole to draw a small circle)
Drill as before and secure the parts together with screws.
The long box needs to lift just enough to give easy access to wiring so it needs a restraining device to prevent it falling too far.
I made a simple device using aluminium bar 15 x 2mm cross section, this will be sufficient if the joints are not too stiff.
Cut two lengths of aluminium 135mm long
Mark drilling centres 15mm in from both ends of each bar and make a dint with centre punch.
To make the bend, each piece in turn should be held in a vise*
Drill holes at both ends of each bar. Choose drill size according to the bolts you are using, they need to turn freely but not be sloppy.
Shape the ends to a radius using a file and clean up with fine abrasive paper.
I put in the z bends thinking the two end pivots might be at the same level when closed, but decided the long box is better balanced opening as shown in the photos. nuts where the two end fixings , see the sketch for details.
The elbow joint is liable to catch on clothing or cables if the bolt is sticking out too much so aim for countersunk machine screw and thin profile nut. I placed a thin fibre washer between the bars at the elbow and all nuts turned up tight enough to make the mechanism free to move but so it will stay where it is put. A little dry-lubricant at the joints will help prevent seizing up.
*NOTE: To prevent the vise jaws marking the soft metal fit soft jaws if you have them or stick strips of masking tape on the face of each jaw.