100W Guitar Amplifier - Speaker Box Details

Note:  This project is superseded by a new version, which has several useful additions.  PCBs are available (but only for the new amp).  This version is included only for the speaker box construction information.  There is no amp that satisfies everyone's requirements, and this offering is not expected to be an exception.

One major difference however, is that if you build it yourself, you can modify things to suit your own needs, experimentation is the key to this circuit, which is presented in basic form, with every expectation that builders will modify just about everything.

The amp is rated at 100W into a 4 Ohms load, as this is typical of a 'combo' type amp with two 8 Ohm speakers in parallel.  Alternatively, you can run the amp into a quad box (4 x 8 Ohm speakers in series parallel - see Figure 5) and will get about 60 Watts.  For the really adventurous, 2 quad boxes and the amp head will provide 100W, but will be much louder than the twin.  This is a common combination for guitarists, but it does make it hard for the sound guy to bring everything else up to the same level.

These are described in the Project 27 article.  The circuits that used to be shown here have been removed, as they are no longer relevant.  The early versions of both PCBs have all been sold long, long ago, and there's no good reason to keep them.  The updated versions are better in all respects and are available for purchase.

The two suggested boxes are shown (in basic form only - you will need to work out the woodworking details yourself).  The first (Figure 4) is a standard 2 speaker cabinet, and I strongly recommend using the open-back box, as this is the preferred option for most guitarists.  Two 8 Ohm speakers are wired in parallel (giving 4 Ohms), and it is expected that with 12" speakers (300mm) this combination will be quite loud enough.  Try to get speakers that are rated for at least 100W each - this safety margin is a requirement for guitar, since the amp will be overdriven for much of the time and this produces up to double the rated output of the amp.

The details of finish, handles (and the actual dimensions) of the boxes I shall leave to the builder, but I will make a few comments:

• Tops and bottoms are shown as being inside the side panels.  This does not really matter, since all corners should be reinforced with 25mm square (1") timber.  All joints should be glued and screwed.  Pre-drill the screw holes to prevent the end grain of the MDF from splitting.
• Use a router if available to round off all the edges and corners, and use corner protectors.
• Vinyl is still the most robust covering for stage gear, but carpet can be used if you prefer.
• Use strong handles, as the boxes will be quite heavy when completed.  Side 'pocket' handles are best for the quad, but a strap handle can be used for the twin.
• The baffle of the twin, and the top section of the quad are angled.  This projects the sound towards the guitarist, and is better than propping the front edge on a brick or similar.
• The baffle is shown recessed.  This is to allow for a grille frame, which should fit neatly inside the recess and be fastened with Velcro or grille mounting clips.
• Speakers should not be held in place with wood screws - use bolts, washers and nuts, or 'T-nuts'.  Wood screws will eventually loosen, and the speakers will rattle.

For those who don't know what a tee nut is, the drawing should give you the general idea.  They are readily available from specialist fastener suppliers.  If you can't get hold of them, use metal thread screws with nuts and washers, and a thread locking fluid.  'Nylock' nuts can also be used - they are the ones with a nylon collar inside the nut.

Generally, one thing to avoid is vented boxes - they just don't sound right for guitar.  Naturally, if you like the sound of vented boxes, then go for it - guitar amps are probably one of the most personal amps in the world, and there is no right or wrong combination, as long as you get the sound you want.

Note:  Although the symbols for jack sockets are shown on the drawings, I strongly suggest that you use Speakon connectors, both for amplifier output and speaker inputs.  Unlike phone plugs and jacks, they don't short-circuit the amp when partially withdrawn, and they are far more rugged.

Figure 4 - Suggested Twin Speaker Box And Wiring

The second example (Figure 5) is the classic quad box, and uses 4 x 8 Ohm speakers in series/parallel.  This gives an impedance of 8 Ohms, so two quad boxes can be used if you really want the amp to be that loud.  You might be able to get 4 Ohm speakers, in which case the series/parallel connection will give you a 4 Ohm box, so only one is needed.  I suggest that the quad box also be open-backed, but this is not essential.  One of the most popular guitar amps around uses closed back quads, and they sound pretty good to me.

Figure 5 - Suggested Quad Speaker Box And Wiring

For the speaker boxes, I recommend MDF (Medium Density Fibreboard).  This is a much better material to work with than chipboard, and is also stronger.  Chipboard has been used (and still is) by many manufacturers because of its one redeeming feature - it is cheap.  MDF will cost quite a bit more, but the end result is worth the expense - a better finish, and a stronger box.  Don't be tempted to use anything thinner than 19mm (3/4"), or the cabinet will resonate too much, and will also lack strength.

The ultimate material is plywood (ideally not less than 19mm thick, but 12mm [½"] can be used) but it's comparatively expensive.  Plywood also has more pronounced resonances than MDF, and that can either make 'the sound' better or worse.  The grade of plywood also makes a difference (e.g. 'normal' plywood vs. marine ply) and you might hear a difference between the type of wood.  Birch, Poplar, Eucalyptus, pine, etc. have different characteristics, but whether these translate to an audible difference is highly dependent on the way the box is put together.  It's worthwhile looking at the article Loudspeaker Enclosure Design Guidelines.  It's intended more for hi-fi enclosures, but it may still be helpful.

Many manufacturers use a thin (typically about 6mm) fibre board at the back of open backed cabinets to provide some protection for the drivers, and a lead storage area.  Don't!  Make the rear protection panel(s) from 19mm MDF (or plywood) too, since this will prevent the unwanted resonances from the thin material typically used.

Speakers should also be fairly efficient if possible (> 90dB W/m), since a 3dB reduction in efficiency will result in the same SPL (Sound Pressure Level) being reduced by 3dB.  You then need an amp with double the power to make up for the 3dB lost by inefficient speakers.  Check out the local dealers for musical instrument speakers - do not use hi-fi speakers, or you will surely be disappointed - they are not designed for musical instrument applications, and usually sound dreadful.  Most cannot handle the continuous (often heavily clipped) output from a guitar amp and they will fail.  Dedicated guitar speakers are generally very efficient, and are rated for continuous power.

Also avoid loudspeakers with aluminium dome dust caps - they often sound bloody awful when a guitar amp is overdriven, with a hard top-end that radiates at frequencies that are discordant.  Any harmonic above the seventh is discordant (out of tune), and an overdriven guitar amp is one of the few instrument combinations that can create the 7^th and higher harmonics with significant level.  As a result, most guitar speakers are designed to roll off the top end above about 5kHz or so to avoid this problem.  An aluminium dome does the opposite, and radiates wildly at the upper frequencies.  This is both unpredictable and unpleasant.

Anecdote:  Some years ago, I was asked by a well known Australian guitarist if I could fly to Melbourne (from Sydney - about 1000 km) to solve this awful problem in the studio.  It didn't matter how they miked the guitar amp, it still sounded terrible on the recording.  It turned out that the aluminium dust cap was radiating so strongly at somewhere between 5kHz and 12kHz that it destroyed the sound, giving a most unappetising metallic edge to the music.  The remedy was to carefully cut away the dust cap, and glue a piece of thin felt in its place.  About an hour later (after the glue had dried), the result was that the recording engineer and guitarist alike were stunned at the difference - the sound was as smooth as silk (well, you know what I mean) and all the nastiness was gone.

Most of the established guitar amp manufacturers use speakers specially made for them by one of a few specialist loudspeaker builders, and they are normally hard to get.  Try music shops (or repair shops) to see if they have speakers that might be suitable.  The second-hand market might be another good place to look - you might even be able to get a complete speaker box for a reasonable price, which saves having to do the woodwork !

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