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 Elliott Sound Products Project 27B 

100W Guitar Amplifier - Speaker Box Details

© 1999, Rod Elliott (ESP)
Updated Feb 2021

PCB   Please Note:  PCBs are available for the latest revision of this project.  Click the image for details.


Introduction

Note:  This project is superseded by a new version, which has several useful additions.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.

Special Warning to all Guitarists

When replacing guitar strings, never do so anywhere near an amplifier (especially a valve amp), nor close to a mains outlet.  Because the strings are thin - the top 'E' string in particular - they can easily work their way into mains outlets, ventilation slots and all manner of tiny crevices.  The springiness of the strings means that they are not easily controlled until firmly attached at both ends.  This is very real - click for a Photo of an Australian mains plug that was shorted out by a guitar string.


Preamp, Power Amp & Power Supply

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.


Speaker Boxes

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:

t-nut 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
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
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.

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 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 awful.  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 7th 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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Copyright Notice. This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is © 1999.  Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro- mechanical, is strictly prohibited under International Copyright laws.  The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference while constructing the project.  Commercial use is prohibited without express written authorisation from Rod Elliott.
Updated 22 Feb 2001 - added IC pinout info and changed effects info./ Feb 2021 - removed all old circuits.