Decibel Dungeon


Introduction

Like many others who have taken up the hobby of DIY hi-fi, I started off looking to improve my loudspeakers. The reason that this path is so popular with novices is that you don't get involved with any dangerous mains voltages. In some ways, it's also a comparatively simple part of the hi-fi chain to begin experimenting with (although the theory is quite complex).
There are two basic choices, either modify some existing 'speakers, or build new ones, either designing them from scratch (not really possible for a complete beginner), or building one of the many kits which are available.
There is so much information on 'speaker building and modification on the internet that I have no intention of making this into a tutorial. An introduction to the subject and some useful links should enable anybody to modify or even build a set of decent 'speakers.
I have found the following books very useful in my understanding of this topic:
  • The Loudspeaker Design Cookbook by Vance Dickason.
  • High Performance Loudspeakers by Martin Colloms.
  • An Introduction to Loudspeakers and Enclosure Design by V Capel.
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Repairing loudspeakers.

I thought a few words on this subject would be in order, based on some of the emails I have received. After all, it is a shame to throw away anything if it can be repaired, especially at this time when the world's resources are diminishing rapidly.
Damage to loudspeakers can fall into three categories: the cabinet, the crossover/wiring and the drive unit. If the cabinet is damaged then it should be fairly obvious what needs to be done in order to make a repair. If you are not handy with DIY tools then you may need to find a friend who is. If the damage is severe, consider building new cabinets but do keep to the same internal volume, even if you change the dimensions.
A crossover or drive unit can be damaged by too much power and this is always worth remembering when you turn up the volume on your amplifier. Another common cause of damage to these items is a short-circuit, often in a cable or plug so never underestimate how important it is to take care when making up new speaker cables or interconnects.
If you have a damaged crossover, it should be possible to replace the damaged components. I say should because you will need to know the specification for each component that you want to replace. Sometimes the value of a component is clearly marked but if not, you will need a circuit diagram for the crossover and that may be a problem if it is an older speaker that you are repairing. Once you know the value of a component, either resistor, capacitor or inductor, you should be able to buy a replacement. The best places to try are the electronics suppliers like Farnells or Maplin, or better still, one of the small companies (see the L'speaker Links section) that supply loudspeaker kits as they usually keep a good range of crossover components as well.
Repalacing a component should be fairly simple. It will involve some desoldering of the existing component(s) which will either be hardwired to each other (often soldered to the back of the speaker terminals) or mounted on a small PCB. The new component is then soldered in position. Hardwired - where the leadout wires of the components are soldered to each other rather than being connected to a PCB
There are two types of damage to a drive unit, physical (usually the cone) or electrical (the coil). If you have a damaged drive unit there are really only two options. If the drive unit is still available, you could just buy a new one. If it is an older unit you will have to get it repaired. At present, I know of only one place in the UK where this sort of service is available.
Dave Smith - one of the few people repairing speaker drive units in the UK
Dave Smith of DK Loudspeakers is located in Essex and has a long history of working with loudspeakers. There's not much that Dave won't try and repair if he has the materials to do so. He has even repaired a unit from a 1920's radio! If you have a drive unit that needs repair, the best thing to do is give him a ring on 01708 447 344. Dave openly admits to having been completely left behind by computers so there's no email or web site but you can fax him on the same number. Please be aware that Dave is a one man operation, and his services are highly sought after. Turn=around times for repairs will likely be measured in months, and may take over a year.
One thing to remember if you need a drive unit repaired is that the repair will most probably involve rebuilding the whole unit. This will slightly alter the speaker parameters so for a stereo system, the other drive unit should also be rebuilt at the same time. This does of course increase the cost of repairs but remember that you will have virtually new drive units so if they are quite old, this will be a good upgrade anyway. Current costs are around 50 per unit and 12 for return post and packaging.
If you are unlucky enough to find yourself with a damaged loudspeaker and need to make a repair, you may as well turn the situation to your advantage by upgrading at the same time. While you have the cabinet 'open' and you are ordering new components, you may as well get the best that you can afford. Also take the opportunity to replace the wiring with something better. If cabinets need repairing/replacing, you should be able to better the original item as you won't be constrained by the manufacturing and transport costs. Every cloud has a silver lining!
And another that has been kindly brought to my attention by Vic Chandler:

Unit A4, Askew Cresent Workshops,
Askew Cresent, London W12 9DP

Telephone: 0181 743 4567

e-mail:paulATwembleyspeaker.demon.co.uk

and yet another place to go for speaker repairs (and parts) is AudioLoudspeakers.
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Upgrading loudspeakers.

There are a number of tweaks that you can make to an existing pair of 'speakers. They are: How feasible it is to carry out any of the internal modifications will depend on how easy it is to gain access to the insides of the cabinets. Access is usually gained by removing the largest drive-unit. In some (rare) instances, you may need a special tool to remove these drive-units.

Replacing the 'speaker cables.

This is one of the easier options as it does not actually involve doing anything to the loudspeaker itself. Better sounding 'speaker cables don't have to cost the earth, try experimenting with some of the comparatively cheap cables available from suppliers such as Maplins. Remember that cables do not sound inherently better or worse on their own but their effect on YOUR system may improve the overall sound quality!

TIP - I found one of the best solutions (in my system) was a double run of RG58 coaxial cable which currently costs 49 pence a metre. I used two types, one with a solid conductor and one with a stranded conductor. I joined the conductors together at each end and used them for the 'hot' connection. The two outer braids were also joined and used for the signal 'return'. When using this type of cable take great care not to get a short between the 'hot' and 'return' conductors. Coaxial cables of this type are not recommended for some types of amplifier such as those produced by Naim.

Replacing the internal wiring.

There is some debate as to how much of an improvement can be obtained by replacing at most a few feet of wire. However, some people have reported that this is worth doing and as the cost involved is minimal, you may as well try it for yourself, especially if the existing wiring is very basic (ie like bell-wire).

TIP - if you can, use the same cable (better still, the same piece) to run all the way from the amplifier output terminals to the terminals of the drive-units.

Upgrading the crossovers.

Crossover filter circuits are built from inductors, capacitors and resistors and in the case of most commercial 'speakers, these components are chosen on price rather than sound quality.
CROSSOVER - Electrical filters which separate the full frequency musical signal into ranges which will be handled by each ' speaker drive-unit (tweeter, mid-range or woofer). Either passive or active.
Replacing them with the best items you can afford can make a noticeable improvement although you should remember that no matter how good the components, if the design is poor, you still won't end up with a good 'speaker. A problem you may encounter is not knowing the values of the components you want to replace. If the value is not marked on the component, you will need to contact the original manufacturer (who may be reluctant to release that information). Alternatively, you could measure them yourself. There are special meters to measure capacitors and inductors but unfortunately they are quite expensive.
Unfortunately, designing crossovers is quite a skill and is probably beyond the capabilities of most hobbyists. For optimum results, crossovers need to be designed for each specific 'speaker system and it is highly unlikely that you will improve on your crossover with one bought 'off-the-shelf'.

TIP - If the crossover has been built on a PCB (often done with mass-produced 'speakers), it is also beneficial to rebuild them, hardwiring the components onto a piece of stiff board.

HARDWIRING - Joining electronic components together by soldering their leadout wires together instead of soldering them onto a PCB. OK for small circuits.

TIP - A further improvement is to remove the crossover from inside the 'speaker cabinet, placing it in its own enclosure, preferably close to the amplifier output sockets.

While rebuilding a crossover, you will also have the chance to provide the facilities (extra connections) for bi-wiring (if they don't already exist). Bi-wiring is where each drive-unit in the 'speaker has its own 'speaker cables connecting it to the output of the amplifier. DRIVE-UNIT - The unit which converts an electrical signal into sound. Consists of a 'motor' which moves a diaphragm back and forward to move air.
There is still some debate whether bi-wiring does improve sound quality but I have never heard it said that it makes things worse. At least if you have the means to bi-wire, you can try it and see if it works in your system. Please note that some crossover designs cannot be split for bi-wiring; if that is the case, it should be obvious when you physically examine them.

Biwiring set-up
Biwiring may make an improvement.

NOTE - Bi-wiring should not be confused with bi-amping. Bi-amping involves each drive-unit being powered by a channel of the power amplifier. This definitely does improve sound quality as I can testify to from my own experience. It has been demonstrated that two cheaper amplifiers used in a bi-amping set-up can outperform an amplifier costing the same as the two cheaper ones.

Bi-amping set-up
Biamping will make an improvement.

Going 'active'

A further improvement is the use of active crossovers. Unlike passive crossovers which are situated between the amplifier and the 'speaker drive-units, the active version is placed between the output of the preamplier and the input of the power amplifier.

Active set-up
Adding active crossovers is even better.

One of the major benefits of active crossovers is that the amplifier has more direct control over the 'speaker drive-unit and can therefore control it more accurately. Think of it like this. If you (the amplifier) sit in the back of your car giving directions to a chauffeur (the crossover), who is driving the car (the drive-unit), you may get on fine at slow speeds, relaying your instructions, but if you were in a race, it would be much better for you to have (direct) control of the car yourself. Click HERE for a fuller explanation of active crossovers.
Changing from passive to active control of your 'speakers will involve the following:
  • A separation of the preamplifier and power amplifier stages. If you currently use an integrated amplifier you will need to know where the circuit for the preamplier joins the power amplifier section.
  • One or more extra power amplifiers (you need one mono power amp channel per drive-unit ie one stereo power amplifier will drive two drive-units) These amplifiers need to have an identical specification as regards gain.
  • An active filter (crossover) circuit. You can either build one from scratch, from a kit or buy one ready built.
  • The removal (or disconnection) of the passive crossovers.
  • The possible alteration of the cabinet size and port tuning.
I am a very big fan of active crossovers, having experienced the improvements they bring. Once you have an active crossover, it is also much easier to alter it to suit different 'speaker configurations than it is with a passive crossover.
On the negative side, an active system costs more than a passive set-up due to the cost of the extra amplification. However, as sound quality is our ultimate goal, I feel that this extra expense is well worth it. And remember that building your own amplification will keep the cost down. See the Build your own section for links to some of the good amplifier kits available.
There's only one thing better for 'speakers than an active crossover, and that is no crossover as I have discovered with my most recent 'speaker project which utilises some full-range drivers. (Click Here for details) However, even full-range 'speakers aren't perfect, often needing another unit to 'help' them cover the full frequency range. FULL-RANGE DRIVERS - Loudspeaker drive-units which can cover enough of the frequency range to be used on their own (without a crossover).

Modifying the cabinet.

Just how much you can modify an existing 'speaker cabinet will depend on how easy it is to get access to the interior. Usually, access is gained by removing the largest drive-unit.
Once you have gained access, the first goal is to try and strengthen the cabinet with some sort of bracing. Don't go over the top here as anything you put into the cabinet will reduce its effective volume which, in turn, will change the alignment. Bracing may consist of adding cross braces which fit between opposite sides of the cabinet, or corner bracing by adding timber or metal angle into each corner to effectively strengthen the joint. ALIGNMENT - All the parameters in a design which produce a 'speaker with the required output eg flat frequency response/bass cut-off etc. Changing one of the parameters can upset the alignment and affect the sound quality.

Adjusting the cabinet damping.

While you have the cabinet open, you could try some further experimentation. It is possible to buy flexible sheets of material which have a ribbed texture. The idea is that when these are fixed to the interior panels of the cabinet, they deflect (and disperse) the sound waves. Despite the claims for these 'Deflex' sheets, they only really work well with the higher frequencies and will not break up the lower ones.
You could also try pieces of carpet. These are particularly effective when placed on the rear panel, immediately behind the woofer as it reduces the amount of sound which is reflected back. Any sound which is reflected straight back can come out through the thin cone where it will mix with the sound radiated from the front of the cone. WOOFER - a loudspeaker drive-unit that produces lower frequencies. Larger than tweeters as they have to move a lot more air.
This causes a problem because the reflected sound is slightly out of time with the radiated sound (due to the extra distance which it has travelled) and also 180 degrees out of phase (due to it being reflected) and therefore 'muddles' the radiated sound.

Replacing the drivers.

There are two reasons for replacing the 'speaker drive-units:
  • They are damaged.
  • You wish to replace them with better quality items.
If the 'speaker concerned is no longer in production, or the original manufacturer has gone out of business, you may not be able to get an exact replacement for a driver. All is not lost though as one of the 'speaker component suppliers listed in the Loudspeaker links section should be able to offer you an alternative providing you can give them full details of the 'speaker concerned.
If your desire is to upgrade the 'speakers by fitting better quality drive-units, you should first consider whether the quality of the cabinets and crossovers justifies the cost of the new drive-units. It may be more cost-effective to build a whole new 'speaker system. Again, advice will be available from the suppliers. Remember, you can't just fit any old drive-units into a cabinet and expect them to work perfectly. Drive-units, cabinet volume and other factors like the design of the crossover must all work together to produce a good sounding 'speaker.

Loudspeaker supports.

One of the current fashions in hi-fi is the floorstanding loudspeaker. Once, the stand-mounter was the norm and great emphasis was put on the quality of the 'speaker stands. It is essential to support any loudspeaker properly, and, in my opinion, this applies equally well to floorstanding types.
This is another area of hi-fi where there are literally hundreds of suppliers, all claiming that their stands are the ultimate. It is also an area where the DIYer can produce something quite satisfactory for a fraction of the cost.
A design for any type of loudspeaker must hold the 'speaker securely, in an upright position so it can't sway in any direction. A variety of materials can be used in its construction but timber is probably the most convenient for DIY construction unless you have access to welding equipment.
If any part of the stand is hollow, it should be filled with something which will damp it and prevent 'ringing'. RINGING - occurs when a material resonates due to the influence of a particular frequency in the music. Metal is particularly prone to ringing, wood is not.
I have covered the subject more fully in the section on Equipment support.
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Building your own loudspeakers

If you have some basic carpentry skills and can do some simple soldering, there is no reason why you should not be able to build a pair of loudspeakers. And there are good reasons for doing so, especially financial ones.
As with most commercially produced hi-fi equipment, a high percentage of the price which you pay for 'speakers goes on marketing and other overheads. One of these overheads is the cost of transport and storage which is particularly high with a large and heavy item like a 'speaker.
You may have noticed that the same loudspeaker can sound quite different when used in another room. Room size, shape and furnishings all affect how the 'speaker performs, as of course does the various hi-fi equipment which it is used with.
When designing a commercial 'speaker, the designer has to produce some sort of compromise which will hopefully sound good in all the different situations it may be used in. Compromise is not really what we are aiming for and when you build a 'speaker for yourself, you can tailor it to get the optimum sound quality in your listening room with your equipment. That gives you a head start even if you have not designed and built a 'speaker system before.
Fortunately, there are many kit 'speakers on the market for those that don't want to go through the long and complicated process of 'speaker design. You usually get the choice of how much of the 'speaker you want to build yourself. You could choose to build the cabinets from scratch or get a ready-to-assemble kit. You can purchase a kit with all the crossover components or just the plan, buying your choice of components.

TIP - if you are building your own cabinets but don't count carpentry amongst your skills, find a timber supplier who will cut your chosen board to the right size. They will have a table saw to ensure accurate cuts and often make only a small charge for this service which will save you a lot of time, effort and mess.

There is such a wide choice of kits you may wonder where to start looking. My advice would be to try and listen to the 'speaker that you are hoping to purchase. So choose a supplier who is geographically closest to you if you can. Most of the kit suppliers have ready-built examples on demonstration for customers to audition.
Another advantage that the home constructor has over most commercial designers is the control over the budget and the opportunity to use better components. Very few commercially available 'speakers use the more expensive and better specified drive-units that many DIYers start out with. I like the Audax range of drivers but most of the units sold by the companies listed in the 'Links' section of this page will have been selected for their high quality.
Before you do anything else, you will need to decide on what type of 'speaker to build. You may have heard a particular type and already decided that you would like to build one. If you haven't any preference, and have not built 'speakers before, I would recommend sticking with a 'sealed box' or 'bass reflex' design to start off with. For more details of these, and other 'speaker types click HERE.
There are really no set rules on what to build 'speaker enclosures from. Just about everything has been used to date; timber, concrete, plastics, polymer filled resins, cardboard tubes and even metal! Experimenting is part of the fun of DIY hi-fi.
Although most 'speakers come in rectangular boxes, there is no real reason for you to stick to the same shape. You could use any shape that you are capable of constructing, and if you are going to the trouble of building your own 'speakers, why not try and produce something that is different to the 'boxes' which stand in most homes?
For some unusual enclosures, try these links:
Anthony Gallus Acoustics
Some people have had success building 'speaker enclosures from tough cardboard 'tubes', the sort used by the construction industry to mould concrete pillars (Sonotubes is one brand name). If you are interested in this approach you can find out more from:

Essex Tube Windings Ltd

Essex Tube Windings Ltd manufacture cardboard tubes in all shapes and sizes for a variety of clients. In construction they are used for pile sleeves, void formers and formwork for columns, but there are many other uses.

Gareth Thomas-Impey
Essex Tube Windings Ltd
Macanie House Dock Rd
Tilbury Essex
RM18 7PT
Phone 01375 851613

Essex Tubes web site

Or:

Plant paper tubes, Slough:
(Sonotubes)
Perth Avenue
Perth Trading Estate, Slough
Berkshire SL1 4XX
United Kingdom

+44 175 352 6565

Contact: Tim Colebeck
+44 175 348 67 01

Sonotubes Tubes web site
Once you have decided what to build, you must purchase all the parts and materials, either as a kit or individually. Go over the project in your mind and work out any difficult construction problems before you begin. Most 'speaker cabinets are really just a box but you can't just throw one together and expect it to sound good. Irrespective of the construction materials used, it must be made as non-resonant as possible and airtight at the joints.
And remember, of all the equipment in your hi-fi system, the 'speakers are the most visually prominent so make them look good. Even if you can't veneer (or can't afford real wood veneer), there are many methods of finishing a cabinet so that it is aesthetically pleasing. You can even match the finish to the decor of your listening room. If you want some ideas or inspiration just do a bit of surfing in the Loudspeaker links section.

TIP - If you want to recess the drive units so that they are flush with the front baffle, there is a method which does not require the use of a router. Get a piece of board the same size as the front baffle and the same thickness as your drive-unit surrounds. Cut out an aperture to suit the driver and glue the board to the front baffle. This job is easier if the board is slightly larger than the baffle to start with. When you have fitted it over the drive-unit (temporarily bolted in position), you can then mark the exact size of the baffle on the board and cut it to the correct size.

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Other matters

In this section I'll cover some other loudspeaker related issues.
The first is the subject of attenuating loudspeakers after the amplifier. Why would anybody want to do this? Well, I received an enquiry from somebody who wanted to connect a second set of speakers to their system in order to listen to music in the kitchen while it was also playing in the living room. The problem was that the kitchen speakers were too loud at the volume setting on the amp that was suitable for the living room.
To remedy this sort of situation, a resistor network is employed between the amplifier output and the speaker(s) to be attenuated.

Circuit for attenuating loudspeakers.
Circuit for attenuating loudspeakers.

Calculations - R1+R2 divided by R2, eg. 8R + 8R = 16R divided by 8R = attenuation of 2.
Power dissipation = Voltage squared divided by Resistance. eg. 10V + 10V = 100 div say 10R = 10W.
Or for an easy way to calculate the attenuation try this online calculator.
Do make sure that the resistors that you use are rated highly enough, especially if you are likely to drive the speakers hard. If in doubt use 10 'watters' and you should be fairly safe!
If you want to get serious about building your own loudspeakers, it will be very useful to know how to take accurate measurements of the drivers, and the loudspeaker's performance. I have always found this a difficult subject to learn about but I recently discovered an excellent web site that explains it all in layman's terms. Take a look here where Hong Nguyen has done a brilliant job at making the process as simple as possible.
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Loudspeaker links

An excellent database of driver parameters.
Loudspeakers101. Very useful site with speaker design tools and advice.
IPL Acoustics. Suppliers of kit 'speakers and components.
Falcon Acoustics. Suppliers of speaker drivers, crossover components and related items.
Wilmslow Audio Suppliers of kit 'speakers and components.
E J Jordan Suppliers of high quality (full-range) drive-units.
BK Electronics BK supply amplification modules and active crossover units for active 'speaker systems.
General speaker building information.
A range of DIY project 'speakers.
Good stuff on crossover design
Vale veneers UK suppliers of wood veneers.
Metrum Acoustics, suppliers of components for (DIY) electrostatic speakers.
Interesting information on the effects of diffraction and loudspeaker cabinets.

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