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Dipole 1   Build a Dipole Speaker.

Introduction

     So would you like to build something a little different? a little more challenging? Well..... here it is. A Dipole speaker project, (electronics to speakers), from start to finish. Interested? then read on.
    Not quite sure what Dipole speakers are? Well, your average speaker is enclosed in a cabinet. A number of differing cabinet enclosure designs are possible but they all have one feature in common: they leave just one Driver face / cone, (usually facing the listener), to directly interact with the environment. The cabinet, also, adds colouration. These designs are known as monopole speakers.
     Dipole speakers have 'two' interacting driver faces i.e. a front 'and' rear driver face. essentially a front baffle with a small surrounding lip / trim but without an enclosing rear panel. By inference, there is little to no cabinet colouration.
    Because of the interaction between the front and rear faces these types of drivers are nearly always corrected for sound level, flatness and phase by using electronic crossovers. Each driver has its own frequency correction / equalisation and power amplifier.
    Historically, multi-channel amplifiers were expensive and difficult to come by, but now integrated chip amplifiers have provided much greater access and the quality of some of them is now very good. 
    If you were to do this from scratch, then it's a chunky project. We have broken the whole into four segments, some of which are self-supporting, e.g. the power supplies and amplifiers. It's worth mentioning here, that this is a long-haul project and, depending upon the choice of components, of modest to expensive cost. Certain elements can be used for other DIY projects, (the power supply and amplifiers for example), so if certain sections are not to your liking you could  either omit or buy them complete. Note complete refers to PCB population / assembly, build and test. It would not at this time include plain wood panels or metal-bashing.

      In order to determine interest, project feasibility / continuance and ultimately price, we need YOU to advise us of your interest. Simply email us at project@qtasystems.co.uk with your thoughts and ideas. An example of the project is given below and shows the power supply used to drive the filter boards, crossovers, (one of which is pictured) and amplifiers. Note, this is not a design by committee exercise, but merely an expression of DIY interest. The closing date for comments will be 30th June.

      The design brief is straightforward, to design and construct a basic dipole speaker system, using readily available parts at a reasonable cost and without being dragged down with all the mathematics of design and operation. This type of philosophy invariably involves taking a few shortcuts, both in design and total performance, but you get 90% of the performance for a quarter of the cost.

      The project is divided into four basic sections.
                (1) Power Supplies
                (2) Power Amplification.
                (3) Electronic Filters, (crossovers).
                (4) Woodwork and Drivers. Items (1) & (2) can be used for other DIY projects so.......



Prototype Dipole Board Prototype Filter In Case
  Fig (1)  Experimental board.   Fig (2)  Prototyping in a 19" rack

  Fig (1)  Experimental board.
Case Mounted Filter Board
  Fig (2)  Prototyping in a 19" rack

    In order to assess your project capabilities, give some thought to the following:-

(1)     You really do need to have had some prior experience in circuit building and cabinet construction. At a minimum you should be competent in identifying component values from their colour codes and have experience in soldering small and large components to PCB Boards.

(2)     The mains transformer and associated wiring must be connected in a practical and safe manner. If you are unclear on this aspect you should seek the use of professional services.

(3)      The woodwork involved is at a basic level, but you still need some tools and to be able to read and follow the drawings.

(4)      The PCB boards are modest in size, but still need housing along with their input / output connection cables and their respective mains powered power supplies, electronic filters and amplifiers. You will need two cases for the left and right power amplifiers and a further case / box for the crossovers. The prototypes destined to follow later were all constructed in 19" rack mount cases.

(5)      Metal working is an expensive business and made more so by small job production runs. Cases can be fabricated from mixed wood and aluminium panels. Food for thought....If you want to buy pre-punched cases then indicate your interest.

(6)      If you've never constructed a project of this nature before or this is your first time at DIY, we suggest a project less challenging as an introduction. Circuit board mistakes are usually difficult and expensive to rescue / repair.


(1) Power Supplies..

      Three modest power supply boards are required to provide power to two sets of power amplifier modules, (L & R Channels) and one filter / crossover assembly. To keep things simple, the same generic power supply layout is used for both applications. The only real difference being in the Transformer Voltage and Current capability. Note, Transformers and mains wiring are excluded from any form of project supply.
      It was decided to use two 'linear' regulated, positive supplies, linked together to provide split rail voltages. This provides a slight improvement in noise, avoids the lower reliability of negative devices and reduces the number of different components.
      The finished PCB's can accommodate various reservoir caps with 10mm pin spacing and can use an assortment of regulator chips ranging from inexpensive to ouch. We're not concerned here with massive power requirements, (remember cost...) so modest component specifications will do.
      The power supplies can be used in a variety of alternative DIY projects, depending upon transformer specifications and regulator choices, a few volts to +/- 35V can be achieved.
      For the Dipole project we use +/-15V supplies for the crossover filters and around +/- 30V for the amplifier modules. Fig(3) is the unpopulated board for the filters. The Assembled Power Supply is shown in Fig(4). with legends. All boards are constructed from FR4 glass cloth / epoxy board, use 2oz Copper, have solder resist and screen printed legends. Rectifiers are SIL, (Single in Line), based 8 Amp devices. A bleed resistor is used to discharge the caps to prevent accidental discharge whilst a transient suppression diode provides a measure of protection against mains power surges. Resistors form an RC filter with the storage caps to help reduce system noise. Reservoir caps shown, are ELNA Tonerex. Screw fitting terminals are provided for power connections. All PCB boards will be available separately.
      The final sound quality will hinge on the choice of regulators and caps but the differences are small and a number of alternative devices could be used.

Printed Circuit Board with legends Completed Power Supply Board
  Fig (3) Basic PCB with Legends   Fig (4) Completed Power Supply
Power Supply Board
  Fig (3)  Experimental board.
Assembled Power Supply Board
  Fig (4) Completed Power Supply

Machined Front Panel Rear View Machined Front Panel Front view
  Fig (5) Dipole Rear Machining   Fig (6) Dipole Front Machining
Dipole Front Panel Rear View
  Fig (5) Dipole Rear Machining
Dipole Front Panel Front view
  Fig (6) Dipole Front Machining
Complete Dipole Assembly Rear View Complete Dipole Assembly Front View
 Fig (7) Rear view Prototype  Fig (8) Forward facing View


Prototype Dipole Assembly Rear View
 Fig (7) Rear view Prototype
Prototype Dipole Assembly Front View
 Fig (8) Forward facing View
Circuit Diagram of Power Supply
Fig(9)    Circuit Diagram of the generic power supply. Widescreen only.

Next time we will look at an outline of the basic system and investigate associated costs.

TO BE CONTINUED..... Watch this space.