Home Studio Active Speaker
High quality monitors for the home recordist
This speaker system was designed specifically to match with the woofer described in the March '81 issue of E&MM. However, it's performance is of monitoring standard and is perfectly adequate when used alone, although the inevitable consequence of it's small size is the lack of deep bass response. As they stand the low frequency cut off of the system is about 70Hz.
Matched with the woofer a full scale active stereo system is formed which responds from 30Hz-20kHz within the -3db points.
To explain why the active approach has been taken it is necessary to return to the basics of speaker design.
It is, unfortunately, impossible for a single drive unit to give a satisfactory performance across the audio band. To produce good bass a heavy, large cone is required that can respond to large movements. For good treble a small moving mass is needed that can respond rapidly. These two requirements are obviously contradictory.
The usual solution is to use two or more speakers which will handle the range, with a passive network to feed the frequencies to the appropriate drive units.
Unfortunately, speakers are far from simple resistive loads. They contain considerable amounts of inductance, for example, as well as generating currents which will feedback into the network producing response anomalies.
For these reasons a typical crossover network is very complex, wastes amplifier power and can only be used with the speakers for which it has been specifically designed. For economic reasons the optimum integration of the drive units is very rarely achieved in practice.
Active speakers, in actual fact, date from 1958! The original designs used filters designed around valve cathode followers. Unfortunately, they didn't catch on because of the then high price of Hi-Fi amplifiers. However, a lot of water has flowed under the bridge since and in relative terms the cost of the electronics now makes a system more viable. This in turn has led to the resurgence of active systems, although their average price is at present in the £1,000 region.
Briefly, an active system uses a filter built around a transistor or op-amp stage. Using only standard C's and R's the crossover becomes very cheap to build. However, the problem comes after the filters because each drive unit now requires it's own power amp.
With this system the advantages over the passive types are amazing. Most of the small response irregularities of the drive units are smoothed out because of the better damping applied to the voice coils. Efficiency is increased by several dB allowing smaller power amps to be used for the same SPL. The speaker's impedance has no effect on the active crossover so that a textbook perfect crossover response is obtained every time. Lastly, it is easy to adjust the relative balance between woofer and tweeter to cater for different acoustics.
The sound of a speaker system depends on several factors, all inter-relating with one another. Firstly, the type of cabinet employed and it's construction. In this system the simple closed box, or infinite baffle (IB) has been chosen. This has two major consequences for the performance, both at the low end. First when a drive unit is placed into a sealed box the resonant frequency rises. In this case to about 75 Hz. Signals above this frequency to the cutoff at the high end are reproduced flat. Below the resonant frequency the response rolls off at 12dB/octave. This rolloff is exactly complementary to the rolloff imposed on the woofer and is within the adjustment range available. This means that the woofer and speakers are easily integrated.
The IB type of enclosure has an inherently good transient response, a necessity for the proper reproduction of music. One of the most serious problems associated with speaker cabinets is colouration. This is because the cabinet usually consists of parallel walls and standing waves are set up when the sound reproduced has a wavelength identical with one or other of the internal dimensions. The cure is twofold: standing waves can be reduced by inserting suitable acoustic absorbants into the cabinet and wall vibrations can be damped by adding extra mass and bracing for rigidity.
In this design BAF wadding is used for internal damping whilst the walls are stiffened and damped with ceramic tiles. In fact the perfect cabinet material would probably be ceramic as it is completely dead and heavy - try flexing a tile! The use of ceramic tiles for damping is at least as effective as using bitumous felt panels but without the problems associated with fixing this material in place.
Experience with large numbers of speaker systems has shown that the stability and depth of the stereo image created by a pair of speakers depends, in large part, on the width of the front baffle.
All else being equal a narrow speaker width allows greater dispersion of the sound than a wide one. For this reason the speakers are only 9" wide. Another important thing is to ensure that sufficient depth behind the drive units is available. The reason for this is that the sound radiating from the rear of the speaker cone impinges directly on the rear panel. Some of this is reflected back to hit the speaker cone. This being flexible will try to move in sympathy. The resulting sound when re-radiated in this way tends to obscure fine detail. The cure is to attenuate this energy as much as possible. This requires a long signal path and plenty of damping material.
To maintain an even response the drive units must both be very linear in response and accurately crossed over, The drive units chosen for this design are the Dalesford D110/30 for the woofer/midrange unit and the Audax HD13D34H dome tweeter for the high end.
Crossover order is the next factor to be determined. A 6db/octave response would be too low to avoid problems with the fundamental resonance of the tweeter and the inevitable response irregularities at the extreme high end of the woofer's response.
Third and fourth order filters are rather complex to design and furthermore have a worse phase response than the lower order types. This leaves the second order which are both straightforward to design and implement and are good at handling transients.
The only suitable filter slope having been determined the remaining problem is to ensure that the 'Q' is set at 0.707. This will ensure the optimum rolloff rate combined with minimal phase ripples in the pass bands.
The circuit of the active filter is shown in Figure 1. Input signals are fed into the active filter by means of the attenuator RV1. Because the amplifiers are internal the units can be driven either from the output of a preamplifier, or from the speaker outputs of an existing amplifier.
If the latter option is used a hidden advantage is that the input impedance of the speakers is such that the output stages of the driving amps will operate in class A, removing most of the distortion usually generated.
The active filter is a standard 'Sallen and Key' type built around TR1. The response is that of an optimally damped Butterworth with a 'Q' of 0.7. R3, R4, C3 and C4 form the feed back loop for a low pass response with a -3dB point at 3kHz.
The treble component of the signal is separated from the full range input by subtracting the inverted low pass response at TR1's collector from the input with a virtual earth mixer, TR2.
In order to prevent any possible instability and hum modulating the outputs, a hefty decoupling network consisting of R12 and C2 is employed.
As with most speaker systems the sensitivity of the tweeter is greater than that of the woofer so some means of equalising the amplitude response is needed. Here this is achieved by attenuating the treble component with RV3 which is brought out for the use of the listener.
Only one amplifier is shown in Figure 2 although two are required in each cabinet. The other amplifier is identical to the one shown except for the output capacitor C12. This component is 100uF for the tweeter and 1000uF for the woofer/midrange. The size differential occurs because the output capacitor and drive unit form a first order highpass filter whose cutoff depends on their relative impedances.
Bias for TR3 is provided by the resistive divider formed by R13, 14 and 15. The base is decoupled from line variations by the capacitor C7.
Output signals from TR3 are taken from the collector across R16 which also provides bias for the pre-driver TR4. TR5 and RV2 form a Vbe multiplier for setting the bias for the output stage. The collector load of TR4 is completed with R19 and R20 whose junction is bootstrapped by C11. This provides the constant current drive required by the output stage if crossover distortion is to be avoided. C9 and C10 put a pole in the high frequency response to achieve stability.
The output stage consists of a pure complimentary pair of discrete Darlingtons. The power transistors have a fairly high ft which helps to produce a low distortion level at high frequencies.
Overall DC feedback is obtained by R18 whilst the AC gain is defined by the ratio of this component to R17 to which it is coupled by C8.
An advantage of the active approach over the passive is that, since no attenuation is used in front of the tweeter, several dB more headroom is available at high frequencies.
The power supply is very conventional consisting of a full wave rectifier followed by a large decoupling capacitor. By connecting the two secondaries in parallel a 'stiff' supply is obtained which means in practice that the amplifier can cope with awkward signals with ease.
A dual connector is taken out to the back panel from across the D110/30 to facilitate connection to the E&MM woofer. If this is not contemplated, these are not required.
The mechanical work involved in making the cabinets has deliberately been kept as simple as possible. Apart from making life easier for constructors I must also admit to an ulterior motive here. I don't count woodwork as my strongest subject so the reader can safely assume that if the writer can build it, so can he (or she)!
Butt joints are used throughout, again for simplicity. When using chipboards it must be remembered that the glue holds everything together and so this must be carefully chosen.
For this job, as with the bass unit thixofix adhesive is specified. For those who have not used this before a brief description is in order. Thixofix is a contact adhesive marketed by Dunlop and a small tin, easily adequate for a pair of speakers, can be obtained from your local hardware store. It will stick a wide variety of materials apart from wood and is really intended for gluing worktops. It adheres well to melamine teak chipboard and works better here than the more usually encountered PVA wood adhesives.
The real advantage, however, is that it is possible to slide the boards around to position them correctly. If pressure is then applied to the joint the materials will stick hard.
Teak melamine board is the ideal material to use for the speakers for a variety of reasons: it is somewhat denser than the real veneer type; it is cheaper and lastly it requires no finishing.
The physical construction of the speakers can be divided into two parts, the electronic and mechanical. Lets start with the mechanics.
Take the cutting list along to your local neighbourhood wood merchant, insisting that the pieces are cut to size accurately - this is worth the price of a pint of beer extra in saved work and temper!
All the pieces can be cut from a pair of 6' by 9" boards with a few inches spare. The battens are hardwood which has the advantage over softwood that it's dimensions tend not to vary from piece to piece. It is also straight edged and doesn't warp.
Having got the wood home mark the worst face with it's corresponding letter as per the cutting list. Take the front panel and mark out the cutouts for the drive units. Note here that the woofer cutout is not recessed as per the instructions supplied for it. For these speakers it is not necessary and saves a lot of work. Don't drill the fixing holes at this stage but position the units and mark the hole positions through the mounting holes themselves. The woofer is then fixed into place as per enclosed instructions. Remember to use the foam gasket supplied under the drive unit for a good seal.
The tweeter is fixed in place with four ½" long No 6 self tap screws. The best way to use these is to take an ⅛" twist drill and wrap a piece of masking tape ½" from the end around the shank. This can then be used as a guide to prevent you drilling right through the panels. The tweeter is then simply screwed against the front baffle with four of these screws.
Now the cabinets themselves can be tackled as follows:
1) Mark out the sides, top and bottom to accept the battens.
2) Glue these into position.
3) Apply adhesive to the butting pieces and the edges of the front baffle.
4) Bring these together to form the cabinet. Don't do anything to the back panel at this stage.
The electronic side of the construction must be done now before the equipment can be finally assembled.
First, the PCB which is straightforward. Insert and solder the components using the overlay shown in Figure 3. Make certain that all the semiconductors and electrolytics are correctly orientated. Having completed and checked the board it can be fixed into position on the back panel by means of ½" self tapping screws. Then screw the transformer, C13 and heatsink into position.
The heatsink assembly should be fitted as shown in Figure 5. Transistors are mounted on to a piece of angled aluminium, which passed through a hole cut in the back panel and is screwed onto the larger flat heatsink.
Lastly, interconnections can be made as shown in Figure 3 and the internal photos.
Having reached this stage it is necessary to set the quiescent current of the output stages. The presets have to be adjusted for zero quiescent current. Connect a multimeter, switched to its lowest resistance range, between the base of TR5 and it's collector. Adjust the preset until a short circuit is read. Repeat this for the other amp. At this stage it is a good idea to cover the mains terminals on T1 and the mains input socket to avoid the possibility of accidental shock. Then remove the earthing link from the board and insert the multimeter in it's place switched to give a clear indication of 50mA quiescent current.
Apply power and the meter should indicate around 10mA. Slowly turn one of the presets until the indication shows 25mA. Now adjust the other preset until 40mA is read. Switch off, remove the meter and reinsert the link.
Switch on again and measure the output voltages at the emitters of the power transistors. They should be about 20V, a couple of volts either way is of no consequence.
Switch off once more and connect the speakers up temporarily with some hookup wire. After switching on again a finger placed on the input should resulting loud buzz from the speakers.
If all is well at this stage switch off, detach the speakers and turn your attention back to finishing the cabinets.
The next task here is to fit the ceramic tiles into the cabinets. You will find that the tiles are a good fit into the spaces between the battens where they should be glued into place with thixofiz.
Having done this solder a couple of feet of 5A mains cable to the drive units and roll up a couple of feet of BAF wadding and insert this into the cabinet(s).
Remembering to observe phasing, connect the speakers to the amplifier outputs. Apply a signal to the input and switch on. At this stage sweet sounds should issue forth. Try the effect of the balance control and check that the tonal balance alters between too much and not enough treble.
Once satisfied, screw the back panel into place with 1½" No. 6 woodscrews around the periphery into the support battens on the sides. The job is now completed and all that remains is to set the balance up to suit your room/taste.
This is best done with a source of voice signals. I suggest that a decent FM broadcast, say Radio 4 is ideal for this purpose. You should find that, with your tone controls in the flat position, voices will sound very natural without chesty boom or overemphasised sibilants. Of course adjusting the balance control on the rear panel will alter what you hear, but speech is ideal for getting it right.
With music the sound should be clear and detailed with adequate bass especially taking into account the size of the cabinet!
If you are going to use the woofer with these use the spare terminals on the back connected to the D110/30. Setting up follows the same procedure as detailed in the March '81 issue.
As with all speakers room positioning is vital. Ideally they should be kept away from corners and placed at least a foot away from the rear wall if the stereo image is to possess depth as well as width.
In conclusion I would like to say that I have yet to hear a speaker system that I would care to swap for these regardless of price.
The PCB for the Active Speaker is available from E&MM, (Contact Details) at £3.75 inc. VAT and P&P. Please order as: Active Speaker PCB.
Feature by Jeff Macaulay
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