How to make a high-end A4 monitor speaker

How to make a high-end A4 monitor speaker

This speaker is reasonably designed, simple to make, and has excellent performance. The only drawback is that it is expensive. The high-end A4 monitor speaker introduced in this article is designed by Mr. R. Smulders specifically for audiophiles who like to make it by hand (see Figure 1). It uses the latest high-end speaker unit launched by Danish can-Speak Company. The sound quality can reach the highest level of similar two-way small speakers at present, and can meet the needs of audiophiles to enjoy music in a small indoor space. Since the panel size of the box is only equivalent to an A4 copy paper, it is named "A4 monitor speaker". The main specifications of this speaker are shown in Table 1.

1. Design and production
Since the indoor space of ordinary families is not spacious enough, the speakers are required to be as small as possible so that they will not attract attention when placed indoors. However, the efficiency no of the speaker has the following relationship with the box volume VB and the low-frequency -3d8 roll-off point f3:
no＝kn·f33·VB
In the above formula, kn is the box coefficient. For a well-designed bass reflex speaker, kn＝2x10-6; for a high-quality closed speaker, kn＝10-6. The unit of the box volume VB is l (liter), and the unit of f3 is Hz (hertz). Therefore, if you want to reduce the box volume, you must increase the -3dB low-frequency roll-off frequency to make the low-frequency response worse, or you have to allow the speaker to work at a lower frequency. Today's high-power amplifiers generally have enough power margin, and it is no problem to use them to drive low-frequency speakers. Therefore, some well-designed small speakers may also have a wider bandwidth and better low-frequency response. Of course, the volume of the cabinet cannot be reduced indefinitely, lest the power limit of the speaker be exceeded and the voice coil be burned or the nonlinear distortion of the speaker be increased.
The designers of this speaker have successfully achieved a proper balance between the two seemingly incompatible attributes of small cabinet volume and excellent sound quality through careful design. The tuning of the bass reflex and crossover filters are calculated very correctly, and the solid cabinet has no resonance. Of course, the use of Scan-Speak's high-end drive unit is also an important factor in the success of this speaker. After listening, the sound quality of this speaker is very close to that of a much larger speaker, so that many listeners (who cannot see the tested speaker) thought they were listening to a larger speaker.

1. Use two-way frequency division
The era when two-way speaker systems were despised and ridiculed is gone forever. Four-way or five-way systems have gradually decreased after a short-lived popularity, and many high-end speaker manufacturers have returned to two-way or three-way systems. However, for many years, the audio industry has faced a question, which is better between two-way and three-way systems? The designers of this speaker believe that the correct answer to this question can only be found when all conditions are known. Even at this point, it is important to remember that any multi-way system is the result of a compromise. The ideal solution is that the speaker uses only one drive unit, which provides a straight phase and frequency response for the entire audio range of 20Hz-20kHz. Since there is no such ideal full-range drive unit at present, two or three drive units are used to reproduce the sound of the audio range. Inevitably, some errors that are difficult to eliminate and remedy will occur at the crossover points of these units, which is why most speaker manufacturers limit the number of crossover points (that is, limit the number of drive units).
The selection of drive units for multi-way systems depends on the sound pressure required for reproduction. For example, if a very high sound pressure level (SPL) is required for the bass, a bass driver with a diameter of more than 25 cm is required. Unfortunately, this large-diameter driver has a tendency to bulge in the low-frequency response and is not suitable for frequencies above 1kHz, nor for two-way systems, because current tweeters cannot cover the frequency range of 1kHz-20kHz. 28mm dome tweeters usually cannot reliably reproduce the sound of 2kHz-2-3kHz, and the low-end limit frequency of 19mm dome tweeters is as high as 4kHz-51kHz. Their low-end frequency response cannot achieve a smooth transition with the high-end frequency response of large-diameter bass drivers.
If the SPL in the bass range is allowed to be less than 100dB, a smaller bass driver can be used. It is usually difficult to dive the low-frequency response below 50Hz at such low SPLs, but the Scan-Speak 15cm bass/midrange driver used in this speaker can produce sufficient sound output at low frequencies and can reproduce the sound of 2kHz-3kHz satisfactorily, so there is no need to use a midrange driver. Some smaller-diameter woofers are designed to reproduce very satisfying bass in smaller speakers, and the bass/midrange unit used in this speaker is a good example.

2. Selection of drive units
The drive units of this speaker are products of Scan-Speak Company of Denmark (see Figure 2). The company was originally an independent manufacturer. After being acquired by Vifa a few years ago, it became the high-end product division of Vifa. Most of Scan-Speak's drive units are handmade high-quality cone and dome products. The D2905/990000 Revelator tweeter launched in the 1980s has won high honors all over the world and is considered to be one of the best dome tweeters on the market.
The 15w 8530K00 15cm low/midrange unit used in this speaker also belongs to the "Revelator" series. Its basin frame is designed to fit the special long-stroke voice coil and facilitate airflow. Due to the absence of resonance, the enhanced paper cone has excellent damping effect and the paper cone support ring has good linearity. This unit can provide the most realistic playback and dynamic performance among similar products.
Until now, all tweeters produced by Scan-Speak have been fabric dome tweeters. Only the 28mm tweeter D2904/980000 used in this speaker is the company's latest aluminum dome tweeter. The sound pressure frequency response of any dome has a series of valleys (zeros) that occur at frequencies where the difference between the path of the sound wave to the top of the dome and the edge is exactly λ/2 times the path of the sound wave in air. As a result, phase differences are generated near these zeros, causing the axial sound output to gradually decrease towards the zeros.
The frequency fn at which the zero occurs is a function of the height h of the dome. For a 28mm aluminum dome with h = 5mm,
fn = C/λ = 345/5 × (10) -3 = 69kHz.
Therefore, the first valley occurs at 34.5kHz. When the dome unit is driven by a voice coil of the same diameter mounted on the edge, the first split resonance occurs near the first phase zero (34.5kHz in this case), which is well above the human hearing range. The tweeter has a low resonance frequency (500Hz) and a separate suspended, specially designed diffuser is used to linearize the sound when reproducing the highest frequencies. Its sound is very open, clear and detailed.

3. Crossover filter
The passive crossover filter of this speaker provides the corresponding high and low frequency bands for the two drive units. It is carefully designed to match the characteristics of the drive unit with the cabinet. Do not easily change the filter circuit and the drive unit during production.

The circuit of the crossover filter is shown in Figure 3. The first-order low-pass filter part of the bass/midrange unit is mainly composed of inductor L1 and capacitor C1, and its frequency response roll-off rate is 6dB per octave. The network R1-C1 compensates for the increased impedance of the unit (LS1) at high frequencies. Without it, the filter cannot work properly. In order to minimize the internal resistance of L1, the inductor is made of 0.5mm diameter enameled copper wire twisted into 1.5mm thick Tritec stranded wire with 7 turns to form a disc-shaped air-core inductor, with an inductance of 2.2mH and an internal resistance of 0.52 ohms. Another advantage of using stranded wire is that the skin effect is small.
The third-order high-pass filter of the tweeter consists of C2, C3 and L2, and its frequency response roll-off rate is 18dB per octave. Due to the different sensitivities of the two units, the voltage divider R1-R3 attenuates the power applied to the tweeter LS2 by 5dB in order to keep the sound output of the two units consistent. C2 and C3 have a great influence on the performance of the speaker and must be filled by high-quality capacitors, at least metallized polypropylene (MKP) capacitors. According to the designer, C2 is preferably composed of a 1pF and a 4.7pF tin foil (KPSn) capacitor in parallel, and C3 is composed of two parallel 10uF MKP capacitors and a 47uF KPSn capacitor in series. However, when listening, no difference was heard between the sample equipped with the MKP capacitor and the other with the KPSn capacitor. The
crossover frequency of the entire filter is designed at 2kHz, the main purpose of which is to achieve a seamless transition between the low/mid-frequency response and the high-frequency response. The frequency response curve shown in Figure 4 shows that the frequency division point cannot be detected, indicating that the design of the frequency division filter of this speaker is successful. Due to the small number of components, the frequency division filter can be installed on a self-made or commercially available frequency division filter printed circuit board, as shown in Figure 5. The disc coil on the left side of the figure is the air-core inductor L1 wound with twisted wire. Note that the two drive units LS1 and LS2 must be connected in phase according to the polarity shown in Figure 3, otherwise the performance of the speaker will be seriously reduced.

4. Cabinet production
The cabinet production of this speaker is relatively simple. It only needs to fasten together 6 boxes with corresponding openings as shown in Figure 6. No reinforcement plate or pillar is needed. The tweeter is installed at a position deviated from the center of the panel and the two sides of the front panel are rounded to obtain the correct radiation direction. In addition, compared with the usual practice, the bass reflex port is not located on the front panel but on the back of the cabinet. The sound outlet pipe connected to the reflex port can be made of a 35mm diameter, 123mm long and slightly tapered polyvinyl chloride plastic pipe. In order
to avoid resonance of the cabinet, 22mm thick medium-density fiberboard (MDF) is used for the two side boxes and the upper and lower panels, and 30mm thick MDF is used for the front panel. It is best to stick 4mm thick lead sheets on the inner surfaces of the 6 boxes (but it is more expensive), or asphalt felt of the same thickness can be stuck for sound damping. The damping material should be carefully cut off at the opening of the box panel to avoid hindering the installation of the drive unit and the sound outlet pipe.
As shown in Figure 6, the two drive units should be flush mounted on the front panel, otherwise the frequency response will have several valleys. The speaker terminal block on the rear panel is also mounted in the same way, but the latter is only for aesthetics. The terminal block has two large terminals for connecting the speaker wires. If you plan to use bi-wir-ing connection, you can choose a speaker terminal block with four terminals. The crossover filter circuit board is installed on the bottom plate of the box, and then it is connected to the drive unit and the speaker terminal with high-quality speaker wire. The appearance of the box can be painted or veneered according to personal preference.

2. Actual test results
In order to truly and comprehensively understand the performance of this speaker, a preliminary audition was conducted first, followed by instrument testing, and finally a careful audition. The results show that the instrument test is very consistent with the audition results, everything is as expected, and the actual audition effect is even better than the instrument test results.
The frequency characteristics of this speaker are shown in Figure 4. Its overall flat response and the slight emphasis near 100Hz will not be repeated. There is a small dip in the frequency response near 500Hz and a slight 1.5dB bulge near 750Hz. These flaws do not damage the actual performance. They are definitely not caused by the low/midrange unit, because the frequency response of this unit is as flat as a pencil. In fact, they are caused by the small size of the front panel, and most small speakers have similar phenomena. The curve represented by the dotted line in Figure 4 is the low-frequency response measured directly in front of the bass reflex port. The reflex port is tuned to 40Hz, and the impedance characteristics of Figure 7 show this more clearly. The figure also shows that the impedance of this speaker is not less than 7 ohms, which means that any appropriate power amplifier can drive it. However, it is best to use an amplifier with a large power reserve because the sensitivity of this speaker is low (about 85.5dB/W at a distance of 1m).

III. Audition and Evaluation
Instrument testing is important, but the performance of a speaker can only be finally evaluated through careful audition. Although this speaker is small in size and unremarkable in appearance, it can produce a very open and realistic sound, and is even better when used to reproduce excellent live recordings. The sound details it reproduces in the midrange and treble range are reminiscent of electrostatic speakers, and the bass is tight, uncolored, and surprisingly deep, with the -3dB point of its low-frequency response located near 40Hz.
As people expect from high-quality monitoring speakers, this speaker is not biased towards any kind of music, and it reproduces classical music, jazz and popular music equally perfectly, mellowly and naturally. The distinctive singing voices of Pavarotti, Reed, Fitzgerald and Brightman are reproduced as realistically as the violin of Perlman, the clarinet of Goodman and the guitar of Williams. In short, this speaker is one of the few speakers that makes you quickly forget about auditioning and completely immerse yourself in the music.
It must be remembered, however, that even a small speaker with this level of performance has certain limitations in its bass performance. The bass limitations of this speaker are not particularly pronounced, however, and only become apparent when compared directly to a larger speaker. This requires a speaker that is 10 times larger than this speaker to reveal the difference in bass performance.
Due to the location of the bass reflex port at the back of the cabinet, this speaker should not be placed directly facing a wall. However, when listening to it, it was found that placing it near a wall gave a better effect than placing it away from a wall, where the bass sounded particularly warm, whereas away from a wall the sound was cooler.