The multi-driver bass/mid array exploits the highly desirable, laterally coherent dispersion characteristic of small radiating cone areas for superb stereo localisation. The 753f driver array provides more than 350 square centimetres of total radiating area, which is responsible for the believable and authoritative portrayal of bass.n n A twin chamber enclosure design allows the upper driver pair to operate under optimum conditions for midband coherence, while providing the correct environment for the "bass pump" lower section of the array. n
Mission 753 freedom Signature Havinb been used this pair of speakers for over 4 years Apart from problems listed below, they are very sweet and musical. Since the upper end is very soft, I guess a pair of tube amplifiers may be better to drive them.
I inspected the compartment of the speakers; the two mid-range speakers are enclosed within a closed partition inside the speaker. The two bass speakers are at the lower part of the speaker with three vents for base reflection. That is why the same pair of drivers produces different range of frequency response. A closed enclosure is good for mid-range and the vented enclosure is good for bass. At this point I realize that the good quality sound of Mission is from the careful design of tuned compartment not from the passive filters. I was thinking upgrading all the filter components inside but later I understand that the improvement may be marginal in a situation that I cannot distinguish the difference. As you can see from the schematic, the bass of 753F is hard to drive with ordinary AV amp, the nominal impedance of this speaker at low frequency can be down to 4 ohm (four drivers connected in parallel-series).
I bi-amp my pairs of modified 753F speakers. 3801 connects to the upper post and pre-out of 3801 connects to the CD (I can configure the amp using CD-direct) input 3530G which connects to the lower post of the speaker. I can easily adjust the bass level by adjusting the volume of 3530G. At this point, I can lower the corner frequency of SW320 down to 50Hz. After that I have one system for both music and movie. I hardly identify the sound from SW320 again as its frequency is no longer directional to me. I can feel the bass lower than 50Hz as turning off SW320 makes difference! Miracle! The quality of bass coming from SW320 is never a concern to me again. I listen to the quality bass from Mission and feel the bass from SW320.
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#1: 由 ch 於 2002-01-14 00:11:10 提供
Modification of Mission 753F
I got a pair of Mission 753F speakers. I use it for both music and movie. However, its bass is not strong enough when using Denon 3801 AV amp. The bass adjustment of the amps cannot suit my taste as after using any bass enhancement, the whole spectrum of sound becomes muddy. I have modified my Yamaha SW320 which is driven by my old Denon 3530G AV amp connecting to the sub-out of 3801. The bass is very good as I can adjust the bass volume under 80Hz in will without any distortion.
Then, I got two problems. Firstly, I can easily locate the SW320 when there is bass (< 80Hz) coming out. Secondly, the bass of 3530G+SW320 is not low enough when watching movie; I have to reconfigure the system between movie and music.
I suddenly had an idea of bi-amp the 753F as I figure out that the speakers got posts for bi-wire and bi-amp. I connected 3801 to the upper post and was thinking to connect 3530G to the lower post later on. The speakers sound strange with only the upper posts connected. Its sound should never be above 400Hz, it is at least above one to two kHz! So what is the point of bi-amp? Only very little energy is coming to the upper post!
I took out the 753F manual to have a look. It is a two-way speaker system with corner frequency at 2.3kHz and 200Hz. What a stupid configuration of partitioning the posts at the frequency of 2.3kHz. It should be 200Hz. I opened the speakers and inspect the filters inside. The schematics is as shown on left of the picture below:
The inductor values are estimated from the corner frequencies. The speaker has one tweeter for frequency above 2.3kHz, two 6” drivers for frequency below 2.3kHz and two 6” drivers for frequency below 200Hz. I realize that it is natural for the Mission engineer to use 2.3kHz as the corner frequency. The upper post connection is a high pass filter at a corner frequency of 2.3kHz and the lower post connection has two low pass filters one has 2.3kHz corner frequency and the other has 200Hz corner frequency. The components used are of ordinary (or low) grade. The inductors use insulated single copper wire. High quality inductor should use a batch of parallel thin insulated copper wires. Low-pass non-polar capacitor of the 200Hz low pass filter is simply two cheap electrolyzed capacitors connected back-to-back in series. I took out a solder iron and reconnect the filters as shown on the right of the picture above.
After modification, it seems that the speaker got a set of full range speakers connected to the upper post and a sub of frequency lower than 200Hz. I connected 3801 to the upper posts and have a test. Wah!!! What a pair of high definition speakers! It sounds lovely! But wait, where is the bass? There is no bass lower than 100Hz (I guess).
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