||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).