AlexanderS

11-09-2005, 02:31 AM

Hi, all,

I have a friend (his nick is Odessit) who is very good in speaker design. His method is based on two principles. This works well especially for tube amplifiers. The point is that typical modern networks cannot have the constant impedance. Consider tube amplifier having 2-3 ohms at the output. Than even simple 1st order network will have changes in impedance say from 5 to 8 ohms or even more, which will change the tonal balance on more than 3 dB. Besides the tonal balance the correct phase of the signal will be lost due to interaction of amplifier with complex impedance.

The method of Odessit is based on two principles:

1. First, he connects a pair of similar drivers for future stereo speakers consequently by wires and connect them to one output of the amp. The input of amplifier is connected to sine generator.

Now, one changes the frequency litlle bit up and down, arround the testing frequency, which makes a "wo-oo-u" sound. The point is to listen and observe the sound image between two drivers. The ideal image must concentrate in one point. The idea is that the human ear cannot tell much about constant frequency signal, but is very sencitive to frequency changes. If the image jumps left or right or in other directions at some frequency this pair is considered as having potential problem at given frequency and might be not good to work together. If one follows the method, he finds that for instance the woofer works well up to some frequency, then he observes the frequency points where the image is detoriated. It is up to You - to accept these frequencies and go further up in the range, or to stop. Increasing the frequency several "potential problem" can be met, and at last You fing the frequency where You decide that You don't want to go anymore up. Finally, one finds the working range of the woofer (and same for the midrange). Now the cross-point is obtained as the geometrial mean value of two boundaries for the woofer and midrange.

The important is that the cross-over point is obtained not only from the driver specs (this is of course still useful), but from the subjective test for a given pair of the drivers.

My friend starting designing new speakers usually finds many similar drivers, and makes the cross-matrix puting in table the test results. Finally, only two units are selected which hold stereo image in the maximum range. :applaud:

2. Point two is how to design the cross-over. He only uses 1st order filters which don't detoriate the phase, and compensate impedances of each driver. Only after compensation - this is important- the woofer is added with the coil, and the midrange with capacitor. The accuracy of the filter tuning is about 1 percent or so, same for active resistances (after compensation) of each drivers. For instance, if LE123-A has 5.5 ohm coil, that means that all drivers must be adjusted to the same 5.5 ohm with serial and parallel resistors (like L-pad). That means all drivers have only real impedances (no complex values), and only after that are crossed. This finally means that entire speaker has constant and only real impedance, with no imaginary part reacting with the amplifier. The tuning must be very accurate.

I am now doing alsmost the same (skiping p.1 with the sound image but I want to try it) for JBL LE123 and LE5-2 and I am very much impressed by the first results.

Alexander

I have a friend (his nick is Odessit) who is very good in speaker design. His method is based on two principles. This works well especially for tube amplifiers. The point is that typical modern networks cannot have the constant impedance. Consider tube amplifier having 2-3 ohms at the output. Than even simple 1st order network will have changes in impedance say from 5 to 8 ohms or even more, which will change the tonal balance on more than 3 dB. Besides the tonal balance the correct phase of the signal will be lost due to interaction of amplifier with complex impedance.

The method of Odessit is based on two principles:

1. First, he connects a pair of similar drivers for future stereo speakers consequently by wires and connect them to one output of the amp. The input of amplifier is connected to sine generator.

Now, one changes the frequency litlle bit up and down, arround the testing frequency, which makes a "wo-oo-u" sound. The point is to listen and observe the sound image between two drivers. The ideal image must concentrate in one point. The idea is that the human ear cannot tell much about constant frequency signal, but is very sencitive to frequency changes. If the image jumps left or right or in other directions at some frequency this pair is considered as having potential problem at given frequency and might be not good to work together. If one follows the method, he finds that for instance the woofer works well up to some frequency, then he observes the frequency points where the image is detoriated. It is up to You - to accept these frequencies and go further up in the range, or to stop. Increasing the frequency several "potential problem" can be met, and at last You fing the frequency where You decide that You don't want to go anymore up. Finally, one finds the working range of the woofer (and same for the midrange). Now the cross-point is obtained as the geometrial mean value of two boundaries for the woofer and midrange.

The important is that the cross-over point is obtained not only from the driver specs (this is of course still useful), but from the subjective test for a given pair of the drivers.

My friend starting designing new speakers usually finds many similar drivers, and makes the cross-matrix puting in table the test results. Finally, only two units are selected which hold stereo image in the maximum range. :applaud:

2. Point two is how to design the cross-over. He only uses 1st order filters which don't detoriate the phase, and compensate impedances of each driver. Only after compensation - this is important- the woofer is added with the coil, and the midrange with capacitor. The accuracy of the filter tuning is about 1 percent or so, same for active resistances (after compensation) of each drivers. For instance, if LE123-A has 5.5 ohm coil, that means that all drivers must be adjusted to the same 5.5 ohm with serial and parallel resistors (like L-pad). That means all drivers have only real impedances (no complex values), and only after that are crossed. This finally means that entire speaker has constant and only real impedance, with no imaginary part reacting with the amplifier. The tuning must be very accurate.

I am now doing alsmost the same (skiping p.1 with the sound image but I want to try it) for JBL LE123 and LE5-2 and I am very much impressed by the first results.

Alexander