What bearing does having 1 8 ohm unit and one 4 ohm unit in each cabinet have

[audio perfectionist]

i have a pair of vintage mission argonauts and i blew all of the original 8" bass units and i could only get 2 of each impedance 2 8's, 2 4's. they dont seem to sound bad for it, but as i have no basis for direct comparison to the originals i cant really say. (very rare speakers. never seen another pair anywhere) i cannot know whether they sound as good as they used to because it has been so long from when i blew them to when i managed to find suitable replacements. incidentally the two 8 ohm units i acquired were originally designed by mission and produced by SEAS and the two 4 ohm units, although never used in any line of mission speakers are very similar in design to the 8 ohm. so i have probably got a fairly similar sound, but i am more concerned about any negative sonic results this setup could have. the bass on my new mission m35i's is much more deep, rich and extended, however i am pretty sure this is to do with 20+ years of research on missions part and not to do with electronic cock-ups caused by units of different impedances. any points of view, advice and opinions would be welcome.

[hermanv]

The crossover will have been designed for a specific impedance, using speakers at 1/2 or two times the impedance will move the crossover as much as one octave for a first order crossover.

Additionally most speakers are designed to have a flat output with the same RMS volatge applied to the midrange drivers and the woofers. Cutting the impedance in half (all else being equal) will cause a 6 dB increase in woofer level or visa versa a 6 dB decrease if woofer impedance is doubled.

Of course your newer speakers might not have the same SPL/volt specification so the previous paragraph might not be accurate.

Dealing with this problem will require equipment and knowledge, get schematics for the old speakers as a place to start.

[audio perfectionist]

The crossover will have been designed for a specific impedance, using speakers at 1/2 or two times the impedance will move the crossover as much as one octave for a first order crossover.

Additionally most speakers are designed to have a flat output with the same RMS volatge applied to the midrange drivers and the woofers. Cutting the impedance in half (all else being equal) will cause a 6 dB increase in woofer level or visa versa a 6 dB decrease if woofer impedance is doubled.

Of course your newer speakers might not have the same SPL/volt specification so the previous paragraph might not be accurate.

Dealing with this problem will require equipment and knowledge, get schematics for the old speakers as a place to start.


you are talking slightly over my head although i am reasonably knowledgable about such things and have got the gist of what you are saying. the two units in each cabinet both recieve the same signal as they are both bass/mid and not one for bass and one for mid and looking at what you said it would seem that you assumed the latter. i would think(although i am no electronics expert) that due to the fact that both recieve exactly the same signal it would make less difference than if say the mid was 4 ohms and the bass was 8 ohms(in which case the midrange would be more prominent than the bass? am i correct?). you seem very knowledgable on the subject and your input would be greatly appreciated.
regards.

[topspeed]

I have a pair of Argonauts in my office. I don't know if it will help, but I'll pull the drivers if you think there might be some info on them that could help. I had to replace a tweeter 15 years ago as penance for my own monumental stupidity (don't ask), otherwise they have performed beautifully. For 20 year old speakers, the Missions can still hold their own :cool:.

[hermanv]

you are talking slightly over my head although i am reasonably knowledgable about such things and have got the gist of what you are saying. the two units in each cabinet both recieve the same signal as they are both bass/mid and not one for bass and one for mid and looking at what you said it would seem that you assumed the latter. i would think(although i am no electronics expert) that due to the fact that both recieve exactly the same signal it would make less difference than if say the mid was 4 ohms and the bass was 8 ohms(in which case the midrange would be more prominent than the bass? am i correct?). you seem very knowledgable on the subject and your input would be greatly appreciated.
regards.It is quite possible that the two drivers do not recieve the same signal, very often these designs feed bass to both drivers but mids to only one of them. Even if they are just wired in parallel, the tweeter will no longer be as loud as the mid/bass causing the voicing of the speaker to change by as much as 3 dB (possibly even more if the driver specs have changed) It is difficult to cover all the issues in a small space but let me give a simplistic example.

Suppose we have a series RL network configured as a low pass where the speaker is the R and a series inductor is the L. This common and simple first order network will allow only bass signals to reach the driver. The frequency at which the driver reaches 3dB is where R = 2 x Pi x F x L . As an example R = 8 Ohms, L equals 5mH, then F equals 254 Hz (i.e. the driver will get half power at 254 Hz.as frequency goes up less and less power will be deliverd to the driver) If we use a 4 Ohm driver in the identical circuit F becomes 127 Hz.

That is but one issue. The second issue is plain old watts. Power equals volts times amps. If we drive a speaker with the same voltage (i.e. the same volume setting on your amplifier) the 4 Ohm speaker will be twice as loud as the 8 Ohm speaker. This is because the current has doubled due to the resistor (speaker impedance) value being cut in half.

All of these things can be compensated for but they require changes in the crossover network. Speaker design is partially science and partially art. We can calculate the changes that need to be made to other components to compensate for a different driver but without measurement tools and a good ear, we may never be able to make the speaker sound good. Conversely it might actually sound better, but this is betting that the original designer did not do a good job, probably a poor bet.

There is no harm in simply trying it., Only you can decide if the end result needs fiurther work or is accepatble as is. The odds are just not in your favor.

[audio perfectionist]

It is quite possible that the two drivers do not recieve the same signal, very often these designs feed bass to both drivers but mids to only one of them. Even if they are just wired in parallel, the tweeter will no longer be as loud as the mid/bass causing the voicing of the speaker to change by as much as 3 dB (possibly even more if the driver specs have changed) It is difficult to cover all the issues in a small space but let me give a simplistic example.

Suppose we have a series RL network configured as a low pass where the speaker is the R and a series inductor is the L. This common and simple first order network will allow only bass signals to reach the driver. The frequency at which the driver reaches 3dB is where R = 2 x Pi x F x L . As an example R = 8 Ohms, L equals 5mH, then F equals 254 Hz (i.e. the driver will get half power at 254 Hz.as frequency goes up less and less power will be deliverd to the driver) If we use a 4 Ohm driver in the identical circuit F becomes 127 Hz.

That is but one issue. The second issue is plain old watts. Power equals volts times amps. If we drive a speaker with the same voltage (i.e. the same volume setting on your amplifier) the 4 Ohm speaker will be twice as loud as the 8 Ohm speaker. This is because the current has doubled due to the resistor (speaker impedance) value being cut in half.

All of these things can be compensated for but they require changes in the crossover network. Speaker design is partially science and partially art. We can calculate the changes that need to be made to other components to compensate for a different driver but without measurement tools and a good ear, we may never be able to make the speaker sound good. Conversely it might actually sound better, but this is betting that the original designer did not do a good job, probably a poor bet.

There is no harm in simply trying it., Only you can decide if the end result needs fiurther work or is accepatble as is. The odds are just not in your favor.

i think you hit the nail on the head with that last paragraph. i think they sound bloody good as it is. and you may not believe me but in certain areas i feel that they actually excel my new mission m35i's and im quite content with how they sound. i realised that for a basis for comparison i could put both 8 ohm units in the one cabinet and have a listen (the originals were 8 ohm.) also, TOPSPEED, theres no need to pull your speakers apart as i have my original drivers (lovely looking things as well arent they. always covers off with my originals, but the new drivers arent as classy looking and also, the two 8s have tranlucent cones. looks tacky. the4 4s are ok though, just not a concave voice coil and no MISSION on the voice coil, obviously)