stupid impeadence question?

[dallaire]

I am designing a WMTMW center channel and woofers will be a four ohm load on amp and so will the mids place a four ohm load on amp. My single tweeter is 8 ohm, so of course that amp will see an eight ohm load. My question is can one just use a simple reststor at the tweeter to make that amp also see a four ohm load?, or am I dreaming again? My system is all electrroniclly crossed using Marchand XM1's. And "if" this can be done will it have any effect on quality of sound!
Thanks,
Dallaire

PS I do have 12 position rotary switches for control but would like to get close inside speaker enclouser if this can be done with a simple resistor....

[jupiterjune]

dallaire-

You could reduce the tweeter resistance by putting an 8 ohm resistor in parallel with the tweeter. This will also reduce your tweeters output level, by 6 dB (need to check that, it could be 3 dB, its been a long time since I've looked at those eq's). But it won't necessarily be a flat reduction across the band -- likely your highs would be rolled off due to voice coil inductance.

But there is really no need for the resistor -- the crossover should have a pot for level matching, and and if your tweeter is run off a separate amp (it sounds like this is your intention from your post), then the 8-ohm tweeter makes no difference.

JJ

[bzfcocon]

What is the problem with the amp seeing a 8 ohm tweeter, if it's a dedicated amp anyway ?
As long as you adjust the tweeter level from the crossover, I don't see any reason why you would add a (parallel !) resistor to the tweeter.

[AndrewT]

Hi,
adding a resistor parallel to the treble driver will increase the loading on the amplifier. The resistor runs warm to hot for no benefit to the sound. The extra loading may make the amplifier perform less well with the result that the sound quality coming from the treble driver is worse with the parallel resistor.

Look at the sensitivity of the different drivers.
Compare them at the same voltage db/m/2.8V not at the same power db/m/W which covers up the difference in impedance and confuses the power/voltage requirement to be supplied by the amplifier.

If you intend using a separate amplifier for each driver then you can adjust the relative sound pressure from each driver by changing the attenuation in front of the amplifier.

[myhrrhleine]

Originally posted by dallaire
I am designing a WMTMW center channel and woofers will be a four ohm load on amp and so will the mids place a four ohm load on amp. My single tweeter is 8 ohm, so of course that amp will see an eight ohm load. My question is can one just use a simple reststor at the tweeter to make that amp also see a four ohm load?, or am I dreaming again? My system is all electrroniclly crossed using Marchand XM1's. And "if" this can be done will it have any effect on quality of sound!
Thanks,
Dallaire

Hi,
adding the resistor is not a good idea.
Amplifiers prefer higher impedences.
Adding a parallel resistor will just give another path for current to flow.
It'll eat power, but do nothing else.
The speaker usually won't stay at 8 ohms anyway.
It's common for speakers to go to 4ohms, 3ohms, 2ohms, sometimes lower, or up to 40-50 ohms.

[fizzard]

Originally posted by jupiterjune
dallaire-

You could reduce the tweeter resistance by putting an 8 ohm resistor in parallel with the tweeter. This will also reduce your tweeters output level, by 6 dB (need to check that, it could be 3 dB, its been a long time since I've looked at those eq's).

Actually the tweater still sees the same signal. The tweeter and resistor are in parallel so both see the output voltage of the amplifier directly. The amplifier will put out twice the power and the tweeter will get exactly the same power as if the parallel resistance were not present.

[dallaire]

Thanks, I guess I should see where things measure acousticlly with software upon completion. What I have now is Deffinitive Tech. spealers "all passives bypassed", mains use two 6.5 inch woofers/mids and one tweeter. All speakers are eight ohm addvertised. I have the woofers parrallel and of course the tweeter is on its own. One would think this would not be a large difference in Db from woofer to tweeter but in my case I have one rotel amp for tweeters at 60 per channel and one amp for woofers at 120 per channel. So then with this setup, I have to turn my 12 position rotarys swithches at full tilt, just to get a measured acoustic balance using R plus D software. So it is OK because I have alot of control with the rotary switch, but if tweeter played any softer in relation to woofer, I would need a switch with more attenuation, and with all one percent resistors and switch they are not really cheap to change.
Thanks,
Dallaire