changing speaker impedance

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71challenger

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Hi I have a pair of the very small Advent speakers using them as rear surrounds. The amp driving these is rated @100wrms x2 @8 ohms. I need to change the impedance of these from 8 to 4 ohms to compensate for my seperate front channel amp rated at 300 wrms to compensate for the difference in volume at lower listening levels. Granted I can't use all the power before it clips or I go deaf ,The loudest I play it for movies is about -58db and -27db for music. Correct me if I'm wrong, but I think I need to Parellel an 8 ohm wire wound resistor across the Advents To give me better balance (from the front and rear amps) at a lower listening level. What wattage of resistor? 75watt?
Thanks in advance
 
I'm completely baffled by what you're asking? - you can individually alter the levels to each channel using the controls on the amp - you just need to do that!.
 
I am using 2 separate amplifiers, the av amp rated at 100 watts is powering the rear surrounds only. I'm using the line out from this amp to an Adcom power amp for the front left and right and center speakers. These speakers are very inefficient and have a 4 ohm load. I tried on my av amp by decreasing the signal to the adcom by -10db to balance the power but it is not enough.
 
You can't change the speaker impedance. Sure, you could put an 8 ohm resistor in parallel with the 8 ohm speaker - and it will look like 4 ohms (more or less) to the amplifier. Note that half the power delivered to the combination is wasted as heat.

An alternative would be to wind and audio transformer to accomplish the task with relative efficiency. Not sure it's a cost effective solution though.
 
stevez said:
You can't change the speaker impedance. Sure, you could put an 8 ohm resistor in parallel with the 8 ohm speaker - and it will look like 4 ohms (more or less) to the amplifier. Note that half the power delivered to the combination is wasted as heat.
Click to expand...

It also wouldn't affect the volume in any way - putting a resistor in parallel will just make the resistor get hot, the speaker will still get exactly the same amount of power.

I still don't see where the problem is? - seperate amps, adjust the volume on each one till they are level.
 
what if I were to get an integraded amp (with left and right volume controls), hook it to the surround channel line out. at a preset volume on the av amp, I could increase the integraded amp volume, to raise the rear level to match the fronts. The preamp connected to the power amp in the av amp is linear, so by using another integraded amp, say 75 watts, in the line out, bypassing the av internal power amp, I could increase it any amount I want. correct?
 
I'm still trying to visualise your set-up?.

You have an AV amp, which presumably includes a 5.1 decoder, how many speaker outputs does this have? - usually it would be six. Does it have line level outputs as well so you can feed external amps rather than the internal ones?.
 

Correct I am using only the speaker out puts of the av amp to power the rear surrounds. The front left, right and center I use the line outs , connected to my adcom 7500 5 channel amp. I am running 8 floor speakes, 4 0n each channel (l+r) hooked in parallel, (two speakers on each channel) This leaves me one channel for the center speaker and no channels left for the rear surrounds, That is why the surrounds are the only speakers hooked to the av amp. When the system is cranking( for music. I like the sound fields) just before clipping the 100 watts from the av amp is sufficient since the rear sound is more of a reflected sound, very little bass, etc. But when watching a movie at lower volumes the rears ar too quiet
 
As I wrote earlier I guess my only option is to aquire a separate 100w interaded amp with volume controls and hook it to the line out of the surrounds on the av amp. This way I could have more adjustment.
 
A simpler solution would be to insert a resistor in SERIES with each of the rear speakers.
This will attenuate the signal, but I don't know whether it will affect the sound quality. I don't see why it should.

It would be a cheap and easy place to start.
 
A resistor in series with a speaker doesn't just reduce the damping factor, it destroys the damping factor. Any resonance of the speaker will be exagerated.

Speakers that were made for vacuum tube amplifiers weren't affected as much as modern speakers because old vacuum tube amplifiers didn't have a good damping factor.
 
Thanks Audio, I did not realise that a low source resistance is necessary to dampen the speakers.

Another way to do it would be to reduce the gain of the output stage.

71challenger, do you have a circuit of the av amp?

The attenuation would have to be after the line out.
 
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