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I have a 2 channel stereo output from my laptop connected to a surround sound amp: all 5 speakers with an impedance of 4 ohms (front, rear and sub) play the stereo sound. The speaker connectors on the amp are marked 4 Ohms. I have four more 4 Ohm speakers that I would like to add in parallel, but doing so would lower the impedance of each to 2 Ohms. Would the following work to keep the resistance at 4 Ohms? (I don't want to open the speaker cabinets for a series connection due to their location.)

Make a box with 4 input jacks and 4 output jacks with a 2 Ohms/25W resistor between each in and out; thus 4 Ohm + 4 Ohm in parallel = 2 Ohms + 2 Ohms added in line through the box = 4 Ohms.

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    \$\begingroup\$ Please can you edit your question and add a schematic instead of describing it in words, I'm afraid it's just too awkward to picture. The better the quality of your question, the better the quality of the answers it could attract. \$\endgroup\$
    – TonyM
    Commented Mar 27 at 20:04
  • \$\begingroup\$ Doing this would convert half the output power of your amplifier into heat in the resistors, which sounds like it would defeat the purpose of adding more speakers. Why do you think you need to open up the cabinets to put them in series? \$\endgroup\$
    – Finbarr
    Commented Mar 27 at 20:15

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While you technically can do that, there might be reasons why you might not want to actually do that.

Assume a speaker is a simple 4 ohm voltage driven load (it's not really, it's basically an inductance with resistance, and even more complex due to possible crossover filters and resonances at some frequencies).

Normally, this 4 ohm load is driven by the amplifier output with very low output impedance, almost zero like an ideal voltage supply. It means that the amplifier can drive the load very precisely, by setting the voltage over the load, and any non-resistive reactive parts of the load that may store energy into capacitive or inductive elements also have this same well-defined low impedance source driving them and amplifier feedback will keep voltage stable by sourcing or sinking the reactive currents, damping the resonances etc.

The output impedance may be in the order of milliohms due to feedback, and if you compare the original load impedance of 4 ohms to for example output impedance of 40 milliohms, that equates to a factor of 100, often called the Damping Factor or DF. A DF of 100 is quite OK and DF might be a few hundred for good and even thousands for amplifier with really low output impedance.

If you parallel two speakers into single 2 ohm load, and put a 2 ohm series resistor to bring the load impedance back to 4 ohms, it means you have a 2 ohm source impedance driving a 2 ohm load as the output impedance of the amplifier is so small it can be ignored. This makes the DF to be 1, which might be considered terrible, and it is 100 times worse than without the resistor. Due to the 2 ohm resistor, no feedback at the amplifier can keep the voltage applied to the speaker well defined, since the feedback is from driving the resistor and speaker, not only the speaker. So any currents caused by driving reactive impedances will cause changing voltage over the speaker and slow down driving it. Not only there is the resistor, but now there are two parallel speakers and their reactive impedances affecting each other.

Even without all that, you will halve the original signal amplitude to the speaker, which means each speaker produces sound at quarter of the power, but on the other hand there are two speakers in parallel so the total power to speaker only halves. The remaining half is just dissipated as heat in the added resistor.

However, if you simply turn up the volume a bit, you might not notice the sound quality being degraded, it depends. But from engineering point of view, it makes no sense to make things inferior than what they are, while consuming more energy and wasting it as heat.

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  • \$\begingroup\$ What if the amplifier specification says that the speaker impedance must not be less than 4 Ohms? \$\endgroup\$
    – PMF
    Commented Mar 27 at 20:50
  • \$\begingroup\$ @PMF I don't understand why you ask that. The original question and my answer are about keeping the impedance seen by amplifier at 4 ohms. It will not be below 4 ohms, allowed or not by the amplifier. \$\endgroup\$
    – Justme
    Commented Mar 27 at 20:53
  • \$\begingroup\$ Then I don't understand you answer, it seems. How can putting two 4 Ohm speakers in parallel still be a 4 ohm load? \$\endgroup\$
    – PMF
    Commented Mar 28 at 7:08
  • \$\begingroup\$ @PMF It means you did not understand or read the question. It was about paralleling two 4 ohm speakers into a 2 ohm speaker as load, and then adding a 2 ohm resistor in series, so the amplifier sees a 4 ohm load. \$\endgroup\$
    – Justme
    Commented Mar 28 at 7:21
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    \$\begingroup\$ @PMF No, my point is that while it is possible to add resitors and parallel the speakers to solve the problem of connecting dual set of speakers to an amplifier not otherwise supporting a dual set of speakers, my suggestion is to not do it as it is not a proper solution. The proper solution is of course something else. Maybe to buy another amplifier. So "can it be done and will it work" is different from "is this even the right approach or are there better approaches". Yes, it will work, it's just not a good solution for many reasons. \$\endgroup\$
    – Justme
    Commented Mar 28 at 8:52
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Yes, that should work. I use series resistors with my speakers as well, because the impedance of the speakers I have is less than the minimum allowed for the amplifier. Has been working fine for 20 years now, and the excess heat is negligible (the amplifier itself generates much more heat). Of course, I'm not running the amplifier even close to its maximum output volume.

As an alternative, you can of course also put the speakers in series, then the resistance sums up and no additional resistors are needed. This could however impact the sound quality a bit, in particular if the speakers have different characteristics.

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  • \$\begingroup\$ Thanks for lending a hand. I love this site...so many kind and knowledgeable contributors. \$\endgroup\$
    – Gerard
    Commented Mar 29 at 22:21
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You sure can, and it will cost you half of your power. The resistors must be high-wattage to handle all the power you threw away. Your 100-watt channels turn into 50-watt channels. Those big, ugly, costly resistors will be too hot to touch; with five channels, you have enough heat to use them as a space heater.

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  • \$\begingroup\$ Thanks to all for the valuable info. Yes, I should have posted a schematic. Sorry. Being a novice in electronics, I will have to study al your answers. I have four Bose speakers lying around, doing nothing. I thought of putting them to use, but my audio amp already has speakers connected to every output. The idea of driving 8 speakers + the subwoofer seemed like a good idea. Since the amplifier is 350W RMS per channel and I play my music at low volume, the heat and audio quality loss due to the added resistors should be minor, no? \$\endgroup\$
    – Gerard
    Commented Mar 28 at 12:27
  • \$\begingroup\$ No more problem than giving me half of everything you own or will own in the future. Both of us will be tickled pink. \$\endgroup\$
    – Dereck
    Commented Mar 28 at 17:17

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