Increasing resistance on speakers

Question

My amplifier is rated at 8ohms for my surround sound but my speakers are 6 ohms. The owners manual says that if the speaker resistance is too low the power protector will blow and that is exactly what happens when I raise the volume on my amp. I want to increase the resistance on my 5 surround sound speakers and my subwoofer by 2 ohms from 6 to 8. I was thinking about inline resistors. My question is do I use 2 ohm resistors to take 6 to 8 and do I put them on both the positive and negative speaker lead or just one of them. If both would I put 1 ohm on each lead to get 2 ohms total?

Answer 1

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Series connection of resistor.

Since you are increasing resistance in a series circuit the amplifier won't know or care where in the circuit the resistance is. Convention would be to put it in the red or '+' wire.

^{simulate this circuit}

Figure 2. Simplified equivalent circuit.

The problem is that your speaker isn't just a resistor. It is largely an inductive load and since it has mechanical resonance (favourite frequencies to vibrate at) the situation becomes more complex. A good amplifier will have a strong drive to force the speaker to the correct position at any instant and overcome the inductance. Low resistance speaker cables are part of this equation. Adding your 2 Ω series resistance will affect how well the amplifier is able to control the speaker and some loss of fidelity will result.

Try it and see if you notice. The resistors will need to be rated to handle the power. Start with ones rated for at least 1/4 of the nominal amplifier output power rating.

Answer 2

Adding a single, \$2\Omega\$ resistor in series with each speaker will work.

Make sure to get appropriate resistors, they will dissipate one fourth of the maximum power your amplifier can output, per channel. If you have 50W per channel your resistors can dissipate up to 12.5W, so you need to get at least this wattage.

Using series resistors is not something I would recommend though. The best option would be to change the speakers, but this of course come at a price, the new speakers' price. But you can just keep your volume low enough. Keep in mind that adding resistors will let you rise the volume up to the maximum, but you will not hear a difference in actual volume since the excess power is dissipated on the resistors, which usually do not emit sounds.

I am guessing you are using the system with a TV set that includes some 5.1 source: maybe you can rise all the volumes, and regulate your amp volume so that the protection does not blow, then you do not touch this volume anymore and just regulate the source.

The resistor option is the safest anyway, since an unaware person can (and will) stop by, rise the volume and blow the protections.

Answer 3

An impedance matching transformer is better than a resistor in series with your speakers. Although they are pricey.

Paul Speltz for speakers.

http://www.zeroimpedance.com/

For guitar amps Ted Weber

https://www.tedweber.com/z-matcher

Hope this helps

Answer 4

Yes you can add a resistor, BUT.

A loudspeaker's impedance depends on frequency. This is the impedance vs frequency plot of a 2-way bass reflex speaker I just randomly googled:

It is a typical plot for a 2-way bass reflex. The bass driver resonance and bass-reflex vent resonance create two peaks at low frequencies, and the crossover between woofer and tweeter is apparent between 1.5k and 3k. This is a 4 ohms speaker. For a 8 ohms speaker the plot would look the same, scaled up by a factor of 2.

Adding a resistor in series will make a voltage divider (see Transistor's answer for schematic), and the voltage on your speaker will depend on its frequency-dependent impedance. This means the frequency response of the speaker will change. The output will be attenuated most where the speaker's impedance is low, but it will have very little attenuation on frequencies corresponding to impedance peaks. Also, damping will change.

So, it will probably sound different, you'll get a bit more bass. This may be pleasant, or maybe not, depending on the speakers, but don't be surprised if you hear a difference.

So I recommend getting a few cheap 2 ohms 10W resistors and making a quick test.

Note that the volume won't get louder, as the extra power will be dissipated into the resistors.

I want to increase the resistance on my 5 surround sound speakers and my subwoofer by 2 ohms from 6 to 8

Well, you don't mention which amp trips its protection circuit. Most likely the amp just shuts down and doesn't tell you which channel is the culprit.

Usually surround speakers don't get that much power. Most of the power goes into the sub, then the left/right/center speakers.

Also usually, the overwhelming majority of the power goes into bass frequencies. Any decent tweeter will make you deaf with 1 watt, but if you want rumble, you need a whole lot more power.

So I'd recommend setting the volume so the amp's protection trips, and then unplugging the sub and/or the main speakers. Does it still trip?

If the main front speakers still trip the protection, try enabling the highpass feature on the amp to direct all the bass frequencies to the sub. This will reduce the current required by front speakers.

Most likely it will trip with the sub plugged in, and will not trip when it is unplugged. This would mean you don't need resistors... it could be that all you need is to lower the gain on the subwoofer a little bit.

If you're a bass addict, that won't be satisfying though. Then you need an active sub, or a sub with higher efficiency. Resistors will not help.