Can resistors add impedance to a speaker

[john11]

Hi Thanks for reading. I have a 4 ohm sub with a dual voice coil. With both coils connected in parallel the nominal impedance is 2 ohms and the minimum impedance is one ohm.
My amp can handle a 4 ohm nominal speaker, so can probably go down to a minimum impedance of 2 ohms.
Can i place a high power one or two ohm resistor in series with the driver to make sure the impedance is not too low which may help to protect the amp
I was told years ago that resistors add resistance and not impedance, so do not raise the total impedance the amplifier sees, what do you think

 

[Blumlein 88]

Impedance can be reactive, meaning resistance plus capacitance or plus inductance, which at a given frequency has a value in ohms. Resistance is pure resistance and over audio frequencies won't change.

You could add a resistor like you are saying, but it would eat up much of the power, and have to be large enough to handle lots of power. You might get away with it for a tweeter where power levels are generally low. For a sub, just not a good idea. Plus any thing you gain from going in parallel you would more than lose going thru the resistor. So if your other choice is series connection at 8 ohms, that probably is still the better thing to do.

 

[voodooless]

Yes, it will change the impedance, but there are loads of caveats:

- you’ll burn half the power in the resistor, so it will need to be able to dissipate a significant amount of power
- the added resistance will change the speakers response, especially for a subwoofer. Usually this is not a desired effect.

 

[Philbo King]

Probably it would be less strain on the amp, and less wasted power, to hook them in series instead. That would give a nominal 8 Ohm impedance.

Amp_neg ---(neg coil1 pos)---(neg coil2 pos)--Amp_pos

By the way
Impedance = squareroot(R²+X²)

where X is reactance in Ohms due to capacitance and coils.

So resistance (R) has a definite effect on impedance. It always increases it.

 

[john11]

Thanks! I'm a low volume kind of guy, run my stereo set up at about 10 watts/channel, the sub will only be getting a few watts which my amp can easily provide
Because of that, power loss through the resistor, a hot resistor and the other points you raise are not a concern, i'm more concerened about protecting the amp in case of a low impedance problem, trying to increase the impedance for long term stability and am wondering if a resistor can do this,
can a one ohm resistor provide the necessary impedance of approximatly one ohm

 

[voodooless]

If you need only low power, just hook them up serially for an 8 ohm load. Cost nothing, no disadvantages at low volume.

 

[Hayabusa]

If you need only low power, just hook them up serially for an 8 ohm load. Cost nothing, no disadvantages at low volume.

or only one coil? (better speaker control by the amp)

 

[thewas]

Also except the above mentioned problems adding just a serial resistor will significantly reduce the damping factor, possibly even to levels where it might make an audible difference.

 

[Penelinfi]

How about, use just one coil. It would act like a woofer with a slightly heavier moving mass.

 

[john11]

Thanks for the replies, much appreciated.
Another reason for the question is out of curiosity. I had an amp a few years ago that only accepted 8 ohm speakers, my speakers at the time were 4 ohm.
I hooked up a resistor in series with each speaker to get the impedance up to 8 ohms, but the amp still blew
At the repair shop I told the techie and he said the amp wants impedance, not resistance, that's the reason the amp blew because the impedance was too low, resistors do not add impedance.
It's bothered me ever since because applying my mediocre electronics knowledge I thought resistors provide resistance in a dc circuit, but in an ac circuit it is exactly the same thing but it called impedance. I didn't challenge the guy, I thought since he is a techie he probably knows best.

 

[DonH56]

Impedance is a complex number, with a real part plus an imaginary part. Pure resistance is a real number so adding a resistor does increase the impedance. Either the techie misunderstood, you misunderstood what he said, or he was having a bad day and got it wrong.

Impedance Z = R + jI where R is real, j = sqrt(-1), and I is imaginary.

Pure resistance is R. Capacitance puts a negative sign before j (so the imaginary part is -j) and inductance is positive (+j).

The impedance of an ideal capacitor or resistor is related to frequency: Zcap = 1/(2*pi*f*C) and Zind = 2*pi*f*L where pi = 3.14159... and f = frequency in Hz. Real components all have a little R, L, and C (plus conductance, G) but the ideal values are usually close enough for audio.

Note you cannot just add terms to get the magnitude of a complex number; you must do some math: Zmag = sqrt(R^2 + I^2).

HTH - Don

 

[Penelinfi]

Not sure what amp would blow just because you used 4 ohm unless it was running at high volume.

 

[samsa]

I don't understand what you are trying to achieve here.

A resistor in series with your speaker implements a voltage divider. To get the same volume of output from the speaker, you'll have to turn up the gain on your amp (because some of the output is "wasted" -- dissipated silently as heat in the resistor). That manifestly makes your problem worse, not better,

As suggested above, hooking up the voice coils in series, rather than in parallel, will raise the nominal impedance to 8Ω (whereas, in parallel, it is lowered to 2Ω). Perhaps that's what you want.

 

[john11]

Thanks!
To continue the subject, and this is a hypothetical question, I’ve never liked the sound of overly complex crossovers. There is some corroboration for this on the net with people reporting similar views that big inductors take away something from the sound, crossovers can introduce crossover distortion, phase reversal, ringing etc.
Simple crossovers with small inductors sound better. As an example, for a given crossover frequency an 8 ohm driver would require a 4mh inductor, a 4 0hm driver would require a 2mh inductor, a 2 ohm driver would require a 1mh inductor. When using low impedance drivers smaller and simpler crossovers can be implemented. Would there be any improvements in sound quality by using simple crossovers with very low impedance drivers, then placing a 2 ohm resistor in series with the driver to make sure the impedance does not go too low.
Sorry, I hope this does not sound like a mad question

 

[unpluggged]

Would there be any improvements in sound quality by using simple crossovers with very low impedance drivers, then placing a 2 ohm resistor in series with the driver to make sure the impedance does not go too low.

This problem, how ever it is real, is long solved by active crossovers.

 

[john11]

This problem, how ever it is real, is long solved by active crossovers.

Would you say active crossovers is the way to go, are they better than passives. would require more amplifiers and more expense

 

[DonH56]

Thanks!
To continue the subject, and this is a hypothetical question, I’ve never liked the sound of overly complex crossovers. There is some corroboration for this on the net with people reporting similar views that big inductors take away something from the sound, crossovers can introduce crossover distortion, phase reversal, ringing etc.
Simple crossovers with small inductors sound better. As an example, for a given crossover frequency an 8 ohm driver would require a 4mh inductor, a 4 0hm driver would require a 2mh inductor, a 2 ohm driver would require a 1mh inductor. When using low impedance drivers smaller and simpler crossovers can be implemented. Would there be any improvements in sound quality by using simple crossovers with very low impedance drivers, then placing a 2 ohm resistor in series with the driver to make sure the impedance does not go too low.
Sorry, I hope this does not sound like a mad question

There's a lot "corroboration" on the net that is completely wrong. Any real filter adds phase shift and the potential for other artifacts, but the purpose is to make the drivers work together better. A more complex crossover can improve the overall performance of the speaker. A simple crossover provides little attenuation either side of the crossover frequency, putting more stress on the drivers to operate linearly outside their design range, and means you cannot correct overall system response issues due to driver placement in the box, baffle step response, etc. using the crossover but must have some external means. Or just live with the problems.

BTW, crossovers have continuous phase, there is no "phase reversal" although at various frequencies the phase may be a factor of 180 degrees differnt than the input, leading to polarity reversal. The big jumps in phase plots from say -180 to +180 degrees are due to wrapping in the plotting routines to constrain the axes to +/-180 degrees for easier viewing. I have had folk (not you) argue those jumps represent discontinuities that are very audible. This is easily disproved mathematically and in listening tests.

Ringing can occur in any circuit, including a digital filter, an din simple first-order crossovers as well as the speaker itself. The argument against inductors seems rather common around the 'net, with the anti-inductor crowd conveniently (or unknowingly) forgetting that the voice coil is itself a large inductor.

If you add resistance you reduce damping, effectively increasing the driving point impedance seen by the drivers. That usually leads to higher distortion and poorer frequency response as the drivers are decoupled from the amplifier and interaction among drivers increases (they can modulate each other's output, leading to additional distortion).

Some of the best measuring and sounding speakers have fairly complex crossovers, like my Salon2's (OK, I admit bias, but there are many other examples). And some of the most highly-regarded speakers like most ESLs have a great big inductor (transformer) in the signal path.

HTH - Don

 

[unpluggged]

Would you say active crossovers is the way to go, are they better than passives. would require more amplifiers and more expense

Just look how well do even the most affordable active speakers measure in comparison to passives of similar design.

 

[DWPress]

This problem, how ever it is real, is long solved by active crossovers.

Active XO is definitely the ultimate way to go but wouldn't solve the OP's problem of 2ohm load presented to an amp.

I used a bridged Crown XLS for awhile for sub duty forgetting my sub was 4ohm so the fans came on constantly as the Crown didn't like 4ohms while bridged. I wired a huge 300 watt resister with the sub load and it did ease the strain on the amp but it's just wasted power so eventually I replaced the Crown with something more appropriate.

 

[dlaloum]

One key thing that people seem to have missed here, is that there are a heck of a lot of amps that aren't stable into low impedance loads - the distortion rises substantially - so sometimes, it isn't an issue of power wasted, but of shifting the amp into its "stable" impedance zone, and thereby providing improved performance.... - so yes there could be benefits from putting a simple resistor into the circuit... (and yes it will waste power, and it will require a "beefy" resistor)