Increase total Speaker Impedance Question

Hi All,

I got a tech question. I have built a pair of speaker using a unique crossover for each speaker. Based on my test, if I parallel wire them, the resulting frequencies are correct, but resulting frequencies are NOT correct when I tried to wire several speakers as a series (series-parallel wiring). I suspect that because series wiring requires daisy chaining two or more crossovers, the resulting frequencies are messed up. However, if I retain current parallel wiring, the total impedance of the speaker system is too low and the amplifier often goes into protection mode. Is there anyway to add a resistor in between the amp and the speaker to rectify the situation? And if I do add a resistor, will that affect the sound quality? Thanks!!
If you combine four speakers by connecting two in parallel and two in series, you will end up with the total impedance equaling the impedance of a single speaker. You can see this by using this calculator and selecting "2 series and 2 parallel" and entering the correct impedance values. If you added a series resistor to increase impedance, it would have to be a power resistor able to dissipate the wasted power it would use in heat, and it would affect the sound. Hope that this helps!
Good comments by Bill, as always.

I'll add that connecting speakers in series will usually result in poor sonics unless the speakers are identical models. The voltage and power put out by the amplifier will divide up between series connected speakers based on the relation between their impedances. If the two speakers are identical the voltage and power put out by the amplifier at any instant of time will divide up equally between them. If they are not identical models, the relation between their impedances will almost always be significantly different at different frequencies, resulting in the voltage and power put out by the amplifier dividing up between them differently at different frequencies, resulting in arbitrary frequency response irregularities in each speaker, as you found.

-- Al
In my response above I was assuming that the four speakers each have identical impedance values. Thanks to Al for kindly confirming my input, as I consider him to be the true authority on any technical issues.
Thank you all for your response and help. Yes, all of the speakers are different models and hence each have its own unique crossover that I ordered from Parts Express. As a results the only way to make it sonically plausible is to wire them in a parallel. However, the current parallel wiring has reduced the total Ohm to 1.6, too dangerous for the amplifier to handle. I have ordered two 3 ohm wirewound resistor rated at 200 watt. It will be placed between the receiver and the speakers. Would that seriously damage sound quality? I'll try it either way and let you know how it turns out. Thanks!!
By the way the receiver is rated at 125 watt each channel, so 200 watt resistor should be sufficient?
The 200 watt power rating of those resistors should be sufficient, but you will then have 3/4.16 or ~72% of your available power being dissipated by the resistors. Only the balance, ~28% of the total power will be available to drive the speakers.
Bill, thanks very much for the nice words. I always find your posts to be particularly knowledgeable, informative, and constructive.

Angelgz2, yes the resistors will certainly affect the sound quality, and most likely for the worse, but the magnitude and character of the effects will depend on the impedance characteristics, damping requirements, and other characteristics of the speakers. If the design of the speakers is such that their impedance and other characteristics are relatively "tube friendly" (for example, if their impedance magnitude doesn't vary greatly as a function of frequency, and if their impedance phase angles are not highly capacitive), their sonics will probably suffer less than they would otherwise.

Also, Bill is right that only a fraction of the amplifier's power capability will be available to the speakers, with the rest being dissipated in the resistors. And his calculation resulted in roughly the right answer, although I think the methodology of the calculation was not quite right.

You didn't say whether the 125 watt rating is for 4 ohms or 8 ohms, but let's assume it is for 4 ohms. And I assume that the receiver is solid state rather than tube-based, at least in its output stage, so that its output impedance can be assumed to be negligible. The receiver would probably be capable of delivering slightly less power than the 125 watts into (3 + 1.6) = 4.6 ohms, but let's ignore that slight difference. 125 watts into 4 ohms corresponds to an output voltage from the amplifier of (square root (125 x 4)) = 22.36 volts. The voltage that would appear across the paralleled speakers under that max power condition would be ((1.6/4.6) x 22.36) = 7.78 volts. That voltage across 1.6 ohms corresponds to (7.78 squared)/1.6 = 37.8 watts. 37.8/125 = 30.2%, close to the bottom line Bill stated.

Good luck. Regards,
-- Al
P.S. to my previous post. Another calculation method, which gives the same 37.8 watt answer but is somewhat simpler:

For many solid state amps, especially those for which the 4 ohm power rating is equal to or close to twice the 8 ohm power rating, the power capability into (1.6 + 3) = 4.6 ohms can be approximated as (4/4.6) x the 4 ohm rating.

125 watts x (4/4.6) = 108.7 watts.

Max power into the 1.6 ohm speaker combination, when in series with a 3 ohm resistor, would be:

108.7 x (1.6/4.6) = 37.8 watts.

-- Al
Instead of killing useful power and also useful bandwidth per given load, I'd use single pair of speakers replacing quantity by quality. Another option is to use zone preamp and drive each pair of speakers with separate identical stereo amp.
Hi Bill and Almarg,

Thanks again for your detailed explanation. I understand the power loss in this setup. However, I am still having difficulty grasping why the sound quality would also take a turn for the worse. Will the frequencies be improperly passed or cut off by the resistor? If not, then, all frequencies will still be passed to the speakers, but at lower power, and that shouldn't affect the sound at all theoretically isn't it?

I did make an error in my initial calculations above resulting in the discrepancy, and Al's math is of course correct. I believe that the primary difference in sound quality will be due to a loss of damping factor from the amp when driving the additional series resistance. The only way to really know how significant the difference will be is to try it with your specific setup. I can't predict it beyond that with any real degree of confidence.
Angelgz2, see this Wikipedia writeup on the voltage divider effect. In Figure 1 consider Vin to be the voltage the amplifier (or receiver) is "trying" to output to the speakers at any given instant. Consider Z1 to be the sum of the amplifier's output impedance and the 3 ohm series resistance. For nearly all solid state amplifiers the output impedance will be negligible, and so Z1 will be essentially 3 ohms. Consider Z2 to be the combined impedance of the paralleled speakers (nominally 1.6 ohms), and Vout to be the voltage applied to the speakers.

As you will see in the writeup, the voltage applied to the speakers will be proportional to Z2/(Z1 + Z2). Since Z2, the combined impedance of the paralleled speakers, will inevitably vary significantly as a function of frequency (to a greater or lesser extent depending on the particular speakers), Vout will vary significantly as a function of frequency. The result will be significant frequency response irregularities.

That will be similar to what happens, as we discussed earlier, when two non-identical speakers are connected in series, except that it will probably not occur to as severe a degree in this situation since Z1 is constant.

Also, whatever damping factor the amplifier may provide will be reduced to the rough vicinity of 4/3 = 1.33 for 4 ohm speakers that may be included in the combination of speakers, or 8/3 = 2.67 for 8 ohm speakers that may be included in the combination of speakers. (The reason I'm saying "the rough vicinity of" is that I'm ignoring the damping effect of each speaker on the other speakers). Those are very low damping factors. Chances are the result will be loose and flabby bass, to a greater or lesser degree depending on how critical woofer damping is for the particular speakers.

Finally, to the extent that woofer damping is degraded by the presence of the resistor it seems conceivable that "back emf" generated by the woofer of each speaker in the parallel combination, instead of being effectively damped by the amplifier's low output impedance, could to some degree enter the other speakers and be reproduced as sound, thereby adding distortion to the sound each speaker should be reproducing.

-- Al
Thanks again Bill and Al,

So in short, there's no way to rectify my current situation with a resistor. Is there possible to redesign a 4 way crossover so that the speakers can be wired as a series or series-parallel? Or, as a last resort, to use separate amplifiers for each channel.....
So in short, there's no way to rectify my current situation with a resistor.
No, we haven't said that. Despite all the potential issues we've cited, using the resistor with the paralleled speakers could conceivably still result in acceptable sonics, and as Bill indicated it does seem to be worth trying.
Is there possible to redesign a 4 way crossover so that the speakers can be wired as a series or series-parallel?
Not sure exactly what this means, but I think you had indicated that all of the speakers that would be used in each channel were different designs, so connecting any two of them in series is unlikely to result in good sonics.
Or, as a last resort, to use separate amplifiers for each channel.....
If you mean using separate amplifiers for each speaker, or perhaps for each parallel combination of two speakers, yes, you could of course do that, as Czarivey indicated earlier. You'd have to make sure that whatever is driving the multiple amplifiers, such as a preamp, or pre-out connections of the receiver if it provides them, would have a low enough output impedance to be able to properly drive the combined (paralleled) input impedances of the amps. Although with most solid state preamps that is unlikely to be a problem.

-- Al
Hi Al and Bill,

Thanks again for all your inputs. Once the resistors arrive, I'll let you know how it goes.

A challenge with using multiple amps for a speaker is that the pre-out connection on the pre-amp cannot be split. If I use a splitter, i.e. a Y analogue splitter, the sound becomes super muddy and awkward. I would need a device that has a single channel input but have several binding posts.

I'm thinking something like the Henry Engineering PatchBox II Stereo Output Multiplier may work. It's got a balanced input with multiple balanced and unbalanced outputs. If that works it actually solves all the problems.
Look up Speltz Zero Autoformer. It's like a gearbox for your system, allowing you to step down the amp output impedance (or step up the speaker input impedance, depending on your viewpoint) without wasting all that power as heat.
Hi Bill and Al,

I would like to thank you for all your invaluable inputs. After thorough testing, I cannot tell a difference (besides loudness) between using a 3 ohm resistor vs splitting the speakers (tri-wiring). Therefore, I would like to say the idea of adding a resistor is a success. Although like you said, there maybe theoretical sonic differences, such difference is probably too small at this level to be perceived by my ears. As I don't have such good ears, I will settle with a resistor in the middle. Again, Thanks for everyone's great inputs.

Ait, thanks for letting me know the Speltz Zero Autoformer as I never knew such stuff exists. However, at $1140 a pair, I am happy to settle with a $26 pair of US-made resistors. When my boss gives me a raise in the future, I'll definitely consider them : )

You can get them for less than half that price without the box (naked).

In any case, glad to hear you found a solution.
I use the naked auto transformers. They are great. With luck you may be able to get a used pair for less than $300.