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The Loudspeaker Crossover Part II: The Brains of your System

by September 30, 2009
Contributors:
Madisound Kit

Madisound Kit

Do Better Quality Parts Really Matter? 

In part one of "Crossover, Brain of your speaker system", we introduced the concepts of inductance, capacitance, and resistance. We then examined how these three basic passive elements relate and combine to create frequency selective networks called High pass and Low pass sections, the building blocks of the crossover network.   We also considered in part one, the effect of real loudspeaker impedance, and how, unlike a resistor, its amplitude and phase vary with frequency to complicate and frustrate the function of constant resistance type crossover networks.  These real loudspeaker impedance variations result in frequency and phase responses which end up being very different than what our textbook equations would have us expect, because they assume a speaker behaves like a simple resistor.  We also made the assumption that the parts used in our crossover networks were theoretically perfect and without flaws. In part two, we will discuss how in the real world, capacitors, inductors and resistors exhibit behavior which is neither ideal nor perfect.  We will determine if better quality parts truly yields better performance.

Recent Crossover Design

Some of my more recent efforts


Real world parts, the kind you will actually find in your own crossovers, suffer from many flaws.  In part two, we will discuss and illustrate the effects of some of these.  We will also examine how simple mistakes, like the physical orientation and location of inductors on the crossover board can result in non-ideal behavior like cross-talk.  This article will allow the reader to gain some insight into the kinds of mistakes made by amateur and professional crossover designers alike, and allow us to recognize compromises in crossovers by simply looking at the networks.  We also hope to gain some understanding into flaws which are not quite so easy to see with the naked eye.  While this article is not going to be an exhaustive study of crossover component parts, it will touch on most of the major flaws present in the three basic components used in all real world crossovers, resistors, capacitors, and inductors.

I am hopeful this light shed on crossover networks will make you all better and somewhat more cynical consumers, ones who understand the importance of the passive crossover parts used in their speaker system.  Reading some of the more ardent audiophile press, one can be left with the opinion that there is all sort of magic going on in this network.  In fact the enemies of these passive components are basically the same as the enemies of all electronic parts; hysteresis, loss, tolerance, insufficient power handling capacity,  insufficient space, and compromises made on behalf of cost.

Resistors & Tolerances

Let’s start by considering the simplest of the three electrical components used in our crossover, the resistor.   It will, in combination with inductors and capacitors create time constants used in frequency selective circuits, although by itself the resistor does nothing other than to consume power.  In a crossover network, resistors are usually used in combination with other components to control either impedance magnitudes or the relative levels between different drivers in a system.  Resistors are most often used in "padding" a tweeter which is more efficient than the woofer, so the overall system frequency response will be flat.  The resistor, in series or parallel with capacitors and/or inductors, is often used as part of a Zobel or impedance compensation network. 

 LCR Meter

A good Meter is the best way to "Trust, but verify"

 Of all flaws with which we must deal, the simplest to understand is tolerance; the allowable variation of the components value, whether that component is a resistor, inductor, or capacitor.  No surprises here, everyone can understand how a part with a small 1% tolerance will lead to a more uniform and reliable frequency or amplitude response performance than a part with a 10% tolerance.  The tolerance issue, while seeming obvious, becomes more critical as we increase the order of the network.   Remember, a first order network has one part with tolerance, while a third order network is going to have 3 which vary with tolerance.  It is for this reason that the higher order the network, the greater the need for a tight component tolerance.  Said another way, for a given amount of  allowable variation in response, (plus or minus 1 db for example), a second order network requires tighter tolerances from its components than does a first order network, and a third order network requires tighter tolerances than a second order network.  As we increase the complexity (order)of the network, the sensitivity of the network to component tolerance increases.  So, as we increase the network order, not only do we add additional parts, for a given crossover frequency, we require both larger size (value) components and tighter tolerance in those components in order to keep the frequency response window tolerance the same as the simpler network.  This is often a hidden and un-calculated cost in using higher order networks.  This exponential rise in part size and cost should explain why crossover networks are almost never found in complexity above fourth order.


Resistors normally deviate from their design values within a window of anywhere from 0.1% to 1%.  If you buy a 5% 10 ohm resistor from an electronic store, you might go back there complaining you measured it with your meter and found is was only 9.5 ohms, but if you get a refund it is because they are hoping you don't return to buy more stuff.  You will find neither the highest or lowest tolerance parts in most crossover networks, as the typical tolerance specification is either 10% or 5 %.  The letters (K) and (J) on the part will indicate if it is 10% or 5% respectively.  The effect of this variation is one of magnitude and is important to hold close enough so that there is not much variation from one speaker system to the next.  Lets consider an example. 


We have an 8 ohm tweeter which is 6 db hotter than the woofer in the system.  If we put an 8 ohm resistor in series with the tweeter, the combination of the 8 ohm series resistor and the 8 ohm tweeter presents 16 ohms as a load to the amplifier.  Since power = V2 /R and since we have doubled R, we have halved the power the entire network (resistor plus speaker) consumes from the amplifier.  The amp is delivering 1/2 the current to the loudspeaker load.  Now half of the power that is delivered is consumed in the series resistor, the 8 ohm resistor in series with the tweeter.  So, we have cut the power in half twice, and therefore get (- 3 db) + (- 3 db) = ( 6 db) attenuation.   Let's say we pick a 10% tolerance for our resistor.  This generally defines a 20% allowed variance, since the specification is +/- 10%.  We are making a stereo pair, and the two resistors we use are 8.8 ohms, and 7.2 ohms respectively, both within our 10% tolerance window.  In the first case, (8.8 ohms in series) we attenuate 6.44 db, and in the second case (7.2 ohms) we attenuate the signal 5.57 db.  This means we have a mismatch between our pair of 0.87 db, and a definitely audible difference.  This same magnitude variation (tolerance) when the part is used in conjunction with a capacitance or inductance will also cause a shift in the frequency corner of the network.


Another very well documented issue with resistors is inductance.  While high impedance, small wattage resistors are most often made from a metal film, higher wattage parts of low impedances (the kind most likely to be used in crossover networks) are often wire-wound parts. (If you have never noticed before, the symbol for the resistor is a bunch of wire scrunched up).  Some old wire-wound types have a inductance high enough to cause issues in a crossover at very high frequencies.  You may often find wire-wound resistors being referred to as "non inductive" to let the buyer or engineer know these parts have eliminated this potential flaw.  Modern day parts are often wound with a serpentine pattern so the windings have self canceling inductance without having an effect on their intrinsic resistance. 


The largest real world problem you will run into with resistors, is a universal problem for all components, heat sensitivity.  As we read the specifications for any component, we must bear in mind that these specifications are only met within a certain allowable window of temperature. 

American Resistor Bank

An American Made Resistor Bank - 3 times 300 Watts

Temperature dependence of resistance

The electrical  resistance of a metal is approximately proportional to its temperature over a limited range.  Resistivity of materials is usually specified at normal room temperature, 20 degrees C (68 degrees F).  If one knows the resistivity of the material at room temperature, and the rate at which it changes, we can calculate the resistivity at other temperatures with the following formula:

Rcalc

Where:

R = Resistance at temperature T

Ro = Resistance at temperature To 

To = Temperature at Reference T (usually 20 degrees C)

alpha (the Greek letter with the bracket and outside parenthesis) = Percentage change in Resistivity per unit temperature

Let’s work one example.  Suppose we put an 8 ohm resistor into a crossover network, and use it to drop

the sensitivity of the tweeter so it will match the woofer.  Lets say we are driving the speaker system pretty hard, and we are heating the resistors so that they rise to be 200 degrees F, (a not uncommon operating temperature for a resistor in a crossover network).   200 degrees F is equal to 93.33 degrees Celsius. (Celsius and Centigrade are the same).  If the resistor is wound from Copper wire, the temperature coefficient would be (3.9 * 10-3)) /  deg C.  Since the resistor was 8 ohms at 20 degrees C, and has now heated up to 93 degrees C, the new value of resistance would be:

8 * [(3.9 * 10-3 / oC(93.33C - 20C) + 1] = 1.286 * 8 = 10.288 Ohms

We can plainly see this increase of 25% is more significant than the component tolerance of 10%.  Suppose we do not use copper in the wire-wound resistor.  If we use a material called Nichrome, which is basically an alloy of nickel and chromium, our change in resistance will be considerably less.  Nichrome has a low resistance variation (alpha) with temperature, alpha = 0.4 e-3/ (deg C)  Using this material to make the resistor gets us an eventual resistance of 8.23 ohms at the elevated temperature of 93 degrees C.  This is an increase of only 3%.  This is no doubt, a small increase compared to the change of resistance in the voice coil which is either copper, aluminum, or a combination of those two. 

As the power dissipated in a resistor increases, so does its temperature. As the temperature increases, so does the parts resistance (as illustrated above). As the resistor is heated, its ability to absorb power is compromised, and its value in the circuit is not as designed. That resistors get hot, and can burn out is a well known phenomena. What is not as well known is that running high power into resistors at or near their limits brings with it audible effects on your music.

Since the resistor manufacturer has no control over how the part is mounted to the printed circuit board (PCB), or its orientation, proximity to other parts on the PCB, they cannot predict with accuracy at what point the power through the circuit will cause the resistor to go outside of its allowable range.  Take a look at the resistor in the photographs below:

R mounted PCB

The 20 ohm resistor shown above (white rectangular part) is mounted directly to the printed circuit board (PCB) which is an effective insulator of heat.  The path for the heat generated inside the resistor to escape has lost 1 of 4 large sides. Now lets take a look at another PCB which has eliminated this issue.

Tall Resistor

On this PCB we see the terminals of the resistor are designed to stand the part off the board.  This has the disadvantage of making the part taller, but the advantage of creating space between the hot resistor (whose only job is to dissipate power) and the PCB (whose only job is to connect the different components without routing the heat from one part to another). With the resistor mounted off the PCB, the hot air can circulate around the part more easily, dissipating the resistors power more efficiently.


Suppose both of these parts shown above are 20 watt resistors.  Which one burns out first?  Now suppose one of the boards is mounted to the bottom of the cabinet so the heat from the resistor can rise into the entire cabinet volume, and escape through a nearby port.  Suppose the other PCB is mounted upside down on the top of the cabinet, inside a sealed speaker box, and under the cabinet stuffing which is fiberglass?  Although both parts are 20 watt resistors, the one in the heat containing environment is going to burn out faster than the one with good ventilation when it needs to handle all 20 watts.  Power ratings on resistors are NOT independent of the way in which they are mounted to a PCB.

 

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Recent Forum Posts:

audioantique posts on June 05, 2010 14:56
More series crossover

Paul,

I wasn't able to post my series links because my post count is only one since I just joined. Too bad. I've got better things to do than be malicious. Let me know and I'll email them to you if you'd like.
Quite frankly with my latest project the crossover sounded great first try. After experimenting with parallel designs for decades, I find the series much easier to implement and test, with only four components for a three-way and two for a two-way. I've even mixed Zeta .5 and Zeta .7 with good results. I personally shy away from Zeta 1 or lower for the same reason as a parallel slow slope, too much overlap, and most drivers don't have that wide a response to handle it. All my testing is by ear with difficult program material, voice, piano, instrumental combos and percussion, mainly classical and jazz. I just forge ahead right or wrong. It all started when I inherited a pair of Dahlquist DQ-10s. One look at that crossover schematic convinced me that almost anything goes, and they sound great. It works for me, and after all, serendipity rules. Shed those prejudices! Hope this is all helpful.
TLS Guy posts on June 05, 2010 10:35
highfigh, post: 723200
Do you have any links to info on these Zeta crossovers?

Here is a circuit of a series crossover.



Series crossovers present a lot of formidable problems, as both sections of the crossover interact.

My rear backs, use series crossover in the passive part of the crossover. These speakers started in 1984, but the crossover did not get to its final form until 1994!

They are very difficult to perfect.

This was a good article, and reiterates points I have made many times over. The sad fact is most commercial designs do use miserable chokes with iron cores and wire of too thin a gauge, and electrolytic caps abound. A speaker is then severely compromised right out of the stating gate then, and I mean severely compromised. A speaker with decent components in the crossover will not be cheap. This leaves money on the table, for active speakers, which I believe with modern production methods, could give much better performance per dollar.

I really believe it is the receiver obsession that is so limiting. If you think about it, it is absurd that a pre pro generally cost more than a receiver. The only reason is production numbers. Burying the receiver is long, long over due now.
highfigh posts on June 05, 2010 09:53
audioantique, post: 723194
Paul,
Very informative and straight to the point, thanks. I suggest that a way to bypass the inductor series losses is to build a series crossover, where there are no components in series with the drivers. One exception to this would be a padding network for a tweeter, but that is only series and parallel resistors. I realize that this solution isn't for everyone, but I've had great success lately with Zeta .7 quasi-second order crossovers. A 3-way is only two caps and two chokes all parallel to the drivers, and the, for me, essential Zobel for the midrange is also parallel to the driver. Of course just like with a parallel crossover, it's essential to use drivers in their usable range and not push their envelopes. I'm not a series crossover zealot, but extensive listening seems to validate my efforts, at least to my ears.
Good luck, all.

Do you have any links to info on these Zeta crossovers?
audioantique posts on June 05, 2010 09:15
Bypass inductor resistances

Paul,
Very informative and straight to the point, thanks. I suggest that a way to bypass the inductor series losses is to build a series crossover, where there are no components in series with the drivers. One exception to this would be a padding network for a tweeter, but that is only series and parallel resistors. I realize that this solution isn't for everyone, but I've had great success lately with Zeta .7 quasi-second order crossovers. A 3-way is only two caps and two chokes all parallel to the drivers, and the, for me, essential Zobel for the midrange is also parallel to the driver. Of course just like with a parallel crossover, it's essential to use drivers in their usable range and not push their envelopes. I'm not a series crossover zealot, but extensive listening seems to validate my efforts, at least to my ears.
Good luck, all.
skers_54 posts on September 30, 2009 18:31
Paul, you did a great job of demonstrating inductor saturation at high drive levels. How would the effect look when the amplifier is providing power on the order of 1 watt? Is it possible to extrapolate or is there too much variance between different magnetic core inductors to generalize when they begin to saturate?
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