CrossOver Series vs Parallel

7/24/2019 CrossOver Series vs Parallel

http://slidepdf.com/reader/full/crossover-series-vs-parallel 1/12

Series vs. Parallel Crossover Networks

Introduction

Despite many of the myths that surround series networks and their acclaimed superiorityover conventional parallel networks for loudspeaker design, both networks can be designedwith identical transfer functions if the load impedance remains constant. Most of theclaims regarding series networks are either grossly overstated or blatantly wrong and maycause deleterious e e c t s on s y s t e m pe r f o r m a n c e . As with a l l a s p e c t s o f d e s i g n , t h e r e a r e compromises that must be made, and it is impossible to make aninformed decision if unaware of the facts.

There are no greatly enhanced features in a series or parallel network - if properlydesigned their performance is essentially identical in terms of response, phase and bye!tension" transient response. #t is unwise to claim that one type of network is superior tothe other, when simple logic dictates that if amplitude and phase response are the same,then all of the filter$s other characteristics are also the same.

There are other factors than %ust the response, and this is where the differences betweenthe network topologies e!ist. &ach has good and bad points that must be considered.

First Order Comparison

'irst order (d)*octave" networks have a strong following amongst many audiophiles, andindeed, they have a number of very desirable features. They have the best possibletransient response, and are predictable and easy to design, but as with all things there isa down side.

The demands on the drivers are e!treme, with significant power delivered to the tweetereven at its resonant fre+uency, and the risk of cone breakup and o-a!is lobbing for the

mid-woofer.

evertheless, at low power, inter-modulation products can be kept within reasonablelimits with careful driver selection, and they can sound very good indeed.

Series and Parallel 1st Order Filters

#llustrated above are e+uivalent series and parallel first order crossovers with k/crossover points for a fi!ed load. ote that resistive loads were used in order to minimi/eanalysis variables. k/ was chosen for one reason - the crossover fre+uency is nicely



centered in the graphs for best display.#nput impedance is e!actly the same for each type, and is essentially perfectly flat, withboth circuits dropping by 0 milliohms at the crossover fre+uency. This is of no conse+uence,and may be ignored.

Response

The fre+uency response and electrically summed outputs are shown in 'igure .0 and it is+uite obvious that they are identical1 since the graphs are perfectly overplayed there are (graphs on the chart, not 2".

Phase Response The phase response of the series and parallel crossovers are also identical as can be seenabove. There are 3 graphs 0 serial and 0 parallel" and again, they are perfectly aligned.

Impedance Variations

'igure .3 shows the variation of high and low pass 4lters and summed response when thewoofer impedance is varied by 5*-0 ohms. 6ed shows the electrical sum of the variation with(7 impedance, and the green graph is for 87. ote that only the low pass filter response isaected.



Parallel, Variale !oo"er Impedance The results for tweeter impedance variations are similar and aect only the tweeter sectionof the 4lter", but have not been shown, since the tweeter is far less likely to undergo anynoticeable change than the woofer.

The graphs below show the woofer impedance was changed from (7 to 87 as was donewith the parallel network. ote that although the crossover fre+uency moves it becomeshigher at higher woofer impedances and vice versa"1 the summed response remainscompletely 9at.

Series, Variale !oo"er Impedance

The two sections have a complementary shift - when woofer impedance changes, iteffects both low and high pass sections, and changes the : of the filter sections. Theresult is +uite obvious - unlike a parallel crossover, the response remains flat

regardless of a shift in the woofer or tweeter" impedance. #f both change in anydirection, the same thing happens. #n theory, this means that the series network is almostimmune from impedance variations in the drivers.

Series, #$%!oo"er, &% 'weeter

)y changing the driver impedances, two things happen. The filter : changes and thereflected change aect the behavior of the other filter section. Although the individualresponse, : and phase varies, the net result is that the eective crossover fre+uency ischanged, but nothing more. This is a remarkable property, and the series first order is the



only crossover filter circuit that has this ability.

6emarkable though it may be it is still advisable to design the series network correctly,and maintain everything as close as possible to the design values. ;hould the wooferimpedance increase with voice coil temperature , for e!ample", the crossover fre+uencywill move upwards, thus providing a small measure of added protection for the tweeter at

sustained high power levels.

owever, all is not completely rosy. &verything in electronics is a compromise, and theselection of a crossover is no dierent. There is one final test that needs to be applied,and that is to e!amine the amount of woofer back &M' that reaches the tweeter. This is anarea where the series network is inferior to the parallel.

Series, !oo"er (ack )*F +ttenuation

<ith a parallel network, only the amplifier$s output impedance plus the impedance of thecable allows any cross coupling between high and low pass sections. <ith a /ero ohmsource, attenuation is infinite, and is not shown above.

A series network relies solely on the isolation of the crossover filters, and as a result, theback &M' from the woofer is not attenuated as well. This may not be a ma%or problem,since the attenuation of back &M' is the same as for amplifier power actually, it is 2d)better", and the latter is at a far greater amplitude. #t is a consideration nevertheless, sobe aware that it may increase tweeter inter-modulation.

Summar

The series network is probably a better choice than parallel for a number of reasons. #tretains a flat response even when the driver characteristics change, and is to an e!tent=self-correcting=. #mplementation is no more difficult than for an e+uivalent parallelnetwork, and the same component values are used.

>n the negative side, woofer back &M' suppression is significantly worse than with a

parallel network - it is up to the designer to determine if this is likely to cause a problem.

'inally, it must be remembered that any first order network dictates that the drivers willhave significant power applied at fre+uencies where their performance will be rapidlydeteriorating, however for a system that will never be operated at high power1 theperformance can be very satisfying.

Second Order Comparison



The design process for a 0d)*octave filter is completely different for series andparallel implementations of the same design. 'or a parallel network assuming a)utterworth alignment", the capacitance and inductance are calculated by ?

C - 1 /# 0 0 " 0 /2 0 3#44 5 - /2 0 3#4 /# 0 0 "4

<here @ is impedance, f is fre+uency, 0 is B .33, and C is B 2.3E"A series crossover design is dierent in terms of the component values ?

C - 1 /# 0 0 " 0 /2 3#44 5 - /2 3#4 /# 0 0 "4

'or this e!ercise, the crossover fre+uency was arbitrarily selected to be k/, and F ohmresistive loads were used. The series network has the advantage of using smallerinductance values, but capacitor values are higher. The dierence is unimportant, butcapacitors for crossovers are more e!pensive than inductors. This is a minor point if thereis an improvement in performance.

The values used for the simulations were as follows ?

Common Values ParallelCrossover SeriesCrossover

Grossover're+uency k/ G H 3.8Iu' G H 0F.2 u'

;peaker#mpedance F 7 J H .F m J H E88 u

Second Order Crossover Value

Series and Parallel #nd Order Filters

Response

As with the previous e!ample using a 4rst order 4lter, when properly aligned, the response isidentical. )ecause the plots look e!actly the same as the previous e!ample other than theroll o slope", there is little point displaying graphs that show two sets of curves that areperfectly matched.

#t can be stated that if two 4lters, regardless of topology series, parallel, active or passive"have an identical fre+uency response, then they must also have identical phase and impulse



responses, since these cannot be separated.

>f course, this only holds true as long as the source and load impedances are also identical.#nput impedance of both 4lters is essentially completely 9at, having a variation of only 3.(md) i.e. 8.883( d)". Due to rounding errors in the component values, there is a tinyvariance between the two 4lters1 however it is completely insigni4cant about 8.I /

dierence".

>ne thing that should not be overlooked is the inductor$s resistance. <hile this causes asmall loss of level with a parallel crossover network KK, it will cause the series network to=shelve= the tweeter roll off. As a result, a DG resistance of say" F88m7 will cause the signalapplied to an F7 tweeter to drop to a minimum of %ust over 08d) below the applied signalregardless of fre+uencyL This includes DG under ampli4er fault conditions.

There is virtually no dierence between series and parallel at about decade belowcrossover i.e. *8th the fre+uency", but below that the dierence becomes apparent.

There may be as much as 08d) more level applied to the tweeter at 08/ with aseries crossover vs an otherwise identical parallel version with an inductor DG6 of 8.F7"

KK Although there is a small loss of level, the parallel crossover$s theoretical response isgreatly disturbed by even a 8.F DG6 in the inductor. This will cause a response anomaly of about d), with the woofer output being 8.Fd) down at one decade below crossoverfre+uency. aturally, higher resistance will create more deviation in response. The seriesnetwork$s overall response remains 9at. ormally, the inductor$s DG6 must be factored intothe design, regardless of crossover type.

Impedance Variations

As was shown to be the case with the first order implementation, by its very nature, thetwo segments of a parallel crossover are separate, and share only the amplifiers outputimpedance, plus the impedance 6, J and G" of the speaker lead. ;peaker leadcapacitance may safely be ignored as it is insignificant compared to the capacitanceswithin the crossover network.

A series network on the other hand, relies on the integrity of the series elements - all of them. A change in woofer parameters for e!ample" therefore aects the tweeter, and viceversa. The tweeter is likely to have smaller and fewer changes than the woofer in apractical system.

#t is interesting to see the behavior of the two network types when the outputs are summedelectrically. This is a severe test, and in 0d) types, neither crossover is significantly worsethan the other in this respect.

Any change in the parameters of the woofer the most likely to change" causes a change inthe tweeter parameters, and the summed electrical response varies with both types.;ince it has been established that the two filter types are identical when all values are

at their design figures, there is no point showing this. The following two charts show thee!tremes - with the woofer impedance at 3 ohms and 0 ohms the latter value being muchmore likely".



Series and Parallel 6 !oo"er at 7%

The red trace is the summed electrical response of the parallel network, and green forseries. The dark green and violet traces with the kinks and bends" are the individualresponses for the series network.

ote that although both series and parallel networks have deviated from the ideal, theparallel network has a flatter and less rapid change. >verall, the dierence is marginal.

Series and Parallel 6 !oo"er at 1#%

ere, the change when the woofer impedance is increased to 0 ohms. The series networkis slightly better, but there is very little between the two. The rise at crossover fre+uencyhas changed from 2d) normal" to 3.Ed) - this will be audible in both cases.

The impedance =seen= by the drivers is also important. This may be referred to as =look-back impedance=. The woofer is e!pected to be eectively short-circuited by theamplifier at low fre+uencies, and both networks achieve this +uite well. #nterestingly, theparallel network loses control at the crossover fre+uency. This is shown in the followingdiagram. The loss of control at this fre+uency is relatively unimportant if the cabinet is welldamped, but may cause coloration with some systems.

#n the following graph, each trace indicates the current generated when a Nsource is connected in series with the woofer. This represents the back &M' generatedby the cones momentum when the signal changes. The red trace shows the current in theparallel network, and as can be seen, it drops to a low value high impedance" at the

crossover fre+uency. A series network maintains relatively good control over this region,tapering off impedance increasing" gradually.



Series and Parallel 6 !oo"er (ack )*F Current

The ne!t test is to see how well each network maintains separation of the signal generatedby the woofer. #t is important that woofer back &M' in particular" is not seen by thetweeter, as this may create inter-modulation. The 0nd order network is the same as a storder network in this respect, e!cept that the slope is 0d)*octave as is e!pected of asecond order network.

Series, !oo"er (ack )*F Re8ection

The amount of this signal reaching the tweeter should be /ero or close to it". The parallelnetwork is not shown, since it is at /ero. ot so good for the series network however,with more than half the generator voltage appearing at the tweeter terminals at thecrossover fre+uency. &ven at 288 /, the voltage is significant at 88 mN 08d) downfrom the full N applied". As with the series st order network, the back &M' re%ection is2d) better than the attenuation of the amplifier signal below crossover fre+uency.

The levels shown are not a real concern, since woofer back &M' will always be much lowerthan the amplifier signal. <hile it would seem ideal to limit such cross-coupling to theminimum possible, the eects are something of an unknown and back &M' can bee!pected to be +uite low with typical drivers - especially where the bo! is well dampedinternally.

Oiven that valve amplifiers typically have an output impedance of ( ohms when operatedwithout global feedback", the dierences between the series and parallelconfigurations become very similar, with the parallel network being only 0.Id) better thanits series counterpart.



Summar

The dierences between second order series and parallel filters are more difficult torationali/e. &ach has strengths and weaknesses, but from the above, the parallel versionprobably has a slight advantage. )oth e!hibit variations in response when the woofer or

tweeter" characteristics change, and they are +uite similar. The parallel filter has betterwoofer back &M' re%ection in the tweeter circuit, while the series crossover has a betterwoofer =look back= impedance characteristic.

Gomponents for a series crossover will be more costly because of higher capacitor values,but it will have lower losses due to inductor resistance, since they are lower values. 'orthose who feel that capacitors change the sound, the higher values may be thought tohave a greater eect

Conclusion

#t is very diicult to make any %udgment of series or parallel crossovers as a generali/ation. The series first order network is probably a better choice in general, due to its flatresponse regardless of driver impedance - this can simplify the design, but at the e!penseof having the crossover fre+uency shift from the design value.

The choice is more diicult for the second order crossover, since both series and parallelhave vices and virtues, with neither standing out as generally superior. >verall, the parallelversion is probably a better choice, if only because it is slightly more tolerant of variations,and will probably have marginally lower losses because there is no series connection of thedrivers this adds the resistive losses in the inductors, whereas they are in parallel in theparallel filter - of course".

As for any claims for better transient response or sound +uality, this is very doubtful -there is nothing to suggest that either version if properly designed will outperform theother to any degree. Parallel crossovers are easier to design, and are simple to convert to asub" )essel response with a : of 8. appro!imating a Jinkwit/-6iley response".

Most constructors who have attempted second order series crossovers have had to spendconsiderable time tweaking to get it right - they are harder to design than their parallelcounterpart, and interactions will always cause problems.

)9periment

As a final e!amination, 'igure 2. shows a series and parallel network, using simulateddrivers. There is no compensation applied for woofer inductance or tweeter resonance, yetboth eects are present.



Fi:ure &.1 6 Series ; Parallel, !ith Simulated <rivers

The grey bo!es are the drivers identical in each version", and the area outside the bo!escontains the generator and filter networks. These are the same in each case, with thevalues deviating from the previous simulations only in that this design is for a realcrossover network. The values are slightly dierent from those shown, but the principle isidenticalL

A transient analysis shows the following outputs, using a nominal 3 k/ crossoverfre+uency as per the circuits above" and an input signal of k/...

'ransient Response, 1 =>? S@uare wave Si:nal

The parallel crossover output is shown in A+ua, and the Niolet trace is the series network$soutput. This is an electrical summing, but it shows clearly that the driver characteristics arefully compensated by the series network, and the output is e!actly the same as theinput. The parallel network by comparison indicates severe waveform distortion, and thisimplies phase and levels are incorrect - remember that no attempt was made tooptimi/e the driver impedance with @obel or notch filters in either case.

This is fine in theory, so to prove the point one way or another, the following arereal impedance and response plots from two identical bo!es, measured under identicalconditions, and within a few minutes of each other.

Impedance Comparison, Series vs. Parallel

The measured impedance dierences are as likely to be the result of slightly



mismatched drivers as anything else. There is not a great dierence at all. The red trace isthe series connection, and black is parallel.

Fre@uenc Response Comparison, Series vs. Parallel

6esponse dierences are a bit more pronounced again, red is series and black isparallel", but are not as should e!pect based on the simulations. ;imulation showedperfectly flat response, but remembers that was an electrical signal only, and fails toaccount for driver behavior. ote that there is a noticeable improvement at the crossoverfre+uency of 3 k/ - the series network is flatter, indicating that the theory does work thedrivers have no impedance compensation".

'inally, after converting the second enclosure$s crossover to series, did another responsecomparison, there are still dierences between bo!es, with one tweeter being moreefficient than the other. This alone would account for some of the dierences seen in theseries-parallel comparison.

Fre@uenc Response Comparison, Series vs. Parallel

The glitch at I k/ appears to be caused by diffraction, probably from the woofer$ssurround which pro%ects slightly from the frame, and is at the correct distance for thatfre+uency". As for sound dierences between the series and parallel connections, there wasvery little that could hear. The microphone is much more sensitive to small variationsthan the ear, and there are +uite dramatic variations in response as one move around -far greater than the dierences measured between the series and parallel connections.

This shows up readily if one moves the measurement mic even a small distance, and thefact that the two sets of response graphs look +uite dierent is evidence of this. The mic



was moved about 8mm further away from the speakers for the second chart.

;pectral decay plots were also done, but are not shown - there are marginal dierences asone would e!pect from the fre+uency response variations, but little else.

Although a simulation shows that a first order series crossover is superior to its parallel

e+uivalent, the fact is that the dierences are slight. The evidence was suicientlycompelling for to change the crossovers, but the huge dierence in sound +uality onemight e!pect was not forthcoming. More revealing drivers may well sound better to acritical listener, but the differences are hardly =chalk and cheese= as some may imply.

Series ; Parallel Networks are /Virtuall4 identical...

Despite the dierences that have been shown, the loudspeaker drivers should always becarefully e+uali/ed with @obel networks to achieve flat impedance. >nce the impedance isflat, it is resistive, and as has been shown above, the two networks are virtually identicalwith resistive loads. Therefore, it follows that properly e!ecuted @obel and a notch filterfor the tweeter resonance" will cause real-world series and parallel crossover networks tobehave in an identical manner, with the relatively" small difference of woofer back &M'applied to the tweeter.

The phase and transient response of both filters will match e!actly withimpedance e+uali/ation, so in a properly designed crossover network, there is nothing tochoose between the two. Gertainly, the parallel variant is easier to design, and thisalone is probably a good reason to stay with a parallel crossover - and probably alsoe!plains why the vast ma%ority of loudspeaker designers use parallel rather than serial. #naddition, a serial crossover cannot be bi-amped or bi-wired.

#t is safe to say that neither crossover is possessed of any magic only skill", so be verywary of any claims that a particular crossover design is =vastly superior= or =infinitely moretransparent= or any other hyperbole that may be thrust upon you" in advertisingmaterial. All crossovers, and indeed, all loudspeakers, are a compromise. <hile the =formfactor= of the crossover is relatively unimportant, the skill and patience re+uired toe!ecute it properly is what really counts.

CrossOver Series vs Parallel

Documents

Transcript of CrossOver Series vs Parallel