Series vs Parallel Networks - First Order Comparison - page 2
Phase Response (deg) Comparison between First Order Series & Parallel Networks
As we can see, both series and parallel implementations of a first order network exhibit identical phase response with 90 degrees phase difference between LPF's and HPF's and a 90 degrees phase change between start and stop band roll off of each filter.
First Order Series and Parallel Networks Comparison with Reactive Woofer Load
For the next phase of our comparison between first order series and parallel networks, we expand our models to include a reactive load such as those typically found in a woofer as illustrated below.
Frequency Response Comparison of First Order Series & Parallel Networks with Reactive Loading
We can see that a reactive woofer load has had a and minor impact on the HPF 3dB point, as seen by the shift to 1.08kHz. More importantly, we note the profound affect the reactive woofer load had on the LPF of the parallel network as evident by the 3dB point shift from 1kHz nominal to 836 Hz, and its degenerative stop band attenuation rolloff. The series network also experiences 3dB point shift to a much lesser degree, with no ill effects on stop band rolloff. Note, Zobel compensation (series R & C) in shunt with the LPF of the parallel network is mandatory to restore comparable filter characteristics and is also recommended for the series network to restore optimal crossover points.
To figure out the correct R and C values for the Zobel network, the following equations are usually used:
Rz = 1.25 * Rs where Rs is the series resistance of the driver
Cz = 1*10^6 / (2*pi*Rs*fd) where fd is the frequency where the impedance of the driver doubles
However, by tweaking these values using trial and error, a more damped response was achieved.
Calculated values: Rz = 10 ohms , Cz = 2.97uF
Frequency Response Comparison of First Order Series & Parallel Networks with Reactive Loading (Zobel Compensated)
As evident in the graph above, both Series and Parallel networks exhibit identical frequency responses under reactive loading when Zobel compensated.
Summed Output Impedances of First Order Series and Parallel Networks with Reactive Loading (Zobel Compenstated)
The series network exhibits a perfectly flat summed output impedance while the parallel network has minor variations, which are mostly inconsequential given their magnitude (44mdB).
Driver Induced EMF
By changing the driver impedances, two things happen. The filter Q changes, and the reflected change affects the behavior of the other filter section. Although the individual response, Q and phase varies, the net result is that the effective crossover frequency is changed, but nothing more. This is a remarkable property, and the series first order is the only crossover filter circuit that has this ability.
Remarkable 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. Should the woofer impedance increase (with voice coil temperature, for example), the crossover frequency will move upwards, thus providing a small measure of added protection for the tweeter at sustained high power levels.
However, all is not completely rosy. Everything in electronics is a compromise, and the selection of a crossover is no different. There is one final test that needs to be applied, and that is to examine the amount of woofer back EMF that reaches the tweeter. This is an area where the series network is inferior to the parallel.
With a parallel network, only the amplifier's output impedance plus the impedance of the cable allows any cross coupling between high and low pass sections. With a zero ohm source, attenuation is infinite, and is not shown above.
A series network relies solely on the isolation of the crossover filters, and as a result, the back EMF from the woofer is not attenuated as well. This may not be a major problem, since the attenuation of back EMF is the same as for amplifier power (actually, it is 3dB better), and the latter is at a far greater amplitude. It is a consideration nevertheless, so be aware that it may increase tweeter intermodulation.
Transient Response Comparison between First Order Series & Parallel Networks
Although not illustrated to save space, both first order series and parallel networks exhibit identical transient response under a fixed resistive load. However, when the load becomes complex, such as a real world loudspeaker load, the results are quite different as illustrated below.
Injecting a 1kHz square wave into each network loaded with a reactive load on the woofer, and looking at their summed response, we see the series network electrically passes the signal unadulterated while the parallel network exhibits overshoot on the rising and falling edges of the square wave. However, again by simply applying a Zobel network (series R and C) in shunt with the woofer load, we see the compensated parallel network can now pass the square wave just like the series network.
These simulations have revealed that first order series and parallel networks can be designed to exhibit very similar transfer functions as evident by their similar input impedance, frequency response, transient analysis, summed output impedance, and phase response. However, under a reactive load such as a loudspeaker, both parallel and series networks must be Zobel compensated to restore equivalent filter responses to their original responses during purely resistive loading. The series network is probably a better choice than parallel for a number of reasons. It retains a flat response even when the driver characteristics change, and is to an extent "self correcting". Implementation is no more difficult than for an equivalent parallel network, and the same component values are used. On the negative side, woofer back EMF 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.
Finally, it must be remembered that any first order network dictates that the drivers will have significant power applied at frequencies where their performance will be rapidly deteriorating, however for a system that will never be operated at high power, the performance can be very satisfying.