How does a Zobel affect a full-range FR?

[majerjack]

I know that the primary use of a Zobel network on a speaker driver is to equalize the impedance so that the crossover network will work properly. I also know that the DIY users of full-range drivers often use a Zobel network to tame the rising response that such speakers commonly produce.

My question is this: is it possible to predict how a Zobel network will affect the frequency response of a full-range speaker? Can one use the Thiele/Small parameters of the driver to do so accurately, and thus, tailor the response of the speaker to one's taste? Or, is it strictly a trial-and-error proposition?

I understand that manufacturers' specifications can not always be trusted, so that if one uses the published T/S values, one can not always be sure of the desired result, but in theory, should it be possible to predict the effect of a Zobel on the FR of a speaker? If so, what are the equations to do so, and can the type of filter be defined as, for example, 6dB/octave, or shelving, or something else?

Thanks to anyone who can educate me on this matter.

[sreten]

Hi,

The consequence depends on the drivers impedance curve, with and
without the zobel, and the output impedance of the driving amplifier.

Normally with most amplifiers the zobel will have no real effect.

With unusual amplifiers with significant source impedance it will.

You need to know what that is to predict the effect.

rgds, sreten.

[DBMandrake]

A zobel compensates the rise in impedance of the driver at high frequencies due to voice coil inductance, as you say usually for the purpose of providing a flatter impedance for a crossover to drive.

If you just place a zobel across a full range driver that has no passive crossover though, and it's connected directly to a low output impedance amplifier, it will cause no difference in the response at all, just the load seen by the amplifier. (In other words its a waste of time unless you have some sort of passive compensation network like BSC in front of the driver)

[Bob Brines]

Originally Posted by DBMandrake

A zobel compensates the rise in impedance of the driver at high frequencies due to voice coil inductance, as you say usually for the purpose of providing a flatter impedance for a crossover to drive.

If you just place a zobel across a full range driver that has no passive crossover though, and it's connected directly to a low output impedance amplifier, it will cause no difference in the response at all, just the load seen by the amplifier. (In other words its a waste of time unless you have some sort of passive compensation network like BSC in front of the driver)

Right. As long as there is significant resistance in the BSC filter, a zobel will help correct a rising response. With a SS amp or low Z tubes,no BSC = no effect.

Bob

[Zero D]

Placing a Zobel in that way on a FR speaker, would actually be putting 2 X Zobels in parallel on the Amp output. As most Amps do have one in place already. Often there can be a wirewound inductor after or before the Zobel, which could make some difference ! How much though ?

[majerjack]

My knowledge of filter theory is very basic, so I am having a bit of difficulty understanding. If the Zobel network is placed in parallel with the driver, does the Zobel not constitute a high-pass filter, especially if the resistor has a lower value impedance than the driver at its lowest value? It seems to me that there should be some effect, even without a BSC filter.

Can anyone elaborate?

[majerjack]

I want to clarify my previous post. What I meant to say was that it seems to me that the Zobel network in parallel with the driver would appear to pass some of the high frequencies to ground, thus bypassing the driver at those frequencies. Is that incorrect, and if so, can anyone explain why?

[AndrewT]

Fraid not.
A filter needs a pass element and a shunt element.
For a low pass filter the pass element is a resistor and/or inductor and the shunt element is a capacitor.
You can change the Filter into a shelf rather than having an extended roll-off by adding a resistor to the shunt capacitor. This is the Zobel you are referring to.

But going back, for the LP filter to be effective you need a pass element. That pass element can be a resistor or an inductor or a combination of these.
If the Power amplifier is the only pass element and it has the usual low output impedance then the pass element is so small that the filtering effect is miniscule.

All the speaker Zobel does is draw extra reactive current through the output stage. This also applies when a crossover is in place. Zobels lower the sensitivity of the crossedover speaker.

[majerjack]

I just finished reviewing the article by Nelson Pass entitled "Current Source Amplifiers and Sensitive/Full-Range Drivers", and this has helped to answer my questions and clear up my confusion.

I had read this article some time ago and had incorrectly recalled some of the ideas presented. I confused some of the techniques illustrated in the article with the use of a Zobel filter and assumed that the Zobel presented some equalization effect where it would not apply.

Thanks to all who responded.