[*Scotty*]

It might help to put this subject into perspective if we consider that most zobel networks used to flatten individual driver impedance curves consist of a 2watt resistor, possibly non-inductive and a capacitor of the appropriate value.
 It should be obvious that not much power is dissipated in a 2watt resistor. In this type of application the zobel is used to stabilize the impedance of the individual drivers in order to have the crossover behave in a predictable manner and achieve the desired crossover slope at the correct frequency. It should be noted that these networks are connected directly across the drivers terminals and are not in series with the driver.
 This is distinctly different than using such networks to bludgeon the complete loudspeakers impedance curve into submission. A network such as this is definitely going to divert significant power through the resistor used.
Scotty
 

[Freo-1]

From wikipedia:
 
 
"  Zobel networks can be used to make the impedance a loudspeaker presents to its amplifier output appear as a steady resistance. This is beneficial to the amplifier performance. The impedance of a loudspeaker is partly resistive. The resistance representing the energy transferred from the amplifier to the sound output plus some heating losses in the loudspeaker. However, the speaker also possesses inductance due to the windings of its coil. The impedance of the loudspeaker is thus typically modelled as a series resistor and inductor. A parallel circuit of a series resistor and capacitor of the correct values will form a Zobel bridge. It is obligatory to choose because the centre point between the inductor and resistor is inaccessible (and, in fact, fictitious - the resistor and inductor are distributed quantities as in a transmission line). The loudspeaker may be modelled more accurately by a more complex equivalent circuit. The compensating Zobel network will also become more complex to the same degree.^[3]
Note that the circuit will work just as well if the capacitor and resistor are interchanged. In this case the circuit is no longer a Zobel balanced bridge but clearly the impedance has not changed. The same circuit could have been arrived at by designing from Boucherot's minimising reactive power point of view. From this design approach there is no difference in the order of the capacitor and the resistor and Boucherot cell might be considered a more accurate description."

[neobop]

You mean it will absorb power that would not have gone to the driver. (Or not used by the driver, or reflected back into the amp, etc.,)

I meant exactly what I said.  The zobel I built and tweaked with different values of capacitor to define the cut/in frequency, changed the sound of the woofer.  Design formulas are inexact and have to be tried.  In general, the resistor (Rc) should be 1.25 x Re, and Cap = L/Rc sq.  L is voice coil inductance in henries.  If the cap value is too high you'll have an impedance dip. You're connecting a parallel load to your driver and it has to coinside with the impedance curve. BTW, 2 watt resistor is not enough power handling for a woofer.

In this case, I had a woofer with flat response to 4K, that I could use full range and augment with a tweeter. I tried different combinations with and without a low pass and low pass with zobel.  I was using a tube amp and zobels are often recommended to present a nice impedance curve.  But my results may not be typical.  Most drivers don't have flat response and results may vary. I don't think you can put a resistor, zobel, attenuator, whatever, in the signal path without consequences.  Series or parallel, it's still in the signal path.  I was using non-inductive resistors.
neo   

[Quiet Earth]

Seems to be a wide range of what defines a zobel and why you would use one in the first place. Maybe that's why there are so many different answers.

[neobop]

A zobel is an impedance compensation network, plain and simple.  It's been the same for the past 60 yrs or more. Look at the impedance curve of a driver with a voice coil.  From the area of nominal impedance it continues to rise with frequency.  This will affect the crossover frequency.  It will also present a load to the amp of varying impedance. 

Maybe it depends on what you have, but I think that sometimes the cure is worse than the disease.
neo 

[*Scotty*]

It seems that if you are building a multi-way system with a passive crossover network you pretty much have to zobel the drivers in order for the system to work as designed.
Scotty

[*Scotty*]

neo,did you try a series crossover topology for your two-way speaker instead of a parallel circuit? For some reason they do sound different from one another even though on paper they should be the same.
Scotty

[Davey]

A Zobel will not effect the sound of a transducer IF the power amp is a voltage source and there are no components in between. 
Obviously, if the power amp is a tube unit and there is a crossover network in between, then you have two large variables that will contribute to audible differences when using (or not) using a Zobel network.

Quiet Earth is correct with his power comment.....assuming the voltage source and no crossover configuration.  In that case the driver will "absorb" the same amount of power regardless of whether a Zobel is used or not.

Cheers,

Dave.

[neobop]

It seems that if you are building a multi-way system with a passive crossover network you pretty much have to zobel the drivers in order for the system to work as designed.
Scotty

Scotty,
If you have testing capability you can adjust x-over values with actual performance.

I've never built a series crossover.  I've only built a few pr and a couple were gutting the cabinet and just using the woofer.  Fried used a series crossover and a lot of people loved them.  I heard something about the values of the components must be adjusted for the drivers in circuit and it's harder to design, but I really don't know. 

I've had pretty good luck with 2 of mine, but I haven't done much lately.  I have 4 - 5" Foster extended range drivers I haven't messed with.  I was thinking of making an open baffle desk spk with a 4 ohm tweeter. Maybe someday.
neo



 

[neobop]

A Zobel will not effect the sound of a transducer IF the power amp is a voltage source and there are no components in between. 
Obviously, if the power amp is a tube unit and there is a crossover network in between, then you have two large variables that will contribute to audible differences when using (or not) using a Zobel network.

Quiet Earth is correct with his power comment.....assuming the voltage source and no crossover configuration.  In that case the driver will "absorb" the same amount of power regardless of whether a Zobel is used or not.
Cheers,
Dave.

I disagree.  A power amp is a current source basically. A tube amp is more of a voltage device and the output must go through a transformer to drive low impedance. 

"assuming the voltage source and no crossover configuration.  In that case the driver will "absorb" the same amount of power regardless of whether a Zobel is used or not."

What are you talking about, a full range driver with no crossover, but with a zobel?

How do you know about the sound of a zobel, have you tried them?

You can't put anything across the terminals of a loudspeaker without its affecting the sound. Try it and see. All the components are reactive, including the voice coil inductance. 
neo

[FullRangeMan]

I disagree.  A power amp is a current source basically. A tube amp is more of a voltage device and the output must go through a transformer to drive low impedance. 

"assuming the voltage source and no crossover configuration.  In that case the driver will "absorb" the same amount of power regardless of whether a Zobel is used or not."

What are you talking about, a full range driver with no crossover, but with a zobel?

How do you know about the sound of a zobel, have you tried them?

You can't put anything across the terminals of a loudspeaker without its affecting the sound. Try it and see. All the components are reactive, including the voice coil inductance. 
neo

You can't put anything across the terminals of a loudspeaker without its affecting the sound.
When a amount of stuffing is placed inside the box it change the sound and also change the cone/VC behavior.
So placing a cap, resistence or inductor will direct change the final electric acoustics result(music);
But some people believe in free lunch...

[Davey]

I disagree.  A power amp is a current source basically. A tube amp is more of a voltage device and the output must go through a transformer to drive low impedance. 

"assuming the voltage source and no crossover configuration.  In that case the driver will "absorb" the same amount of power regardless of whether a Zobel is used or not."

What are you talking about, a full range driver with no crossover, but with a zobel?

How do you know about the sound of a zobel, have you tried them?

You can't put anything across the terminals of a loudspeaker without its affecting the sound. Try it and see. All the components are reactive, including the voice coil inductance. 
neo

No, I'm afraid not.  Audio power amplifiers (even tube amplifiers) are much closer to voltage sources than they are current sources.  The output impedance is nearly zero in most cases which means voltage applied to the transducer will not vary (much) whether a Zobel is attached or not.

Sure, many single full-range driver systems utilize Zobel networks and no crossover.  The impedance leveling characteristics of a Zobel can (possibly) be of benefit even if no crossover is used.

If I put a 10k resistor across the terminals of my speaker system do you think that would affect the sound?? 

Cheers,

Dave.

[*Scotty*]

I think what Davey is saying is that a SS amp with a big enough power supply and output capability will deliver the same voltage to its output terminals regardless of the load impedance, if it can do this it can be described as a constant voltage source. A tube amplifier does not supply the same voltage to all load impedances. It has usually has more than one tap on its secondary to accommodate loads of varying impedances, it tends to deliver the same amount of current to each tap and corresponding load value, in this way it can be described as a constant current source amplifier. Davey feel free to correct me here.
Scotty

[Pete Schumacher]

Solid state amplifiers with a decent amount of negative feedback to maintain a low output impedance (99.9% of modern SS) are pure voltage amplifiers.  An example would be if your SS amp puts out 100W into 8 Ohm, it will put out 200W into 4 Ohm with the same input signal.  If that amp is connected directly to the speaker, as is the case for the OP's active setup, then a zobel is pointless.  SS amps can easily handle the load presented by any transducer that uses a voice coil.

Tube amps that employ output transformers and lower amounts of negative feedback tend to have higher output impedance.  In the case of a tube amp, the output will vary more with load variation than the solid state amp.  In that case, a zobel can be useful to keep the output of the tube amp from rising as the load impedance rises.

Zobel networks are not always required in a crossover.  Their use depends on the drivers and the crossover topology used.  The electrical transfer function is used to shape the acoustic transfer function so that it follows a prescribed slope.  In many cases of crossover design, a zobel is used to provide control of filter "Q"around the crossover point. 

As to whether crossovers add "distortion" to the pass band, consider that whatever distortions they might add are at least 40dB smaller than the distortion present in the driver itself.  Drivers run full range, those asked to produce bass frequencies requiring large excursions of the voice coil, with high frequency undulations on top of that, will have the highest distortion of any component in the signal chain. 

[brj]

Might I suggest that we short circuit, as it were, the current vs. voltage source debate with some references?  To start, I'd suggest Nelson Pass, specifically the current source related white papers on his First Watt website.  (The First Watt F1 and F2 amps are solid state "current source" amplifiers, rather than the more common voltage source amplifier.  The manuals for those amps have additional background.)

For what it's worth, I would suggest that sweeping generalizations aren't likely to advance this conversation significantly.  Everything is a compromise, and putting something in the signal path is not necessarily a bad thing if it solves more problems than it creates.  (Though if you can avoid a problem via another method, then so much the better.)  More significantly, "in the signal path" is somewhat relative.  Not all parts are "in the path" at all frequencies, much less components in a shunt path.  In the end, what works for you is what matters.  Again, I'm currently just seeking education, not trying to change minds or even address an immediate need.

Quiet Earth, to address your point on the last page, I didn't mean to imply that a Zobel couldn't be used in an active system.  Your points are valid, though without a power dissipating passive crossover in front of the drivers and the limited range over which you need to run the drivers (vs. a single full range driver), most of the amps I've seen in active systems are loafing.  That said, I'm sure we could still find a sufficiently challenged active system driver/amp pairing if we put some effort into it!

Thanks for the additional comments!

[Davey]

I think what Davey is saying is that a SS amp with a big enough power supply and output capability will deliver the same voltage to its output terminals regardless of the load impedance, if it can do this it can be described as a constant voltage source. A tube amplifier does not supply the same voltage to all load impedances. It has usually has more than one tap on its secondary to accommodate loads of varying impedances, it tends to deliver the same amount of current to each tap and corresponding load value, in this way it can be described as a constant current source amplifier. Davey feel free to correct me here.
Scotty

Nope, no need for correction.

I think what has neo confused is the concept of vacuum tubes being voltage amplification devices and solid-state being current amplification devices.  That's essentially correct, but the final (larger) product they're used in here....audio power amplifiers....most closely match a theoretical voltage source and not a theoretical current source.
Most amplifiers have output impedance well below one ohm.....or at least a small fraction of the load (speakers.)

I didn't think it would be long before someone mentioned the Pass amplifiers.    However, even those are not true current sources since the output impedance is not HIGH enough.    And of course, they're the exception in a vast world of audio power amplifiers that execute a different concept.

Pete is of course correct that, theoretically, a Zobel should not be required with an SS amplifier connected directly to a driver.  However, real world issues.....maybe an RFI issue, maybe an amplifier stability problem, etc, etc, might still be helped by using a Zobel network to terminate the far end of the speaker wire for high and above-audio frequencies.

Cheers,

Dave.

[neobop]

I think what has neo confused is the concept of vacuum tubes being voltage amplification devices and solid-state being current amplification devices.  That's essentially correct, but the final (larger) product they're used in here....audio power amplifiers....most closely match a theoretical voltage source and not a theoretical current source.
Cheers,
Dave.

I don't think it really matters.  You're looking at it from the perspective of a power amp driving an impedance load, not how the driver will sound.  A zobel is a shunt cap and resistor - 2 reactive components in parallel with the voice coil.   A 10K resistor doesn't have anything to do with it,  the value of the capacitor coinsides with rising impedance of the driver (often in the middle of its band), and the value of the resistor is close to the drivers impedance. 

In a conventional low pass crossover 12dB/octave or higher, a cap going to ground certainly has consequences.   Do you think an L pad, a resistor in series and another going to ground, has no audible consequence other than to attenuate a driver?  The net result might be better, but it has other consequences.  Just because a zobel  might flatten rising impedance, doesn't mean you won't hear any difference in the driver.   A zobel is often used to smooth out the high end of a driver.  Is this because it has no affect on the sound of the driver?   I don't think it makes much difference to a reasonably powered amp if a zobel is there or not.  If you're arguing that a zobel has no affect with a full range driver or one used with an active crossover, I think you're wrong. 
neo

[Davey]

You're missing (or not understanding) the points that are being made.  Not only by me, but Pete, Scotty, and Quiet Earth as well. 

Anyways, the 10k resistor comment was a retort to your comment that "you can't put anything across the speaker terminals without its affecting the sound."  I think you missed the point there too, which was....that a 10k resistor will not affect the sound.  It's completely innocuous in that position.

Cheers,

Dave.

[*Scotty*]

Interestingly enough, I have consistently preferred the sound of conventional tweeters with a zobel in place to the sound they have when used without one regardless of the type of amplifier driving them, tube or SS. I suspect there is less distortion being produced by the combination of driver and amplifier when a zobel is in place.
Scotty

[Davey]

Yeah, that may be a case where something being picked up by the speaker wires is being returned and interacting with the feedback gain of the amplifier.....possibly.

And/or the Zobel is changing the characteristics (load) seen by the upstream crossover network.  I thought I made clear (maybe I didn't) to neo in my earlier comment, that a crossover network is a "variable" and would certainly account for an audible difference if a Zobel was used or not.....all other things being equal.

Cheers,

Dave.