Zobel network with LMS/DSP?

[Bakert]

Hi all,

I'm reading about Zobel Networks in relation to passive crossovers, and they sound very necessary. However on here and on speakerplans, I have not seen any discussion on their use as part of a system that uses a LMS. Does an LMS negate their use, or are they simply not as critical to sound reproduction as I think?

Also, as an aside, does using a low value cap on my compression driver to block DC thumps from the amplifier, introduce any phase shift? Or does that only occur when using 2nd order or higher crossovers?

Thanks for any advice you might have.

 

[jan.didden]

I think the Zobel is of benefit to the amp, not the xover, unless I am missing something?
When I design an amp I often put in a zobel for stability.

A cap always causes phase shift. The final phase shift in the system depends on the size of the cap in relation to the impedance it is connected to. A series C will have less phase-shift the higher its value is.

BTW What is 'LMS'?

Jan

 

[DF96]

People often say 'Zobel network' when they mean a series CR in parallel with anything. Maybe the OP is asking about impedance compensation for drivers, so a naively designed passive crossover can work a bit better? If so, this is irrelevant to active crossovers.

 

[Bakert]

@Jan: Thank you, is there a formula for working out the phase shift?
And LMS is Loudspeaker Management System. Like a DBS Driverack or Behringer DCX 2496. I don't know if there are any important differences between DSP and LMS.

@DF96: Yes I mean it for impedance compensation, to flatten the rising impedance that you get higher up from the fs. Surely to the speaker and it's impedance, it doesn't matter if the crossover is passive or active the benefits are the same?
Also, I assume when you say a naively designed crossover, you mean that drivers should be used crossed over before the impedance even starts to rise?

 

[jan.didden]

@Jan: Thank you, is there a formula for working out the phase shift?
And LMS is Loudspeaker Management System. Like a DBS Driverack or Behringer DCX 2496. I don't know if there are any important differences between DSP and LMS.

@DF96: Yes I mean it for impedance compensation, to flatten the rising impedance that you get higher up from the fs. Surely to the speaker and it's impedance, it doesn't matter if the crossover is passive or active the benefits are the same?
Also, I assume when you say a naively designed crossover, you mean that drivers should be used crossed over before the impedance even starts to rise?

LMS describes the function. It can be implemented in DSP but also with analog filters. So an active xover DSP is 'just' one kind of LMS.

The speaker doesn't care for the Zobel which normally is between the amp and the (passive) xover/speaker. As DF96 mentioned, it is used to flatten the impedance versus frequency curve seen by the amp. The idea is that the amp is 'happier' with that; I haven't seen any proof for that and I don't believe that. But that's another story ;-)

Another use is for an individual driver, to flatten its response so that the passive xover where it is part of, is easier to design. I think DF96 calls that the 'naive way'. It costs more parts, eats into your available power, but does work.

For the phase shift, the formula for 45 degrees shift is at F = 1/(R*C*2*pi) where R is the impedance in series with the C. This is also the frequency where the response is 3dB down (or up in case of a high pass).

There's a lot of online stuff explaining this.

Jan

 

[DF96]

Yes, the speaker doesn't care about impedance compensation. The amplifier doesn't care about impedance compensation. The only thing which cares is a passive crossover designed on the assumption that the speakers have flat impedance curves. An alternative is to design the passive crossover taking account of the driver impedances. Anyway, no passive crossover means no need for impedance compensation.

Of course there is a need for a Zobel network at the output of an amp, but that is for another purpose.