Hi all,
I'm trying to start understanding something about Zobel netwoks and notch filters, and I'm reading anything I can find on the Net.
My problem is that I find so many different formulas, giving so many different results, that I'm convinced that the thruth (if any) should be somewhere else.
To test the formulas I'm using Speaker Worshop (it could be THE problem), and I was unable to check any of the (all different) result I've got.
Is there something more "scientific" on the Net ?
Thanks
I'm trying to start understanding something about Zobel netwoks and notch filters, and I'm reading anything I can find on the Net.
My problem is that I find so many different formulas, giving so many different results, that I'm convinced that the thruth (if any) should be somewhere else.
To test the formulas I'm using Speaker Worshop (it could be THE problem), and I was unable to check any of the (all different) result I've got.
Is there something more "scientific" on the Net ?
Thanks
I think the reason for the many equations is that a RC zobel network cannot give anything but an approximation to a constant load. The voice coil inductance does not behave like an ordinary inductance.
The ordinary inductor has an impedance
Zl=jwL
On the othe rhand, a typical voice coil inductance can be approximated by
Zlvc=K*w^n * (cos (n*pi/2) + j * sin (n*pi/2))
where n= 0.6..0.8, typically. Note that if n=1, the equation becomes that of the ordinary inductor. The equation looks complicated, but the important part is w^n, ie the impedance is not proportional to the frequency, but increases slightly slower. Such a component is impossible to compensate for perfectly with a RC network, but with a bit of tweaking, a reasonably good compensation can be acheived. I suppose that this tweaking part is the reason for the slightly different "rules of thumb" that you have found.
HTH
The ordinary inductor has an impedance
Zl=jwL
On the othe rhand, a typical voice coil inductance can be approximated by
Zlvc=K*w^n * (cos (n*pi/2) + j * sin (n*pi/2))
where n= 0.6..0.8, typically. Note that if n=1, the equation becomes that of the ordinary inductor. The equation looks complicated, but the important part is w^n, ie the impedance is not proportional to the frequency, but increases slightly slower. Such a component is impossible to compensate for perfectly with a RC network, but with a bit of tweaking, a reasonably good compensation can be acheived. I suppose that this tweaking part is the reason for the slightly different "rules of thumb" that you have found.
HTH
Agree ...
I agree with Svante, the formula
Z=Re+Le*(jw)^n
(it's the same as yours) was given by Leech on purely physical grounds, and I have "tested" it, using Excel and Solver (that is Mean Square Root methods), on many loudspeakers impendance curves.
My problem is that even the formulas given by Leech on a different paper seem not to work, and the rest of the humanity ... gives 6 different interpretations: none working !
I agree with Svante, the formula
Z=Re+Le*(jw)^n
(it's the same as yours) was given by Leech on purely physical grounds, and I have "tested" it, using Excel and Solver (that is Mean Square Root methods), on many loudspeakers impendance curves.
My problem is that even the formulas given by Leech on a different paper seem not to work, and the rest of the humanity ... gives 6 different interpretations: none working !
Teodorom:
If you are thinking of the same article as I am, with a number of RC links, the approach should work. I have not tried it though even if I wrote a related computer program once, that filtered a signal over a wide frequency range at a constant tilt anywhere in the range +/- 6dB/octave.
Could you tell us something on why you are interested in this, is it a purely theoretical interest, or are you trying to do a practical design? And why do you want to flatten the impedance curve?
If you are thinking of the same article as I am, with a number of RC links, the approach should work. I have not tried it though even if I wrote a related computer program once, that filtered a signal over a wide frequency range at a constant tilt anywhere in the range +/- 6dB/octave.
Could you tell us something on why you are interested in this, is it a purely theoretical interest, or are you trying to do a practical design? And why do you want to flatten the impedance curve?
Both theoretical and practical
Hi Svante,
my interests are both theoretical and practical.
I have a couple of speakers I made a long time ago using Coral/Peerless drivers (and a Coral project).
The sound is pretty good, but it can be improved.
First I made calculations using the MJK worksheets for the vented box, and I discovered that a light stuffing could help.
It was true ! Now basses are deeper and the mediums are clearer.
Now I'm adressing the cross-over.
For sure I could use the same schema and values, using better components (now I have non-polarized electrolic caps and inductances with core: http://www.coralelectronic.com/Home/Filtri/NT_850_C.htm).
This schema seems too generic (it's used even with different woofers) so I would like to check the values too.
Why: It is said that it is worthwhile to control the impendace rise in the cross-over region.
Thanks
Hi Svante,
my interests are both theoretical and practical.
I have a couple of speakers I made a long time ago using Coral/Peerless drivers (and a Coral project).
The sound is pretty good, but it can be improved.
First I made calculations using the MJK worksheets for the vented box, and I discovered that a light stuffing could help.
It was true ! Now basses are deeper and the mediums are clearer.
Now I'm adressing the cross-over.
For sure I could use the same schema and values, using better components (now I have non-polarized electrolic caps and inductances with core: http://www.coralelectronic.com/Home/Filtri/NT_850_C.htm).
This schema seems too generic (it's used even with different woofers) so I would like to check the values too.
Why: It is said that it is worthwhile to control the impendace rise in the cross-over region.
Thanks
teodorom, it seems like you are determined to get things right 
However, the fact is that zobels are based on the assumption of a simple driver model used to generate T/S parameters. Since the curve fitting criteria may be different, each different software will generate different parameters. So what I would just recommend is to use the equations for first estimate and adjust the values until you get what you want.
However, the fact is that zobels are based on the assumption of a simple driver model used to generate T/S parameters. Since the curve fitting criteria may be different, each different software will generate different parameters. So what I would just recommend is to use the equations for first estimate and adjust the values until you get what you want.
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