View Full Version : Importance of Zobel ?
Tom Loizeaux
08202006, 11:43 AM
I have some stage monitors that I am refurbishing. The 16 ohm compression driver/horn is set up with a first order, high pass network. There is a 33 ohm Zobel resistor in place. I'm changing the values of the cap and series resistors because I'm going to an 8 ohm 12" driver (running full range). I'm going to use a 10 ohm series restitor with a 2.2 uf cap on the compression driver.
What advantage is stayng with this Zobel resistor going to give me in real terms in this application? Any disadvantagers?
Thanks,
Tom
Earl K
08202006, 04:56 PM
I have some stage monitors that I am refurbishing. The 16 ohm compression driver/horn is set up with a first order, high pass network. There is a 33 ohm Zobel resistor in place. I'm changing the values of the cap and series resistors because I'm going to an 8 ohm 12" driver (running full range). I'm going to use a 10 ohm series restitor with a 2.2 uf cap on the compression driver.
What advantage is stayng with this Zobel resistor going to give me in real terms in this application? Any disadvantagers?
Tom,
 Perhaps I'm not following your textual explanation properly .
 So, can you draw this out and post it ?
 To date, what I understand to be present on your horn circuit is a two resistor Lpad , preceeded by a single capacitor of some value.
 I can't see from your text any sign of an actual Zobel existing in that circuit .
:o:
Tom Loizeaux
08202006, 06:22 PM
Does this drawing help?
Tom
Earl K
08202006, 06:35 PM
Does this drawing help?
 Yes it does .
 That 33 ohm resistor is simply part of a reversed Lpad . It's not a Zobel .
 The 33 ohm resistor is paralleled across the following load ( series resistor & drivers impedance ) to give a desired target impedance / while giving some attenuation.
:)
Tom Loizeaux
08202006, 06:53 PM
 Yes it does .
 That 33 ohm resistor is simply part of a reversed Lpad . It's not a Zobel .
 The 33 ohm resistor is paralleled across the following load ( series resistor & drivers impedance ) to give a desired target impedance / while giving some attenuation.
:)
Earl,
I thought a Zobel was a resistor of at least double the driver impedance put accross the leads to smooth out high frequency impedance.
Isn't that what this is?
How does this improve the performance of the compression driver?
Tom
Earl K
08212006, 04:32 AM
Earl,
I thought a Zobel was a resistor of at least double the driver impedance put accross the leads to smooth out high frequency impedance.
Isn't that what this is?
How does this improve the performance of the compression driver?
 No, a "Zobel" is a specific type of an impedance equalizer used to flatten a rising impedance curve ( in the upper frequencies ). Its constituent parts are a resistor and a capacitor wired in series to each other and then wired across the load ( transducer ) .
 The capacitor value chosen determines the Fo point ( essentially that point where one wants to start flattening out a rising impedance curve ). The resistor value chosen is generally the "target" impedance that one wishes was the overall impedance ( in the range ) .
 An inductor and resistor similarly configured ( and employed ) is another form of an impedance equalizer that works in the lower end of an impedance curve. To my knowledge, it doesn't have a handy name like "Zobel" attached to it .
 What you are calling a Zobel is simply a load resistor . It will average down to some degree, the worst of the peaks and ambulations found in an impedance curve.
 Back to your monitor retrofit ;
 I'd recommend simply using 100 watt variable Lpads on the horn drivers / preceeded by ( at least ) a 2 pole LC network on those compression drivers.  If you liked the FR curve that a 2.2 uF cap gives you with a 16 ohm load / then simply double that value to 4.4 uF for the newer 8 ohm load ( that irepresent an approx 5000 hz F3 point ) .
 Adding in an inductor ( wired in across the line after the cap ) that has a value chosen to work at around 800hz to 1000hz will offer you added driver protection and power handling. Either a 1.6 to 1.3 mH coil ( 16 or 18 gauge ) will offer cheap protection while having very minimal effect on the final horn curve ( since the active elements are more than 2 octaves apart ) .
:)
Tom Loizeaux
08212006, 05:55 AM

... I'd recommend simply using 100 watt variable Lpads on the horn drivers / preceeded by ( at least ) a 2 pole LC network on those compression drivers.  If you liked the FR curve that a 2 uF cap gives you with a 16 ohm load / then simply double that value to 4 uF for the newer 8 ohm load ( that irepresent an approx 5000 hz F3 point ) .
 Adding in an inductor ( wired in across the line after the cap ) that has a value chosen to work at around 800hz to 1000hz will offer you added driver protection and power handling. Either a 1.6 to 1.3 mH coil ( 16 or 18 gauge ) will offer cheap protection while having very minimal effect on the final horn curve ( since the active elements are more than 2 octaves apart ) .
:)
Earl,
I don't need or want an L Pad on these horns.
The horns are staying, so they are 16 ohms.
Are you suggesting I use a 2nd order,L/C network, or are you suggesting something else?
Please explain.
Thanks,
Tom
Earl K
08212006, 06:24 AM
Earl,
I don't need or want an L Pad on these horns.
 Though ,as your circuit presently exists, you already have a "fixed Lpad" within that horn circuit. It just happens to be wired in reverse ( from the norm ). In your case the buildout resistor is "following" the load or conjugate resistor .These two resistors still function ( and are defined ) as a form of Lpad. In this case, the direction of impedance matching (offered by an Lpad) is reversed to the norm .
The horns are staying, so they are 16 ohms.
 Oh, my mistake. For some reason I thought both component types were being replaced by 8 ohm types.
Are you suggesting I use a 2nd order,L/C network,
 Yes, that's what I'm suggesting / for the sake of long term component safety .
 "If" the horn circuits' working impedance is 16 ohms / then the coil size will need to double from what I previously suggested.
:)
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