Filtering the facts

[Earl K]

JohnY,


- This thread ( up until quite recently ) has been about whether or not one can "splitup" the LP & HP legs of the typical parallel-type crossover ( as opposed to the series topology ) . The answer was "yes" the legs can be split as long as they are truly split. Splitting is especially important when 2 different amplifiers are invovled.

Will the Rotel drive a 4 ohm load ?

If so, I believe your least expensive option is to just rebuild one 3115 crossover - or - make the woofer leg from scratch ( the easiest & least instrusive).

The above quote ( of my words ) is not a confirmation that a single ( or twin ) 8ohm woofer(s) would achieve a 500 hz crossover point from the Low Pass leg of an "unaltered" 3115 network.

FAR FROM IT: It won't since the Inductor & Capacitor values are set to operate with a higher impedance woofer.


You've made incorrect assumptions and drawn the wrong conclusions from my words.

Earl K

[John Y.]

Earl K.,

From your comments:

Quote:
Will the Rotel drive a 4 ohm load ?

If so, I believe your least expensive option is to just rebuild one 3115 crossover - or - make the woofer leg from scratch ( the easiest & least instrusive).
</B> The above quote ( of my words ) is not a confirmation that a single ( or twin ) 8ohm woofer(s) would achieve a 500 hz crossover point from the Low Pass leg of an "unaltered" 3115 network.

FAR FROM IT: It won't since the Inductor & Capacitor values are set to operate with a higher impedance woofer.

"You've made incorrect assumptions and drawn the wrong conclusions from my words."

Let me dig myself out of this one. You stated that the woofer leg of a passive crossover can be independently used as a filter between an amp output and a speaker. I used the example of the 3115 without the implication of use with an 8 ohm woofer. JBL literature states that it may be used with 8 - 16 ohm woofers, which always bothered me. I think you are correct in your warning that with a 4 ohm load, the crossover point would not be 500 Hz. The woofer knee would shift. That said, I have a 3115A, which should be correct for my application.

In any event, two woofer legs that are paralleled (if they are separated from the tweeter legs) should result in halving the inductor values and doubling the capacitor values, which is exactly what we want in order to have paralleled woofers at the same crossover point as the single woofer with only one crossover. Right?

Seems logical that this might be done just paralleling two crossover lower legs at the inputs and connecting their outputs individually each to an 8 ohm woofer. Should give the same result as paralleling each component (inductor, capacitor, woofer) internal to the crossover. Agree?

What do I do about the Zobel? Use the same values, or parallel them?

Thanks, John Y.

[Earl K]

Hi JohnY

I'm buried in work for about a week or so. I'd like to address your questions but in fact I can't even really focus my eyes on the computer screen properly. So, to make sure all the "nuances" of the printed words are addressed properly, I'm going to have to put this off till next Sunday when the skies clear ( metaphorically ).

- Maybe someone else in the meantime will address this . ( I do think you have the logic correct, but I need to revisit your comments about the Zobels ).

regards < Earl K

[John Y.]

Hi JohnY

I'm buried in work for about a week or so. I'd like to address your questions but in fact I can't even really focus my eyes on the computer screen properly. So, to make sure all the "nuances" of the printed words are addressed properly, I'm going to have to put this off till next Sunday when the skies clear ( metaphorically ).

- Maybe someone else in the meantime will address this . ( I do think you have the logic correct, but I need to revisit your comments about the Zobels ).

regards < Earl K

Earl,

I have had no further input to the discussion so I intend to proceed with a simplistic approach of figuring the inductor/capacitor values for my situation and replacing (adding) these to a 3115A crossover then testing to assure I have everything right for a 500 Hz crossover of 8 Ohm woofers in parallel.
Thanks for all your help.

John Y.

[duaneage]

Earl K.,

From your comments:

Quote:
Will the Rotel drive a 4 ohm load ?

If so, I believe your least expensive option is to just rebuild one 3115 crossover - or - make the woofer leg from scratch ( the easiest & least instrusive).
</B> The above quote ( of my words ) is not a confirmation that a single ( or twin ) 8ohm woofer(s) would achieve a 500 hz crossover point from the Low Pass leg of an "unaltered" 3115 network.

FAR FROM IT: It won't since the Inductor & Capacitor values are set to operate with a higher impedance woofer.

"You've made incorrect assumptions and drawn the wrong conclusions from my words."

Let me dig myself out of this one. You stated that the woofer leg of a passive crossover can be independently used as a filter between an amp output and a speaker. I used the example of the 3115 without the implication of use with an 8 ohm woofer. JBL literature states that it may be used with 8 - 16 ohm woofers, which always bothered me. I think you are correct in your warning that with a 4 ohm load, the crossover point would not be 500 Hz. The woofer knee would shift. That said, I have a 3115A, which should be correct for my application.

In any event, two woofer legs that are paralleled (if they are separated from the tweeter legs) should result in halving the inductor values and doubling the capacitor values, which is exactly what we want in order to have paralleled woofers at the same crossover point as the single woofer with only one crossover. Right?

Seems logical that this might be done just paralleling two crossover lower legs at the inputs and connecting their outputs individually each to an 8 ohm woofer. Should give the same result as paralleling each component (inductor, capacitor, woofer) internal to the crossover. Agree?

What do I do about the Zobel? Use the same values, or parallel them?

Thanks, John Y.

I have a simple answer for you, maybe it will work or not.

Wire the woofers, tweeters, the crossover, and anything else you have. Connect a signal generator to this with a voltmeter and sweep through the desired range. Record what the impedances are and what the voltages are across all the drivers. After this is mapped out you can see what effect changes will have. I always do this no matter how I "model" crossover designs. Then you will know what works.

You are looking at quite a bit of interaction and phase relationships and we have time delay caused by driver distances affecting the polar response at the crossover frequency, with driver polarity reversed or not, depending on what slope and cutoff combined with filter Q and parasitic resistance caused by inductors.

Whew that was a long sentence, eh?