Resistor before or after cap?

fatmarley · 6th May 2009, 08:05 AM

Hi all,

I managed to get hold of some old AR18s (2 way, 8" woofer 1-1/4" tweeter), re-foamed them and replaced the electrolytic cap with a polyprop, but they were rather forword sounding after fitting the new caps.
I downloaded some test tones and found there was a jump in volume at 2000hz (The crossover point, according to specs), so I experimented with series resistors before the tweeter and found 3.5ohm gave the flattest frequency response.

Question: I fitted the resistor after the polyprop cap because it was easiest, but wonder if it makes any difference before or after the cap?

The only thing I could find with google was an L-pad calculator. Would there be any benifit having a shunt resistor as well as the series resistor?

fatmarley · 6th May 2009, 08:08 AM

Here's the thread I started on another forum: CLICK HERE!

pacificblue · 6th May 2009, 09:44 AM

If it is a 6 dB filter, i. e. only capacitor, resistor and tweeter in series without parallel components, it makes no difference.

You must however be aware that fitting a simple resistor to that configuration will change the cross-over frequency.

An L-pad after the capacitor can change the roll-off slope and could probably help you with that peak, if it is produced by the resonant frequencies impedance peak. An-L-pad before the capacitor will maintain roll-off slope and cross-over frequency the same.

If the cross-over has other components, e. g. an inductor going from behind the capacitor to ground, it does matter, where you put the resistor. Although there is no general rule, which is best. It depends on the speaker.

fatmarley · 6th May 2009, 10:42 AM

It started life with an electrolytic cap (4.7uF) and no resistor. I think the lower ESR of the polyprop cap I fitted, is what caused the tweeter to be too loud.

pacificblue · 6th May 2009, 11:52 AM

3,5 Ohm is a lot to compensate for the ESR of a 4,7 µF capacitor.

Those speakers were designed as bookshelf speakers. Give them a wall in their backs to support the woofer, and the tonal balance will probably be okay without the resistor.

fatmarley · 6th May 2009, 04:26 PM

The speakers are about an inch away from the back wall, and sound even worse when pulled out further.

6th May 2009, 08:05 AM	#1
fatmarley diyAudio Member Join Date: Aug 2003 Location: Cheltenham	Resistor before or after cap? Hi all, I managed to get hold of some old AR18s (2 way, 8" woofer 1-1/4" tweeter), re-foamed them and replaced the electrolytic cap with a polyprop, but they were rather forword sounding after fitting the new caps. I downloaded some test tones and found there was a jump in volume at 2000hz (The crossover point, according to specs), so I experimented with series resistors before the tweeter and found 3.5ohm gave the flattest frequency response. Question: I fitted the resistor after the polyprop cap because it was easiest, but wonder if it makes any difference before or after the cap? The only thing I could find with google was an L-pad calculator. Would there be any benifit having a shunt resistor as well as the series resistor?

6th May 2009, 09:44 AM	#3
pacificblue diyAudio Member Join Date: May 2008	If it is a 6 dB filter, i. e. only capacitor, resistor and tweeter in series without parallel components, it makes no difference. You must however be aware that fitting a simple resistor to that configuration will change the cross-over frequency. An L-pad after the capacitor can change the roll-off slope and could probably help you with that peak, if it is produced by the resonant frequencies impedance peak. An-L-pad before the capacitor will maintain roll-off slope and cross-over frequency the same. If the cross-over has other components, e. g. an inductor going from behind the capacitor to ground, it does matter, where you put the resistor. Although there is no general rule, which is best. It depends on the speaker. __________________ If you've always done it like that, then it's probably wrong. (Henry Ford)

6th May 2009, 11:52 AM	#5
pacificblue diyAudio Member Join Date: May 2008	3,5 Ohm is a lot to compensate for the ESR of a 4,7 µF capacitor. Those speakers were designed as bookshelf speakers. Give them a wall in their backs to support the woofer, and the tonal balance will probably be okay without the resistor. __________________ If you've always done it like that, then it's probably wrong. (Henry Ford)

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6th May 2009, 08:08 AM	#2
fatmarley diyAudio Member Join Date: Aug 2003 Location: Cheltenham	Here's the thread I started on another forum: CLICK HERE!

6th May 2009, 10:42 AM	#4
fatmarley diyAudio Member Join Date: Aug 2003 Location: Cheltenham	It started life with an electrolytic cap (4.7uF) and no resistor. I think the lower ESR of the polyprop cap I fitted, is what caused the tweeter to be too loud.

6th May 2009, 04:26 PM	#6
fatmarley diyAudio Member Join Date: Aug 2003 Location: Cheltenham	The speakers are about an inch away from the back wall, and sound even worse when pulled out further.

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