Question about JBL Pro III Plus passive subwoofer crossover...

[ijosef]

Hi all -

Longtime lurker and very infrequent poster here. I picked up a passive JBL Pro III Plus subwoofer from goodwill. It was only a couple of bucks and I wanted something to pair with some bookshelves or satellites in my little office, so I thought "why not?". I've emailed JBL's customer support on this, but they haven't been very helpful - consumer audio (JBL Lifestyle) says I need to contact JBL Pro, and JBL Pro tells me I need to contact the consumer products. Oh well.

Anyway, I would like to know what the crossover point is and what impedance the satellites should have. The reason I ask is because the Pro III plus satellites I see for sale on the used market are all 4 ohm units. Since this is a passive crossover, I don't want to alter the crossover point by using 8 ohm speakers. The label on the unit says "8 ohms total system impedance" so I'm guessing that the network presents an 8 ohm load to the amplifier even when connected to a pair of 4 ohm satellites?

I've attached all the info I can find, so any help would be greatly appreciated. My questions are, in short:
1. What is the proper impedance of satellite speakers for this sub (4 or 8 ohm)?
2. What are the crossover frequencies and slopes, using the correct satellites?

 

[donprice]

From the spec sheet, the DCR of 1.8 to 2.2 ohms for the woofer leads me to believe the sats are closer to 4 than 8 ohms...and the spec sheet says 4 ohms....so 4 ohms.

The sub driver is 8 ohms and that is where the 8 ohm system load comes from.

The spec sheet spells out 125Hz crossover and the cap & coil on the sub driver makes it second order or 12 dB/octave to the sub...cap only to sats = first order 6 dB/octave.

Plug in a pair of 6 or 8 ohm book shelves and see how it sounds if you don't have a 4 ohm pair. You could change C2 to work with 8 ohm sats....got a photo of the crossover?

 

[ijosef]

Thank you! I'll take a pic of the crossover when I get a chance to open it up (it's at home right now).

I took a little bit longer to look at the diagram and what you said makes sense (I was actually reading it backwards... oy). An inline inductor (L1) on the + of the woofer and a cap (C1) across the + and - leads for a 2nd order low pass filter on the sub, then a simple cap on the + of the satellite output for the first order.

According to the various crossover calculators on the web, a first order high pass for a 4 ohm speaker at 125hz would be 318uF or thereabouts, and a 159uF for an 8 ohm load. If it's indeed meant for 4 ohm satellites, using 8 ohm satellites could shift the crossover point from 125hz to about an octave lower.

Right now all I have on hand are a pair of old Realistic MC 500s, a set of surprisingly good sounding Insignia something-or-others, and a pair of Infinity SS 2001 bookshelf speakers. I can swap 'em in and out and see if I like anything.