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Ducatista47
05-09-2010, 01:34 PM
I searched a bit, but I am going to risk starting a new thread. A friend has a really nice pair of C38s and we would like to replace the crossovers. They have the 001 load - D130A, N1200 and LE1750LH potato mashers. We know the crossovers are not worth rebuilding and want to build simple replacements with modern parts. Does anyone have a diagram/schematic with values? Just steer me to the post/library entry or make suggestions. All help greatly appreciated! We are looking for minimalist/simpler, not charge coupled or anything like that.

These speakers get played every day, fed by top flight equipment.

Clark (& Dave)

Earl K
05-09-2010, 02:23 PM
Clark,

Here's the schematic for the N1200 (http://www.jblproservice.com/pdf/Network%20Schematics/N1200.pdf) .

Also , the 3120a (http://www.jblproservice.com/pdf/Network%20Schematics/3120A%20Network.pdf) is worth a look see , as is it's predecessor, the 3120 (http://www.jblproservice.com/pdf/Network%20Schematics/3120%20Network.pdf) .
One can see by the Zobels that they were meant for different generations of JBL ( so called ) 8 -12 ohm woofers .

A common problem with all three , is that they use a tapped inductor ( instead of an L-Pad ) in the horn circuit for attenuation .
- These tapped-coils are near impossible to come by and are easiest replaced by a fixed (L) pad ( composed of 2 resistors ) .
- The tapped-coil ( when used for attenuation ) also allows some HF to bleed through it. ( See ; Tapped-Autotransformers (http://www.audioheritage.org/vbulletin/showthread.php?5376-Tapped-Autotransformers)to chart how much ( theoretically ) .
- Forgoing the tapped-coil means forgoing the HF contouring that comes with the attenuation . This loss of HF will need to be replaced with a more normal RC , HF contour circuit ( built across the fixed L-pad ) .


<> cheers

Ducatista47
05-09-2010, 09:01 PM
Thank you, Earl! We will start doing our homework.

I am guessing that if we calculate a standard 1250hz first or second order crossover we would be ignoring something important? I don't see any asymmetric slopes, but I am brand new at this.

Clark

Earl K
05-10-2010, 05:20 AM
Clark,

You say the existing N1200 networks are not worth rebuilding . Why is that ?

Are they trashed or is this ( articulated sentiment ) a judgement call to maintain vintage value vs building new ( or is something else at play here ) ?

<> cheers

Earl K
05-10-2010, 05:31 AM
I am guessing that if we calculate a standard 1250hz first or second order crossover we would be ignoring something important? I don't see any asymmetric slopes, but I am brand new at this.

I highly doubt that standard secord order values will work here ( btw, the N1200 schematic I linked you to is 2nd order ) .
- I think there was an ancient 3rd order type ( way back when ) / though I've never seen a schematic for it .

Really, you need to have one of the guys ( with LinearX Crossover Workshop ) take an interest in recreating the voltage drive of the vintage N1200 while using the more typically available set of L,C,R, components . This may involve $$ .

In the absence of that happening, wire up your best guess ( kludge ) .

- I'd use the LowPass section of the N1200 ( since there are no unobtanium parts within it & it's about the right vintage/impedance for your woofer ) .
- At best, the HiPass portion of your network would need to be an educated guess .

Do you or your friend have any way of measuring the AC impedance ( of the DLH175 ) ?


<> cheers

Ducatista47
05-10-2010, 11:04 AM
Clark,

You say the existing N1200 networks are not worth rebuilding . Why is that ?

Are they trashed or is this ( articulated sentiment ) a judgement call to maintain vintage value vs building new ( or is something else at play here ) ?

<> cheers
Our thoughts are (1) very old caps that are no longer in spec and not of the quality of current reasonably priced parts, and (2) reading maybe fifty times in these forums that vintage JBL - and everyone else's as well - crossover designs were built to a (low) price and were not as good as newer designs and are easily improved. Perhaps the "easily" part was not mentioned often.:banghead:


I highly doubt that standard secord order values will work here ( btw, the N1200 schematic I linked you to is 2nd order ) .
- I think there was an ancient 3rd order type ( way back when ) / though I've never seen a schematic for it .

Really, you need to have one of the guys ( with LinearX Crossover Workshop ) take an interest in recreating the voltage drive of the vintage N1200 while using the more typically available set of L,C,R, components . This may involve $$ .

In the absence of that happening, wire up your best guess ( kludge ) .

- I'd use the LowPass section of the N1200 ( since there are no unobtanium parts within it & it's about the right vintage/impedance for your woofer ) .
- At best, the HiPass portion of your network would need to be an educated guess .

Do you or your friend have any way of measuring the AC impedance ( of the DLH175 ) ?


<> cheers
We would settle for dropping in newer caps & maybe resistors if the original design is good enough. If no big improvement is forthcoming and this entire effort is misguided, keeping the N1200 intact is fine.

By the way, I happen to be a fan of simple 1st order crossovers where they will work. I have a system - my best, actually - where the only crossover component is a 2 microfarad cap in line with the tweeter.

I will see if he can measure AC impedance at work. He is in charge of maintenance for a paper mill and sports a brand new EE degree, so anything is possible. He is also a decorated war hero and slips off occasionally to distant locales to be a military consultant for a movie about one of the very famous missions he was on, but he is here now. He is certainly several times the man that I am, but we have a lot of things in common. Like myself, music keeps him reasonably sane.

Thanks again, Earl,

Clark

jcrobso
05-10-2010, 12:45 PM
I ran a LE175 and a D140F with a standard homemade 12db x-over without problems.
Some info on this.
http://sound.westhost.com/lr-passive.htm#s4.0
And a quick calculator.
http://www.diyaudioandvideo.com/Calculator/XOver/

Earl K
05-11-2010, 06:09 AM
Hi Clark,

My first inclination would be to partially rebuild the N1200 .

- I would replace all the parts in the LF portion of the circuit ( L1, C1, C2, R1 ). ( L1 is such a small value that replacements won't break the bank )

- I would only replace C3 in the HF leg of the network .

- I would "add" most of the HF "boost circuit" from the 3120A network (in this case comprising C3, C4, L2, & S1 ) for the extra UHF it makes available . This HF bypass circuit is then wired into the HF portion of the N1200, ( functionally ) direct from the networks + input over to the "hot" terminal of the driver ( whatever color on the driver is presently being used as hot ) .

<> cheers

Ducatista47
05-11-2010, 09:36 PM
Thank you, gentlemen! Sometimes the massive experience of the membership here shows. I bet that between the two of you there is a hundred years of audio experience and knowledge. I, on the other hand, have better than half that experience but not much knowledge.

jcrobso, Mr Elliot does impart a lot of passive crossover knowledge for a guy who wants everyone to go active.

Clark