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Rectilinear III crossover confusion - Should this even work at all?

tgmartin

tgmartin

AK Subscriber
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I recently acquired a pair of Rectilinear III (111?, 3?) "Highboy" speakers with "MIDRANGE" and "TWEETER" controls on a recessed black plastic back panel. One has serial #44967, the other is missing the tag.

I found that one (#1) had no audio from the mid driver and the other (#2) had no audio from any of the four tweets.

I've pulled #1 apart and found that the Phillips mid makes audio and reads 6.2 ohms DCR when disconnected from the crossover. It looks like I'm the first person inside this thing. All the other drivers check out fine as well. I made a schematic of what I found in the crossover and, for the life of me, can't see how it would have EVER worked. Anybody want to hazard a guess?

During web searching I found a drawing of one that somewhat resembles what was found in mine. It's been characterised by some as a "series" circuit. I'm not clear on how this is supposed to work so I thought I'd do a pre-emptive strike and plead ignorance as I wander into uncharted (for me) territory.

Here's the drawing I found that resembles my #1 speaker with the changes in red to reflect what I found:

RectilinearIII-Xover-Series-EDIT-TGM_1.jpgMyRectilinearIII-Speaker1_12.JPG

The main difference between this schemo and my #1 crossover is that mine is missing the 2uf cap entirely. It's just not there. There's only one 60uf/25V cap. Also, the pots are 15 Ohms instead of 32 Ohms and the one for Mid is a higher wattage wire wound while the Tweeter control appears to be a standard carbon trace pot. The inductors seem to be roughly in the same ballpark. Of course the symbol "T" for tweeters represents four drivers in series/parallel.

I haven't pulled speaker #2 apart yet but will do so shortly.

Seems that there is mention of QC trouble with some years of Rectilinear product and I was wondering if this may be what happened here, as opposed to being done on purpose. Guess we'll see.

I tried to model this in LTSpice but, so far, I can't make it go ... I'm sure it's probably a combination of my ineptitude and lack of sleep!
 
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I have restored two pairs of these but did not find anything out of the ordinary besides bad potentiometers, out of spec capacitors, and blown mid range drivers.

Below are 'as found' pictures of the crossover and the wiring. Yours sound like they were either hacked up by a previous owner or manufactured on a Friday afternoon. Rewire to the schematic and you should be fine.

IMG_3258.JPGRectilinear III Highboys (6).JPG
 
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It sure looked like no one had been into this thing before me. It's a puzzler! The old feller I bought them from said they had been in storage for many years but he thought "they sounded fine" when he put them away, because they became too big. Didn't seem like much of an audio dweeb. Got the feeling he was the original owner. Inside view of your front baffle looks pretty much the same but mine has a CTS (137) woof that looks like they sourced the same parts for. Ya gotta love the cottage cheese container for the Mid compartment!
 
This is a series style crossover, not such a good thing but fairly simple.

Consider if the drivers were in series without any crossover components - all drivers would get a fraction of the full FR output.
The woofer gets a cap across it to shunt mids and highs to the other drivers.
The inductors shunt lows and even DC away from the mid/tweeter and to the woofer.
The 2uF directs the highs away from the mid and to the tweeter.
Note that all of the caps and inductors have the driver impedance across them making them less effective at filtering.
The tweeters are not very well protected with this type of XO.
 
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Thanks Pete! I appreciate your explanation. Do you have an opinion on how I might silk purse this sow's ear? If I just add in the MISSING 2uf cap and don't hit it with 1KW might I have a chance at witnessing something like what these things sounded like when they came out? That was my hope when I got them. I suppose now that I've pulled it apart and exercised what few connections there are maybe the Mid driver will wake up. Will the Tweets be protected somewhat with the added cap? The pots seem to be working according to my meter ...
 
Adding the cap will not protect them but should give you more extended highs, and it is 2.5uF, use 2.7 should be fine.

I'd recap both of them with tweeter caps, use a 50-100V for both. Then listen to them side by side in mono to see if they sound the same.
You could also do resistance checks to see if the drivers are the same and healthy side to side.
I can show you a minor mod that should better protect the tweeters, but I'd do one and compare to make sure that you like it.
First make sure that they sound the same.
 
Thanks again Pete! I'll check back in again after getting the second one apart. It currently has "silent tweeter disease" and I'm hoping it's not because they've been toasted. I only have room to host one at a time in disassembled condition so it might be a couple of days. I would be interested in the protection mod.
 
The problem with the tweeter XO is that the 60uF has the DC resistance of the woofer across it, that branch
passes highs but also lows and DC. The 2.5 uF has the midrange inductor and midrange across it so it also
passes lows and DC.

The attached mod gives the tweeters a real second order crossover.
The 2.7 is X ed out but of course is required, you might have to increase the value to get enough tweeter output
since we've removed the leakage across it.
The mid is run wide open on the top end - it also is not very well protected with the "leaky" 60uF feeding it.
I read that people really rave about these so I'm trying to keep the changes to a minimum.
RectilinearIII-Xover-Series-PLB MOD.jpg
 
I've also read many positive reviews but have yet to HEAR the dang things. I hope there's enough left of these to get them back up and running as stock so that I can judge for myself. If so, I like the idea of having a little more protection for the tweets as they are said to be relatively fragile, there are so many of them, and they're made of unobtainium! Seems like a reasonable "mod" for potential durability.

Just from the looks of these, they're not what I would be drawn to as they seem to be kind of counterintuitive with tweeters all over the place. I would expect lots of phase cancelling and comb filtering ... maybe that's considered a good thing with these? I dunno ... I typically like relatively simple 2-ways.
 
The 8 ohm resistors on the wiper across the 15 ohm pots limit the resistance to 8 ohms max. That iss not so complicated.
The missing capacitor is necessary to protect the 4 tweeters from low tones. I don't recall the correct value; interesting that you included the inductor values
The woofer and mid circuits are in a series configuration while the tweeter is in a parallel configuration with the woofer-mid circuit. This is something Rectilinear worked out in the nearly 70s for a couple of their models.

I've found a couple of Rect III crossover circuit diagrams on AK; it is a good idea to compare those with the one presented here. As shown, this one has been modified a bit.
 
Pete B said:
I've provided links on here for Visaton paper cone tweeters that I think will work as replacements if you end up needing them.
Click to expand...
I need all 8 tweeters in my recently acquired rectilinear III low boys. I had come across some visaton options myself, and was wondering if anyone had the same idea
 
I haven't tried them yet. Just found them today, and thought I'd come here to see if anyone else was as smart as me. Pricey buying 8 of anything. I might pull the trigger now though
 
EDIT: Just saw "Rectilinear Highboy Tweeter Replacement" thread here and will check it out ...

PeteB, I was able to locate one of the tweeters you mentioned on the Mouser site but they only had one in stock, promising more to come. On paper, this looks like a close fit for the 2.5" Peerless but I couldn't find a 2" that is also needed for the Rectilinear III. There are 2 of each size in each cabinet. So, for a pair of speakers one would need 4 of each size.

HOWEVER, Mouser shows an out-of-stock 2" Visaton cone tweet that looks promising with a delivery date of 09/2026. It claims 1KhZ to 30KhZ 91dB - Mouser Part #243-TW6NG-8OHM. Is this unit one that you were also suggesting?
 
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tgmartin said:
EDIT: Just saw "Rectilinear Highboy Tweeter Replacement" thread here and will check it out ...

PeteB, I was able to locate one of the tweeters you mentioned on the Mouser site but they only had one in stock, promising more to come. On paper, this looks like a close fit for the 2.5" Peerless but I couldn't find a 2" that is also needed for the Rectilinear III. There are 2 of each size in each cabinet. So, for a pair of speakers one would need 4 of each size.

HOWEVER, Mouser shows an out-of-stock 2" Visaton cone tweet that looks promising with a delivery date of 09/2026. It claims 1KhZ to 30KhZ 91dB - Mouser Part #243-TW6NG-8OHM. Is this unit one that you were also suggesting?
Click to expand...
I'm about to order 4 of each to match the original sizes and achieve some of the "virtual crossover" that I think was the original intent of the two types of tweeters
 
Back at the Rectilinear IIIs. Took the baffle out of unit #2 that had no sound from the tweets and found that the 4 of them together in series/parallel show 7.3 Ohms so that's encouraging! The funniest thing is that this unit appears to have ALL of its crossover parts in place, unlike #1 that had the missing 2uf cap ...

MyRectilinearIII-Speaker2_1.JPG

Guess the other one I have was put together on a Friday, or a Monday as the case may be. Now I gots some more 'splorin' and solderatin' to do ...
 
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