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Old 2nd March 2013, 09:59 PM   #11
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Thanks again Steve.
I'll try a few things and see how I go. I'll definately lose the tank capacitor on the woofer first. (The brochure suggests the 18s and 38s tweeters are ferro-fluid cooled, and I can't see evidence that any has leaked.)
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Peter
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Old 3rd March 2013, 09:35 AM   #12
system7 is offline system7  United Kingdom
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Ferrofluid oil tends to dry out and turn solid (the ferrite particles) more than anything. That makes sense if you think about it, because it's working at high temperature at parties which shorten its life.

We can only hope for the best.

You don't really have many options with the AR-38S. I think harsh bass cone-breakup is always going to be the problem. But I do want to hear how it goes.

My college friends often had the lively AR-6, whereas I was more into the dull smoothness that was bextrene plastic cones or B&O electrically filtered paper cones. Nothing is perfect.
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Old 1st April 2013, 09:21 PM   #13
system7 is offline system7  United Kingdom
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I spent some more time on the AR-38S, Graveyboat. It was tougher than I expected.

Unless you add Inductance to the circuit, which will make it bassier, you really can't do much with it.

The 6uF 3kHz Tank does a good job of rolling off the bass and aligning phase IMO. But it was deeper than necessary, and in fact only the non-polar capacitor resistance stops it being a huge pit. I found adding a 3.3R resistor made it smoother. But for all that, the mod might damage phase alignment. I don't know if AR's circuit is brilliant or stupid! Without the tank, you get quite a peak at 4kHz anyway.

There is also the possibility of doing a second order tweeter without dropping the impedance much at the top end. Say 5 ohms goes to 4...worth a go, I'd think. The tweeter ought to sound smoother. Hope that helps.
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File Type: png AR-38S_PossibleModifications.PNG (6.3 KB, 94 views)
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Old 2nd April 2013, 04:30 AM   #14
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Hi Steve. Thank you for the effort you have put in. I thought about a 2nd order for the tweeter, but decided to do this: I removed the tank cap completely (ignoring the phase shift). I then included a 1.5ohm 10watt resistor in series with the tweeter, increasing impedance, and changed the cap to 4.7uF. The caps are nice polypropylene ones made in the UK, which sound quite smooth. I also lightly doped the tweeters with some polyurethane. The crossover point shifts slightly and the tweeter is down about 2db. Resistor is inductive, but I can only hear to 12K, so not much difference there. The bass then sounded a little bloated, but this was remediated by de-coupling the speaker on stands, away from the wall. The stands just sit on the carpet, hence the decoupling. I actually think the best move was buying good quality MKP caps. The last replacement ones were stated to be polyprop, but the sound was so harsh I expect they were polyester, and poorly designed at that. So, overall I now have a quirky sound I can live with, but it was always going to be a compromise, wasn't it?
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Old 25th June 2013, 09:35 PM   #15
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Hi guys. I just bought a set of AR38s 1981 speakers that have a little damage on the outside and the 10" woofers need refoaming. I have started the refoaming process but I was thinking if I should take this a step further and try to improve their sdesign in any way....
I have been looking online for a picture or drawings of how they are constructed and how they look on the inside so that I can know more before I make up my mind what to do, but I have not found much of any use. I am a noob in speaker refurbishing but I have a lot of professional experience in making and refurbishing furniture. So Im good with my hands and have to tools but am not very knowledgeable about the science of making speakers.
Can you give me some advice or maybe some links that would help ?
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Old 28th June 2015, 03:07 AM   #16
mress is offline mress  United States
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Sorry about the thread necromancy... this thread is the Web's best information source about the AR38s speaker!

I recently trash-picked a pair of AR38s. After re-foaming them they play pretty nicely. I really like that fast sealed-box bass sound. They're kinda shouty.

I hate notch filters, just on aesthetic principle. I decided to go ahead and replace the crossover with a 2nd order Bessel filter at about 1800Hz to see what happens. The steeper attenuation should help reduce the woofer's cone break-up...

To help anybody else in my situation, here's the values I (crudely) measured from my AR38s drivers:

Tweeter Impedance: 6.68 Ohms, no resonance behavior above 700Hz. Thanks, ferrofluid.

Woofer Impedance: 8 Ohms up to about 350Hz and rising at higher frequencies

Woofer DC Resistance: 6.3 Ohms
Woofer frequency where impedance is twice DC resistance: About 1250Hz
(I think this is a good stand-in for the frequency used in Rod Elliott's paragraph about Zobel network calculation at Passive Crossover Network Design)

I plan to put a variable L-Pad on the tweeter so I can tone it down if it ends up too bright. I'll post back with my impressions after the crossover replacement.
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Old 28th June 2015, 07:31 PM   #17
system7 is offline system7  United Kingdom
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I'd forgotten about this thread!

FWIW, I don't think those AR cone tweeters had ferrofluid. Just impressively flat on a 6uF capacitor from 1kHz.

If it was me, these days, I'd try a circuit like below. Tweeter coil between 0.45 and 0.6mH.

10" bass likes that 3kHz notch. Leak used it in the 16 ohm sandwich speaker. The capacitors are 1,8 and 16uF. The resistor is 22 ohms. The coil will be 3mH I'd guess.
Attached Images
File Type: png AR-38S Trial Circuit.PNG (7.0 KB, 13 views)
File Type: png AR-38S Rough Sim with better crossover.PNG (24.2 KB, 13 views)
File Type: jpg LEAK Sandwich Construction.JPG (41.5 KB, 11 views)
File Type: jpg LEAK Sandwich Crossover 1 8 16uF.JPG (46.6 KB, 11 views)
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Old 28th June 2015, 08:19 PM   #18
mress is offline mress  United States
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I can confirm that at least one of these AR tweeters has ferrofluid in it.

One of my tweeter cones had come unglued, so I partially disassembled it while re-gluing. There's ferrofluid in there -- it's old and thickened but I didn't bother to clean it out or replace it. I also didn't bother to open the other one, which was still firmly glued together.

I'll have a think about that schematic you posted. Thanks a bunch!

Last edited by mress; 28th June 2015 at 08:31 PM. Reason: Remove inaccurate "quote" tag
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Old 28th June 2015, 08:37 PM   #19
system7 is offline system7  United Kingdom
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Hmm, dried out ferrofluid is NOT GOOD really.

It's just lumpy brown ferric oxide gunge that sits in the gap spoiling the sound after the oil has dried out. Bit like the magnetic stuff they used to make audiocassettes out of.

The phenolic ring tweeter claims to be a good replacement:
Phenolic Ring Tweeter Replacement for AR-4X CTS Marantz and More 8 Ohm

All I did with that modelling is add a Zobel and a tweeter coil really. Ought to get more rolloff. Have a play with Boxsim to get some idea of what does what.

Once you get a circuit that works, you can tune it by ear. Our own speakerdave did a similar sort of thing to the AR-4X, and he knows what he's doing.
Attached Images
File Type: jpg Acoustic Research AR-4X crossover.JPG (30.4 KB, 6 views)
File Type: png Acoustic Research AR4X Mod.PNG (45.0 KB, 6 views)
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Last edited by system7; 28th June 2015 at 08:45 PM.
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