Acoustic Research AR90 recap?

Hi,

I hope someone can help with my crossover questions? Specifically, I have original Acoustic Research AR90's, bought directly from the AR factory, when I once worked at a Teledyne company. They're in very good shape. I've refoamed the woofers and the lower midranges. But, the cabinets need a bit of a clean-up, and I'd like to recap the crossovers, being that the speakers are around 35 years old.

Now, my questions. Are certain crossover caps more important in regards to sound than others? Series caps more important than parallel? Tweeters, over mids, over woofers? I want to use decent parts, but I am admittedly on a tight budget. I want to change what counts, using the best that I can afford, and the rest with 'good enough', if there's little sonic impact? Lot's of NPE caps in my 4-way 90's: 4, 6, 8, 24, 40, 30, 80, & 350uf, so I want to budget my money where it'll make the most difference. I'll attach the crossover schematic, for illustration.

Oh yea, the 4, 6, 24, and 80uf caps are the caps wired in series to the drivers. Are they the most important? And should I use only capacitors of the same brand, for consistency in sound? I've looked at Dayton, Audyn, Jantzen, Sonicap Gen 1, Mundorf MCap EVO, and others, but these seem to get consistently good reviews, and aren't 'too' expensive. But, I've been driving myself crazy over this, and just need some guidance, and a 2nd opinion!

Thanks for any suggestions!

Stimpy


AR90 post-101175-1200137398.gif


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eriksquires

Member
2013-05-10 4:11 pm
Hi Stimpy,

Series caps are more important, but I've had reports of parallel cap upgrades also being worthwhile. In terms of overall longevity, film caps are also ridiculously stable compared to electrolytic caps.

My usual suggestion when some one wants to experiment for themselves is to try a Mills Resistor and Mundorf MKP cap upgrade. They're both very good, but on the budget side.

I would try the series resistor in the tweeter, both of the series caps (4uF and 6uF) to start with. If you replace them and hear no difference then it's a good time to stop. You may have to get parallel caps to get the exact values you want. Like 3.3uF + 0.68uF to get the 4uF values, or 3.9uF + 0.1uF or some other combination. Just make sure you wire them in parallel and keep to the same brand/type.

Best,


Erik
 
Hi Stimpy,

Series caps are more important, but I've had reports of parallel cap upgrades also being worthwhile. In terms of overall longevity, film caps are also ridiculously stable compared to electrolytic caps.

My usual suggestion when some one wants to experiment for themselves is to try a Mills Resistor and Mundorf MKP cap upgrade. They're both very good, but on the budget side.

I would try the series resistor in the tweeter, both of the series caps (4uF and 6uF) to start with. If you replace them and hear no difference then it's a good time to stop. You may have to get parallel caps to get the exact values you want. Like 3.3uF + 0.68uF to get the 4uF values, or 3.9uF + 0.1uF or some other combination. Just make sure you wire them in parallel and keep to the same brand/type.

Best,


Erik

Thanks Erik,

That helps a lot. And also reinforces most of what I've been reading. So, I'll at least spend for good 4, 6, and 24uf caps (Mundorf MKP or Sonicap Gen I), and probably parallel caps for the 80uf. If that goes well, I can spring for the other parallel shunt caps next. Maybe I'll splurge on those 60 cent Madisound poly caps, and go all out!

I already have Erse NPE's for the 350, not much choice there. Plus, I picked up 20 Russian paper in oil 0.01uf 1000v caps. And now that I think about it, I have 8 0.1uf 600v Auricap's floating around here somewhere too. I can try those as bypass caps. I'll even try some cascoded bypass combinations and see how that sounds as well.

Thanks again, and take care,

Stimpy
 
I have AR92s in storage (indoors) and wonder if the dome tweeters and mids will work properly when I hook them back up. How are yours sounding?

The only thing that would worry me about your 92's and my 90's, is that both use ferofluid in the mids and tweeters. KEF speakers seem to have a lot of issues with aged fluid gumming up the voice-coil, and I just wonder if that could be an AR issue too? But thankfully, the guys over at the Classic Speaker Pages say it shouldn't be a problem. AR used a different formula of fluid.

When I last played my 90's, they still sounded awesome. But, I have to admit, that was at least 6 years ago! Moving, and building a new home, got in the way. That's why I want to recap before I fire them up again. Then I'll probably start at a very low level, to let the new caps charge, and the drivers warm up a bit, before cranking.
 

TMM

Member
2007-09-01 8:37 am
Australia
I would recommend you get yourself an ESR meter. If you search ebay for "Mega328 ESR meter" you will find bare board meters for less than $15 which will do the job.

Measure the existing caps out of circuit (disconnect one leg before measuring and check them against an ESR chart - google images: "capacitor ESR chart"

If any have exceedingly high ESR or the capacitance is way off them replace them. If they measure OK, then leave them in. Replacing them with new electrolytics is fine - AR likely factored the typical ESR of electrolytics into the crossover design so if you replace them with film caps (with ultra low ESR) you may get a slightly different response to how AR designed them.

Small value (<0.1uF) film bypass caps have been proven not to make any audible difference - you only get the 'benefit' of low ESR through the audible frequency range if you replace the entire capacitor with a film version and this may not always result in a desirable outcome - if AR designed for a cap with ESR=1ohm and you replace it with a cap with ESR=0.01ohm then the frequency response is going to change for better or for worse. The difference between a healthy electrolytic and a film cap is usually only a fraction of a dB at best. Can it be audible? yes if that small change in level spans a wide frequency range, but it is only a frequency response change so it is not 'better', just different. Non-linear distortion from electrolytics is typically much lower than the non-linear distortion from the speaker drivers themselves.
The typical scenario is that people replace an old tired electrolytic (ESR>10ohm) with a film cap (ESR<1ohm), suddenly their tweeter level goes up 3dB and they praise the 'clarity' of the film caps. If they just replaced the old electro with a new one they'd get the same effect.

Using different brands of caps doesn't matter. Film caps or quality electrolytics will have better longevity than cheap electrolytics. Even a cheap electrolytics should give 10+ years service under normal use.
 
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TMM is absolutely correct regarding replacing electrolytics with film caps. The best option is to replace the existing BPE's with the best BPE's currently available. I would expect some deterioration of the originals after 35 years and replacing them with new bipolar electrolytics shouldn't break the bank. I used to lust after those speakers back in the day!
 

eriksquires

Member
2013-05-10 4:11 pm
Well, I guess if I have nothing better to do than to take caps out, measure them and put them back in that makes sense. However, caps are cheap. If you are going to go through the trouble of measuring them you might as well replace them with caps that will last another 4 decades rather than spend the same amount of money on a meter, say I! :)

Plus, this way Stimpy will have his own ears to judge the value of capacitors. Those who know there is nothing to find will invariably always find nothing. :)


Best,


Erik
 
Well, I guess if I have nothing better to do than to take caps out, measure them and put them back in that makes sense. However, caps are cheap. If you are going to go through the trouble of measuring them you might as well replace them with caps that will last another 4 decades rather than spend the same amount of money on a meter, say I! :)

Plus, this way Stimpy will have his own ears to judge the value of capacitors. Those who know there is nothing to find will invariably always find nothing. :)


Best,


Erik

My Hero,

Thanks for that, and I totally agree. While I could measure everything, why should I? The parts are suspect, so why the extra effort, when I know I want to change the caps regardless. And while I do believe in measurements, as a minimum standard, measurements still can't predict sound quality. I trust my ears for that. They're well trained, both professionally and domestically, and they know what to do.

Take care, and Thanks for all the responses!

Stimpy