I am working on a pair of AR3a's that will need crossover caps, and I have a question on replacement type. Hopefully this isn't a very tired old topic or will start an opinion battle - but I think the fundamental question is fairly simple....
In place of the 150uf (and instead of a NP electrolytic), is it acceptable to use a Polypropylene Film Motor Cap? These are AC rated - which I understand has to do with duty rating and conditions under AC power more than anything. But would this be suitable for use here? I understand that replacing caps in a cross-over is not just simple value matching - and that considerations for changes to the overall circuit inductance need to be accounted/adjusted for - but if these considerations were made, is there a technical reason this is not a good option?
In place of the 150uf (and instead of a NP electrolytic), is it acceptable to use a Polypropylene Film Motor Cap? These are AC rated - which I understand has to do with duty rating and conditions under AC power more than anything. But would this be suitable for use here? I understand that replacing caps in a cross-over is not just simple value matching - and that considerations for changes to the overall circuit inductance need to be accounted/adjusted for - but if these considerations were made, is there a technical reason this is not a good option?
An externally hosted image should be here but it was not working when we last tested it.
I don't see a problem using that motor cap. Do you have a pair already you can use? If not, I wouldn't spend a bundle purchasing any. There are some 150uF NPE's you can buy cheap that will do the job fine.
It's just a woofer LP cap and won't have any noticeable effect on the sound just because it's a film cap.
It's just a woofer LP cap and won't have any noticeable effect on the sound just because it's a film cap.
Thanks - that helps. The motor caps are running about $7 a piece.
So the LP cap is 150uf, the Mid cap is 50uf and the tweeter cap is 6uf.
Maybe I will stick with tradition and go with NPE 150 and 50 and a good film for the tweeter.
Any recommendations on audio-grade NPE's?
BTW - I did test the originals and they have drifted considerably, and are leaky.
So the LP cap is 150uf, the Mid cap is 50uf and the tweeter cap is 6uf.
Maybe I will stick with tradition and go with NPE 150 and 50 and a good film for the tweeter.
Any recommendations on audio-grade NPE's?
BTW - I did test the originals and they have drifted considerably, and are leaky.
Those original AR caps were made by the Chicago or Illinois Capacitor company IIRC. I'm not too sure what they were exctly. Perhaps foil and oiled paper.
But there is a pitfall in putting modern film caps to replace NP electrolytics in general. That is that bass often needs some resistance in an LC filter to avoid peaking and get good phase alignment. Non Polars have a sort of ohm or so built in. I've never successfully modelled any of those old filters without adding series resistance to the capacitor arm. Whether you'll really hear it, I don't know. But just sayin. I'd expect a generally brighter sound with modern film types. 🙂
But there is a pitfall in putting modern film caps to replace NP electrolytics in general. That is that bass often needs some resistance in an LC filter to avoid peaking and get good phase alignment. Non Polars have a sort of ohm or so built in. I've never successfully modelled any of those old filters without adding series resistance to the capacitor arm. Whether you'll really hear it, I don't know. But just sayin. I'd expect a generally brighter sound with modern film types. 🙂
Yes - I've read that too - and loosely my concern above about impacting the overall circuit inductance/impedance. I understand the recommendation is to add a ~.3ohm/10 watt non-inductive resistor in series with the 6uf tweeter cap - and a ~.3ohm/20 watt non-inductive resistor in series with the 50uf mid cap.
Bennic and ERSE sell very good NPE's. Their D.F. is usually <=5%
I've worked on many 3a's. Over time, the cap mfgr's changed. The tweeter cap stayed the same (Chicago Industrial), but the mid and woofer caps changed at least 3 times. If you find any with Sprague Compulytics, they don't need replacing. I've tested many of these and they are remarkably stable in the uF and ESR properties.
IMO, the original 3a tweeters could use some brightening up. Therefore, I don't recommend adding series resistance.
Thanks - I'll check with my friend who owns these whether he wants the East coast sound (no series resistor) or West coast sound (add a series resistor). 🙂
The cross-overs I'm working shown below. These are late '69 AR3's and use the large dual box cap (oil?).
The cross-overs I'm working shown below. These are late '69 AR3's and use the large dual box cap (oil?).
An externally hosted image should be here but it was not working when we last tested it.
I don't think the absence of the series resistor with the 6.0 uF cap will make the 3a a west coast sounding speaker. Just a bit more open and most likely, much like they sounded when new. It is well known the original 3a tweeters lost efficience over time.
The 50/150 cap in your pic is constructed pretty much the same as the 6.0. IIRC, its a paper/wax construction.
Getting rid of those 'boat anchor' cap blocks will make the speaker noticeably lighter.
The 50/150 cap in your pic is constructed pretty much the same as the 6.0. IIRC, its a paper/wax construction.
Getting rid of those 'boat anchor' cap blocks will make the speaker noticeably lighter.
Understood - the W/E comment was my attempt at humor... but that is an excellent tip.
The measurements of the originals are as follows:
150uf: 303uf, ESR 1.5
50uf: 97uf, ESR 1.5
6uf: 16uf, ESR 1.8
Interestingly, there is a reading of 68uf between sections (green and black) of the 150/50uf cap. Blue is common. All of my measurements were with all wires disconnected.
Reference: http://www.classicspeakerpages.net/library/acoustic_research/original_models_1954-1974/original_models_schematicss/restoring_the_ar-3a/restoring_the_ar-3a_full_pd.pdf
The measurements of the originals are as follows:
150uf: 303uf, ESR 1.5
50uf: 97uf, ESR 1.5
6uf: 16uf, ESR 1.8
Interestingly, there is a reading of 68uf between sections (green and black) of the 150/50uf cap. Blue is common. All of my measurements were with all wires disconnected.
Reference: http://www.classicspeakerpages.net/library/acoustic_research/original_models_1954-1974/original_models_schematicss/restoring_the_ar-3a/restoring_the_ar-3a_full_pd.pdf
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Yup, you've got some real BAD boys there!
Those ESR's are very high. Talk about 'laid back' NE sound! 🙂
Those ESR's are very high. Talk about 'laid back' NE sound! 🙂
A muffled mess I'm sure.
Another question...
It looks like others have elected to leave the original box cap in place during cap replacement. I assume that is desirable so as not to change box volume. The box caps in these cabinets are clamped and secured with foam, which is deteriorating. I'm thinking eventually it may deteriorate to the point that the cap is loose and possibly will rattle. Maybe I should simply remove, replace the foam and reinstall?
Another question...
It looks like others have elected to leave the original box cap in place during cap replacement. I assume that is desirable so as not to change box volume. The box caps in these cabinets are clamped and secured with foam, which is deteriorating. I'm thinking eventually it may deteriorate to the point that the cap is loose and possibly will rattle. Maybe I should simply remove, replace the foam and reinstall?
An externally hosted image should be here but it was not working when we last tested it.
Every 3a I've worked on with those boat anchors - they have been tossed out. I install new fiberglass. About 1.6 lbs/speaker IIRC. If you can measure Fc, it shoud be about 40 hz after re-stuffing.
I don't agree with the worry worts who obsess over the box volume. Having a little more FG behind the woofer is an improvement anyway. It helps dampen the upper bass/lower mid freq. reflections within the box.
I don't agree with the worry worts who obsess over the box volume. Having a little more FG behind the woofer is an improvement anyway. It helps dampen the upper bass/lower mid freq. reflections within the box.
Ok - if I have time, I may experiment. I have the Smith & Larson Woofer Tester 2 but I'm not sure I can measure box Fc with that... need to check.
Yes you can! That's exactly what I use. Just hook up the test leads to the terminals at the back of the speaker. The Fs reading will be your Fc reading. The Qts reading will be your Qtc reading. My Qtc's range from 0.7 to 0.9 in 3a's.
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