Are film capacitors directional?


I'm rebuilding a crossover for some Dali speakers. I'm using a mixture of Sonicaps Gen. 1 and Mundorf Supreme caps.

Anyone know if these caps are directional?

Thanks,
Bryon
bryoncunningham
They are not polarized(as electrolytics are), but- YES; there is a preferred direction, with regards to orientation. Some helpful insights: (http://jimmyauw.com/2010/04/24/observing-inner-and-outer-foil-of-some-popular-capacitors/) (http://www.audience-av.com/capacitors/a_applications.html)
Thanks Rodman. I don't have an oscilloscope. Any other way to measure?

Bryon
Good info, Rodman.

Bryon, if you have a good digital multimeter, which can read AC voltage down into the millivolt region, you could probably use that instead of an oscilloscope. Note in the jimmyauw writeup that the amplitude scale on the scope displays is apparently 5 mv per major division (there being 8 major divisions on the entire vertical axis of the display).

Best,
-- Al
Also, Bryon, if you use a battery powered DMM instead of an AC powered oscilloscope, it may be that instead of using your hand as the "signal source" you might have to drape an AC power cord (that is conducting current) over the capacitor. The 50 Hz waveform that is shown in the scope photos (he's undoubtedly in a 50 Hz rather than a 60 Hz country!) is probably contributed to by leakage between the scope's AC input and the ground of the probe, that is in turn connected to one end of the capacitor.

Best,
-- Al
Thank you, Al. My multimeter doesn't go to AC millivolts. I will borrow a better meter from a friend. Please forgive my ignorance, but once I have the meter, what exactly do I do?

bc
Please forgive my ignorance, but once I have the meter, what exactly do I do?
There's no ignorance to be forgiven, aside perhaps from my own :-)

Thinking about it a little further, I believe that to get meaningful readings with a battery powered DMM you MIGHT have to provide it with an AC ground reference. That could be done by connecting a piece of ordinary hookup wire between the screw on the wall plate of an AC outlet, and the capacitor lead to which the DMM's "common" terminal is connected. I'll describe the procedure as if that wire is necessary, although I'm not completely certain that it will be necessary.

What you would do is as follows:

Set the meter to read AC voltage, on a fairly sensitive scale that would resolve a millivolt or so. If the meter has a "LoZ" (low input impedance) provision, disable it. Connect the meter's com (common) input lead (or whatever the equivalent nomenclature may be on the particular meter) to one end of the capacitor. Connect the wire from the wall plate screw to that same point, perhaps holding it in place with an alligator clip that may be provided with the meter, or that may be attached to the end of its lead. Connect the other lead from the meter to the other end of the capacitor.

Put your hand on the capacitor, and see if you get a reading. If not, drape the AC power cord of some powered up electrical device across the capacitor, and see if that gives you a reading. Note the value of the reading.

Remove the three connections from the capacitor. Connect the com lead of the meter and the wire from the wall plate screw to the opposite end of the capacitor than the end they were originally connected to. Connect the other lead of the meter to the opposite end of the meter than it was originally connected to. Repeat the voltage measurement.

The end of the capacitor that was connected to the com lead and the ground wire when the lower of the two measurements occurred is connected to the capacitor's outer foil.

Best,
-- Al
Hey Al....once again, thanks for your valuable input on all technical subjects. I don't mean to sidetrack this thread - but since audiogon disabled the ability to communicate otherwise I'd like to ask & hopefully this is valuable to others as well...is it worth buying a really quality grade multimeter? I've used a $20 Radio Shack digital multimeter for years to bias my amps & often wondered if I'd get different readings from a better grade meter?
Hi Pehare,

Thanks! I don't know the answer to your question, though, as I haven't ever used a RS DMM. FWIW I did use a RS FET-input (high input impedance) analog multimeter many years ago, and it seemed to work well, and lasted a long time.

If the opportunity arises, perhaps you could borrow a Fluke or other good meter from someone, and compare readings.

Believe it or not, the only meter I've had here in recent years is a not particularly accurate Triplett analog multimeter, supporting both my audio and antique radio endeavors. I do have a low end oscilloscope, though. I'll have to get myself a good Fluke DMM at some point, probably one that also measures capacitance and perhaps inductance as well.

Best regards,
-- Al
Thanks, Al! Very helpful. I bought a better meter today, so I will try the procedure you described tomorrow and report back.

Bryon
Okay, Al, I think I'm following your instructions exactly, but I'm not getting consistent results. I've tried several different capacitors, and I get a different reading every time. Don't know what I'm doing wrong...

-I checked the outlet and it is definitely grounded.
-I've drained the capacitors by connecting a wire between the two leads and then taken readings.
-I'm using clips to ensure that I have a good connection.
-I've draped power cords from 2 different devices over the caps.

...and still I get constantly changing results. Some of the time, the meter WILL come to rest on a specific number. But when I remove the meter's leads and check again, the number is different. Sometimes the number is positive, sometimes negative, even for the SAME end of a cap. I'm confused. Any ideas?

Bryon
Hi Bryon,

Are you using a setting for AC (not DC) volts? Not sure how you could sometimes be getting a negative number for AC volts.

Also, charge that is stored in a capacitor can sometimes "bounce back" to some extent after a connection that has been used to discharge it has been removed. That can occur as a result of what is known as dielectric absorption. So it would be a good idea after discharging the cap to use the meter to verify that no DC voltage exists across it.

Beyond that, it may be a good idea to tape the power cord in place on the capacitor, to be sure that its exact position relative to the capacitor doesn't change as the leads are being removed and applied. And also try to keep the test leads as far away as possible from the power cord.

Also, let us know the make and model of the meter, and I'll try to take a look at its description, which might give me some further ideas.

Best,
-- Al
Are you using a setting for AC (not DC) volts? Not sure how you could sometimes be getting a negative number for AC volts.

I had it set for DC. My only defense to the charge of idiocy is that the meter is new to me as of yesterday, and I accidentally threw away the instructions with the packaging, and the manufacturer's website has been down all day. Even so, I find myself guilty as charged. I am an idiot.

The meter is a Gardner Bender GDT 3190. I don't think it's sensitive enough to perform this test. So I need to go out and buy another meter. For the love of...

bc
At the defense electronics company where I spent my career, in the forms that were used to report the results of investigations into test failures or other equipment problems there was a failure category entitled "operator error." That category was frequently applicable, despite the strong qualifications of those performing the tests :-)

I can't get through to the Gardner Bender site either, but it seems evident looking at the photo that for AC volts only 500V and 200V scales are provided. I don't know how many digits are provided, but yes, it seems a safe bet that the meter isn't sensitive enough.

Being a dedicated audiophile isn't easy ...

Best,
-- Al
Operator Error? Write 'em up.

I wrote more PDRs than the law allows. (Process Deviation Report) and even dinged some supervisors who were NOT certified on the process they screwed up.
If you plan on getting serious about your electronics: Bite the bullet, and buy a Fluke(http://www.ebay.com/itm/Fluke-114-True-RMS-Multimeter-/250994535268?pt=LH_DefaultDomain_0&hash=item3a7070ab64). The 114 will resolve down to 0.1mV. I've had my Fluke 6020B forever(well, way over 25 years anyway), and never had the first problem with it. Of course; now Fluke is made in China also, BUT- automatically selects for AC/DC =;^) (http://support.fluke.com/find-sales/Download/Asset/2634055_6116_ENG_B_W.PDF). I just bought the same one for my son at Grainger, and BOY did I get screwed on the price. Double what they sell for(new) on eBay(he was in a hurry, and it was the weekend). This one is more versatile, also reading mA(nice, if you get into tubes), and capacitance: (http://www.dealextreme.com/p/fluke-17b-2-6-lcd-digital-multimeter-meter-2-aa-included-28659)
2x on the fluke.
We had one at work....model# unknown, but we had to reliably measure 8mv with several digits left over.....and this one did with great accuracy and repeatability......Our spec was 8mv +-.001
Hi Al,

I borrowed a Fluke 115 from a friend. Here is the owner's manual. I tried the procedure again, and again I got confusing results. I think I should seek medical assistance for a severe multimeter learning disability.

At the risk of testing your considerable patience, can you try to walk me through this?

Bryon
Hi Bryon,

I looked through the Fluke 115 literature. The one notable thing I found is that I think you should perform the measurement using the "AC Volts" mode, not the "AC Millivolts" mode. The meter's input is AC-coupled in the "AC Volts" mode, meaning that any DC that may be present (which might otherwise confuse the measurement) will be blocked in that mode. "AC Millivolts" is DC-coupled, which is undesirable for what you are trying to do. The "AC Volts" mode can resolve down to 1 mv, which should be more than adequate.

On the Model 115 the "AC Volts" mode corresponds to the large rotary mode select switch being at the position that is immediately to the right of the "Off" position.

Also, given my uncertainty about the need for connecting the ground wire to an outlet screw, try it both with and without that wire, if necessary and if you already haven't.

Hope that helps. Best,
-- Al
Since Al's valiant effort to help me determine the directionality of these caps has failed to produce an answer, I emailed Parts Connexion, who sold me the Mundorf caps. They said this...

Regarding the caps...while caps aren't polarized.....some people "feel" they sound better, if the outer foil is connected to the INPUT side....and the input side is on the left, when reading the case printing from left to right.
.

But this seems to contradict what I read on the auricap website, which said...

The idea is to always have the outside foil connected to the lower impedance to provide outside foil shielding to noise. Circuit OUTPUTS are always lower impedance than inputs and should be connected to the outside foil.

So does the outer foil connect to the INPUT or the OUTPUT?

You can see a picture of the crossover I'm building, prior to soldering, here. The + outputs, which haven't been added yet, are marked "OUT" in red. Assuming the outer foil is connected to the LEFT lead of each Mundorf cap in the photo, do I have the caps oriented correctly?

Thanks,
Bryon
Hi Bryon,

Yes, the Mundorf's are probably oriented correctly. The two wordings are consistent, although confusing. The lowest impedance in this context is the output impedance of the amplifier. So by orienting the outer foil of the capacitor towards the input of the speaker, which is the same as orienting it towards the output of the amplifier, you are orienting it towards the lowest impedance point.

The reason I say "probably" is that if the values of R2 and R4 are particularly high, conceivably the lowest impedance path to the amplifier's ground could be through the corresponding driver and its interconnected crossover components. What are the values of R2 and R4?

Best,
-- Al
YES- in the photo; you have the Mundorf outer foil leads connected to the x-over's inputs, which is correct(for a series circuit). Reread point three, of the auriCAP advisory, regarding connection in a x-over circuit.
You owe me a Coke, Al. We must have hit, "enter" simultaneously!
Thanks guys.

R2 = 10W 3.3Ω
R4= 5W 3.9Ω

bc
3.3 and 3.9 ohms seem low enough to be unlikely to change the stated conclusions.

Rodman, I'll make it a beer rather than a Coke. Next time you're in Connecticut!

-- Al
Thanks, Al!!
Hehe....Next time? OK; you're on! Happy listening guys!
Just to add my experience, I've found it much easier to discern the orientation of the outer foil on film caps using TrueRTA spectrum analyzer software on my laptop. The most expensive version of this software (1/24 octave resolution) is only $100, lower resolution versions cost less.

I use the line in connection on the laptop with a simple cable consisting of a mini jack on one end and bare wires for signal and ground on the other, then watch the 60Hz peak on the spectrum analyzer go up or down depending on cap orientation. I also own a Tektronix 20 MHz scope and the TrueRTA method works much better for me.