Dave Bullet said:
I would setup a LP filter with a resistor and the capacitor. Set the HP point at say 2 kHz or so. Put in a 100 Hz. tone at a very low level and look at the voltage spectrum across the cap. An electrolytic is likely to show distortion products from the zero crossing change in capacitance. This is the problem that people talk about with these types of caps. A film cap should not show anything.
I use old fashioned techniques. For two caps, I'd make a bridge with a cap in each leg. Balance it. Drive it with the frequencies of interest and look for anything other than the fundamental in the bridge residual. For a single cap, put it in an old commercial bridge (various GR units, depending on the cap value). These used polystyrene or mica for reference caps. Again, drive the bridge, balance it and look for anything other than the fundamental in the residual signal. Inability to get a deep null is indicative of distortion and shows up harmonics in the residual. For practice in what to look for, check any non-C0G ceramic!
Various methods have been devised to skin cats, some better than others. The bridge method does definitely work, though you won't see the difference between Teflon and polystyrene. I do have both PC-based and hardware SA capabilities, but sometimes I find just looking at a distorted residual, the output of a THD meter for example, is sufficient to my purposes. If I see mostly second harmonic, I go happily on my way. If there's something more complex, then the SA is the way to go.
Laziness my good fellow, laziness. I always have several bridges and a scope set up on the bench, but SA requires dragging one out or setting up the computer for that particular task. Hmmm... I wonder how much progress in how many things has been hindered by man's skill at taking the easier path?
gedlee said:
If not visable, these kinds of "aberations" will be audible? That's my point.
Here's the most simple test: use electrolytics in a crossover and listen. These caps are cheap and it would not be a big deal to swap in better ones.
I'm not a member of the better cap, better sound tribe. I still don't use electrolytics, thinking my projects deserve at least decent poly caps (I use Solen metalized poly). These are relatively cheap and I assume that I can count on their performance.
As for equipment, I have as much as I need for now - dedicated lab computer with a good quality soundcard, speaker testing jig (with built in mic preamp amd high quality power amp) that I use with Speaker Workshop and Arta. I have used Right Mark Audio Analyzer more than a few times, some of the results posted here in some of my threads, including a distortion and performance test on a next-to-no-cost equalizer here. More than a few caps in that unit.
gedlee said:
By "aberation" do you mean distortion? If so, you are being a little inconsistent in your message to us. On one hand (in another thread) you say up to 25% distortion is undetectable yet here you can differentiate between two caps, as little as 1%.
Or should I take the comment above as an offhand dismissive remark?
MJL21193 said:
You should carefully reread what I write because you obviously did'nt understand it.
Up to 25% distortion in a loudspeaker, typically low order, was not audible.
.1% crossover distortion, as found in a poor cap or amplifier, is highly audible. The % THD numbers are meaningless, its the type of distortion that matters.
You should make sure that you correctly understood before accusing someone of being "inconsistant".
Thats why I used the word "aberation" and not "distortion", although "distortion" would still have been correct. "Harmonic distortion" would NOT have been correct.
SY said:
An electrolytic cap requires a voltage across it to polarize the electrolyte. The electrolyte doesn't function the same non-polarized as it dooes when polarized. In a cap this means that as the voltage swings through zero, it goes into a state where the capacitance is changing. Actually the capacitance is constantly changing from the voltage effects, but in a non-polarized cap the major effects can be cancelled by placeing two caps in opposite polarities, but there are minor effects that do not cancel exactly as the voltage swings through zero. This will create distortion whose percentage will rise as the signal level falls. This is the most audible form of distortion.
By apply a constant polarizing voltage across an electrolytic cap, like JBL and others do, you can have an inexpensive cap that has a very stable capoacitance. This is a cool trick.
Cloth Ears said:
I hate to keep bringng this up over and over, but this is NOT true. It all depends on the type of distortion. 25% 2nd order distortion IS NOT audible. One simply cannot say that X% is audible and Y% is not because there is no correlation bewteen % THD and audibility. Read my papers (posted on my website). This has been completely confirmed by everyone who has looked at the problem. Its not even controversial anymore.
OK, my apologies for being a bit thick, but... you're saying that if I use a bipolar electrolytic (the only kind I ever see in crossovers) as a series element in a high-pass filter, I should be able to measure zero-crossing distortion? At what sort of voltage level? Suggested frequency with respect to to the fo of the high pass?
Frankly, I've never seen it and I'd like to look. I've got pretty decent measurement capability.
Frankly, I've never seen it and I'd like to look. I've got pretty decent measurement capability.
I have not read your articles on your web page, but I did a lot of listening and testing. I have testing cd, which has series of signals, sine waves, which go from 0% distortion up, for both even order distortion, just second harmonic, and other series has odd order distortion, just 3rd, and later mix of higher odd order harmonics. You know, one of those audiophile testing cd's.
I can clearly detect 3% second order distortion. /That why I like my sweet magnavox se el84 amp./
When it comes to higher degree odd harmonics, I can pick it up at about 0.3-0.5% as buzzing.
Now common 25% second order distortion IS NOT audible?
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