Thanks Mooly,
What about the replacement? Same (do they still make them after 41 years) or modern?
Mike
What about the replacement? Same (do they still make them after 41 years) or modern?
Mike
There is a very detailed series of reports on capacitors including Polarised and bipolar.
Both of these types show the very small amounts of distortion generated by the capacitor when it was used as a filter of the test frequency.
The report goes on to show a reduction when both types are used in the back to back format indicated in Mooly's diagram.
If your crossover is using the capacitor as a filter then use the back to back for lowest measurable distortions.
But when using capacitors as AC coupling to block DC voltages then if you arrange for the capacitor values to be big enough such that the capacitors do not act as a filters for the signal/s you need to pass, then the very small distortions become unmeasurable.
Both of these types show the very small amounts of distortion generated by the capacitor when it was used as a filter of the test frequency.
The report goes on to show a reduction when both types are used in the back to back format indicated in Mooly's diagram.
If your crossover is using the capacitor as a filter then use the back to back for lowest measurable distortions.
But when using capacitors as AC coupling to block DC voltages then if you arrange for the capacitor values to be big enough such that the capacitors do not act as a filters for the signal/s you need to pass, then the very small distortions become unmeasurable.
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The manufacturers seem to have assumed the same. I would expect them to know their product better, than apply the wrong labelling.
I've spent a couple of minutes looking for a reference to polarised plastic film capacitors. None found.
Some types like MKT & MKS have a small polarised effect, whereas others like MKP and Teflon are truly non polar.
It's something to do with the chemistry of the plastic.
All electrolytics are strongly polar. But manufacturers have found a way to make a Bi Polar electrolytic. That has an insulating film on both sides of the foil.
These are not Non Polar, even though many/most manufacturers label them NP. Some do use BiP (= Bi-Polar)
I've spent a couple of minutes looking for a reference to polarised plastic film capacitors. None found.
Some types like MKT & MKS have a small polarised effect, whereas others like MKP and Teflon are truly non polar.
It's something to do with the chemistry of the plastic.
All electrolytics are strongly polar. But manufacturers have found a way to make a Bi Polar electrolytic. That has an insulating film on both sides of the foil.
These are not Non Polar, even though many/most manufacturers label them NP. Some do use BiP (= Bi-Polar)
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Found some references to polar and non polar plastics.
I don't know enough to confirm if any of the answers about the chemistry are correct.
https://en.wikipedia.org/wiki/Chemical_polarity
http://www.appstate.edu/~clementsjs/journalarticles/zeus_dielectric.pdf
https://en.wikipedia.org/wiki/Polyethylene
I don't know enough to confirm if any of the answers about the chemistry are correct.
https://en.wikipedia.org/wiki/Chemical_polarity
http://www.appstate.edu/~clementsjs/journalarticles/zeus_dielectric.pdf
https://en.wikipedia.org/wiki/Polyethylene
https://answers.yahoo.com/question/index?qid=20080206160640AAuII88
Good info above.
Most manufacturers look to keep costs down, so many crossover networks are rather simple, using the minimum number of components necessary to get the job done. Tweeters absolutely need to be protected from large low-frequency signals. If the woofer and tweeter are closely matched in sensitivity and don't have any glaring frequency response issues that need correcting a simple crossover may be all that is required.
Electrolytic capacitors are used because they are physically smaller and cost less than film capacitors. Electrolytics are known to distort the signal (non-polar are measurably better than polarized in this respect) and also degrade over time. As the electrolyte dries out the value of the capacitor changes. Any electrolytic more than 20 years old should be considered suspect simply due to age.
Film capacitors (polypropylene or even polyester/mylar) are known to be superior to electrolytics for audio. Replacing the old capacitor would be a good choice, and an upgrade to a polypropylene cap should be preferred.
Most manufacturers look to keep costs down, so many crossover networks are rather simple, using the minimum number of components necessary to get the job done. Tweeters absolutely need to be protected from large low-frequency signals. If the woofer and tweeter are closely matched in sensitivity and don't have any glaring frequency response issues that need correcting a simple crossover may be all that is required.
Electrolytic capacitors are used because they are physically smaller and cost less than film capacitors. Electrolytics are known to distort the signal (non-polar are measurably better than polarized in this respect) and also degrade over time. As the electrolyte dries out the value of the capacitor changes. Any electrolytic more than 20 years old should be considered suspect simply due to age.
Film capacitors (polypropylene or even polyester/mylar) are known to be superior to electrolytics for audio. Replacing the old capacitor would be a good choice, and an upgrade to a polypropylene cap should be preferred.
A 4uF electrolytic cap has a substantial ESR, while an equal capacitance film cap has much lower ESR. Replacing an electrolytic cap with a film one needs some resistance added.
Ralf
Ralf
I really like the idea of replacing my 4 MFD 50v Electrolytic caps with Polypropylene but cannot find an exact replacement. I know I can up the voltage but from what I've read I must stick with the 4 uF otherwise I risk altering the frequency. Is that right?
Any recommendations?
Mike
Any recommendations?
Mike
Mike,
a 3.9 uF polypropylene will do perfectly as it is within a 2.5% tolerance of 4uF which is easily close enough when it comes to passive crossovers.
Your loudspeakers now being 40 odd years old are not the same loudspeakers when you first bought them.........just as we are not the same as we were then, 40 years ago.
Any manufacturer that uses a cheap non-polar electrolytic capacitor as a series capacitor for a 12db/oct filter is not overly interested in close tolerance components being used in his crossovers.
The 3.9 uf polypropylene will no doubt be an upgrade.
C.M
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You could also use two 2 uF capacitors in parallel. Even four 1 uF. See what is available and make a combo. You are not mass producing these - spending a few cents more isn't going to be a deal-breaker.
add a 1u8F and 2u2F for 4uF.
Use MKT, smaller and cheaper than MKP.
You may have to add 0r1 to 0r22 to match the original crossover transfer function.
Not really important, but the older KEF coils are ferrite cored, not iron. The caps are n.p. electrolytics . I forgot the name of the -to my best of knowledge British- manufacturer of the caps. Many British speaker companies used these in the 70's en mid 80ties.
I used to have some in my spare parts collection. They keep up (and thus measure) surprisingly well even after 35 years of age.
Good luck,
Eelco
I used to have some in my spare parts collection. They keep up (and thus measure) surprisingly well even after 35 years of age.
Good luck,
Eelco
MikeBP
Since you are new to electronics, some basic info.
When combining capacitors to arrive at a particular value, remember that they must be connected in parallel (side by side) and not in series (end to end). Series connected capacitors will reduce rather than increase the total capacitance. This is the opposite of resistance or inductance, where series connections increase the total.
Since you are new to electronics, some basic info.
When combining capacitors to arrive at a particular value, remember that they must be connected in parallel (side by side) and not in series (end to end). Series connected capacitors will reduce rather than increase the total capacitance. This is the opposite of resistance or inductance, where series connections increase the total.
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