Scrounging caps from old circuit boards to make crossover

[nobb]

Hi.

This will be my first time designing a crossover. The inductors I can wind myself, but the hard part is matching the exact required capacitance value. For my design, I need 4 2.95 uF capacitors. Rather than going out to buy them, would it be a good idea to just use capacitors from old circuit boards (I have a ton) and wire them in series, parallel...or whatever to get the right capacitance? Are there any disadvantages?

My circuit boards have alot of electrolytic and mylar caps. The mylar caps are difficult to read. But the problem with the electrolytics is that they might be polarized (how can I tell?). I was thinking to make these polarized electrolytics useable, to just wire them in series (each cap with double the capacitance required) and have the legs with the same sign connected. I read this on another forum.

So what is your guy's opinions?

[Hartono]

Do not use polarized electrolytic for speaker passive crossover
the reverse voltage will be too much.

I wouldn't use any back to back polarized electrolytic, it still distort considerably. and definitely not for passive crossover.

Even if it's not polarized the rating/age might not make it suitable for safe operation in crossover.

If it says +/- in on the body near the leg then it's polarized.

I wouldn't comment on the mylar, as power rating and quality varies a lot.

Yes you can use capacitor series or parallel to make up the value you want, just make sure the voltage/power rating is suitable.
have fun with your new project !

[OzMikeH]

2.95uF!? 3uF is close enough. Typical tolerance on a very good capacitor is around 10%

How close is the tolerance on your speakers, I bet it's far worse than 5 parts in 300. More like 10 %.

Capacitors get old. Electrically leaky ones often still measure ok. Buy new ones.
Mail order in Australia high quality 1uF Metallized polypropylene capacitors (best for crossovers) are $2.60 each - retail price including tax. To put that in perspective: Half the price of a reasonable cup of coffee.

Electros have a black band down one side, this is the more negative side. Tantalums (little peas) have a tiny + sign near the + leg.

To make larger value of capacitors you wire them in parallel. Series will make capacitance smaller.

so three 1uf in parallel will make 3uF.
three 1uf in series makes 0.33uf (but with three times the voltage rating)

[sreten]

Hi,

Non-polar electrolytics are back to back electrolytics in one electrical case.

/sreten.

[I_Forgot]

Electrolytic capacitors usually have very wide variation in values-
-20 +80% is not uncommon. If you are going to use salvaged electrolytics, do as recommended and connect them back to back to make a "nonpolarized" part, and try to actually measure the capacitance. You wouldn't want one channel to have 6 uF and the other 2 uF when you are trying to make a 3 uF caps. Many inexpensive multimeters can give reasonably accurate measurements of capacitance.

I_F

[nobb]

Thanks for the replies.

I've decided to not get my caps from old circuit boards as I want to do this properly the first time. I planned on winding my own inductors too, but seeing the low prices on partsexpress for what I need, I think I will just buy my capacitors and inductors from them.

I am planning on building a custom component system for my car using the audax 5.25 aerogel:
http://www.partsexpress.com/pe/showd...number=296-155
and the dayton nd20:
http://www.partsexpress.com/pe/showd...number=275-030

I will cross them over at 4200hz using a 2nd order linkwitz-riley. I picked this frequency because it is within range for both my drivers, and the exact cap/inductors can be bought from partsexpress.

I made another post about this, but no one replied =(.
So is this a good configuration? Or is there something I didnt take into account yet.

[Hartono]

you can buy slightly larger value inductor than you need, that way you can reduce it's value by unwinding a few turns to get desired sound. If you do unwind it, be careful and keep the winding tension tight afterwards. You might need some plastic cable strap and generous amount of glue using glue gun .

also notice that inductor have series resistance, which act like resistor in series with the inductance. Larger AWG inductor have lower series resistance.

I would use air core inductor , even with it's larger series resistance than iron/ferrite core, air core inductor doesn't saturate and doesn't have hysterisis, which translate to cleaner and more dynamic sound.

Inside car, try to put the inductor/ the whole crossover as far away from any metal/ car body, it will affect inductor magnetic field, especially important if using air core. air core inductor doesn't have iron/ferrite to confine it's magnetic field. 10 cm away from car body might be sufficient.

[I_Forgot]

If you use multiple inductors in one crossover be sure to orient the axes of the inductors orthogonal to each other so they don't "talk" to each other.

I_F

[planet10]

The plastic caps that you will find in old dead monitors & in switching power supplies are generally of very high quality (they have to be to do the job they were intended to). 1.0, 2.0 & 2.2 uF are very common values. I wouldn't hesitate to use them. The problem is going thru enuff boards to make pairs.

Forget about the elcos for XO work. The high voltage ones can be used for a no-budget tube amp thou.

dave