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Crossover construction questions.


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10 replies to this topic

#1 of 11 OFFLINE   Brett DiMichele

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Posted January 21 2003 - 10:40 PM

Hey all,

I just have some simple questions about replacing crossover
components.

If you have prebuilt crossovers and you want to replace a
capacitor and you know the value and say it's a 40.0uf and
all you can find is a 39.0uf and a 1.0uf you can use the
pair together to make 40.0uf correct? The capacitance value
comes out the same by using two to arrive at the figure.

Next question...

Can an Iron Core Inductor be replaced with a regular Air
Core Inductor? Why would they use an I-Core and an Air core
on the same side of a board, was this done to thwart magneto
striction between two air core inductors?

Third question...

Say the Air Core Inductor you need is a 0.31mh and all you
can locate is a 0.33mh, can you unwind some of the windings
to get the 0.31mh figure? What do you need to test the inductor
with to make sure you arrive at that precise value?

And Fourth... Why in gods name is a 200.0uf Capacitor so
danged expensive.. YEESH $90.00 for a pair of Solen 200.0uf's

Posted Image

A rough shoppers list of components to rebuild my existing
XO's comes out to $256.16 from PE!

That's all Solen Caps, Dayton Non Inductive Resistors, Alpha
Foil Inductors (where I could find them that matched the
specs..)

Crossovers get pretty expensive it looks like Posted Image
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#2 of 11 OFFLINE   Patrick Sun

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Posted January 22 2003 - 03:15 AM

1. Just parallel caps together to arrive at the value (additively).

2. Iron Core is used for space considerations (smaller for larger inductance values vs. air core of the same wire gauge). Iron core, rule of thumb, should only be used in the lower frequencies due to saturation effects as frequency increases into the midrange region. Some people dig the clean sound you get from foil inductors, but those are very very expensive in relation to air core and iron core inductors.

The 2 easiest way to minimize magnetic fields between inductors is to space them apart as far as possible (3" or more), and/or keep the inductors at right angles from each other.

3. Sure, you could unwind a larger value inductor to get a smaller value inductor. But you need a LCR meter to correctly measure the newly unwound inductor. A 0.33mH inductor won't make all that much difference from a 0.31mH inductor.

4. This is why companies use the cheap electrolytic caps when large values are needed for the crossover, at the expense of using caps whose values can change as a function of frequency.
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#3 of 11 OFFLINE   Brett DiMichele

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Posted January 22 2003 - 03:22 AM

Pat,

Thanks for the info!

Well I don't have an LCR Meter but I am friends with an EE
and he probably has one, I am going to have to ask him
before I order something that I can't figure out without
the appropriate test equip.

I was fairly certain just paralelling the Caps with additive
values would give me what I needed.

I understand that the higher quality caps will be more
expensive it's just a fact that quality costs more. But
what I don't understand is how a 1.0 Farad Cap can be had
for as little as $70.00 but this dinky 250uf deal costs
$45.00 each!

The 1.0Farad's used in car audio are most likely low quality
inside since all they need to handle is incoming voltage
and not a signal with constant changing frequencies.

Again MANY thanks for the information!

(Now I need to think about whether it's worth it to spend
$250.00 on parts for the crossovers in a pair of speakers
that I only paid $580.00 for....) I am thinking nah!

I would be just better off to save that money to spend on
the Line Arrays...

Besides they don't sound bad at all, I am just in this Tweak
everything ta hell mood untill I can start on my projects!
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#4 of 11 OFFLINE   Mark_E_Smith

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Posted January 22 2003 - 01:20 PM

The 200 uF is used in the woofer section to roll in the 5" speakers around 150 Hz. You don't need a high quality one there, but in the tweeter circuit you bet. You can also use a small uF "bypass" cap that will greatly improve a lower quality large cap. IE 30 uF and a 1 uF in parallel will sound very close to a 31 uf expensive cap. use foil core inductor in the tweeter. Be advised that the gage of the inductors makes a difference in the x-over as it changes the resistance of the circuit. So try to stay with the same gage inductors. Foil inductors usually have a little lower resistance than air cores. ONLY use iron core inductors in the woofer circuit and in your application I wouldn't use them if you are not going to use the 10" woofer.

What are the values in your x-over?
Is it a 3 way or a 2 way?
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#5 of 11 OFFLINE   ThomasW

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Posted January 22 2003 - 02:09 PM

Brett

You certainly don't need the high priced foil inductors. The Solen Perfect-Lay are fine for your speakers

My experience with the Dayton non-inductive resistors is that they don't meet the advertized tolerance. So go with the Mills.

Madisound has some GE film caps that are really quite good. The largests value is 10mfd. You could make a fairly inexpensive 200mfd cap with a cluster of these. The down side of course would be the size of the cluster.

#6 of 11 OFFLINE   Brett DiMichele

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Posted January 22 2003 - 02:29 PM

Mark,

Here are all the values (not including the polyester caps
they have on the board (I didn't check the value on those)

Front of Crossover:

Capacitors:

Capacitor F 4.3J MT100V
Capacitor F 40.0K MT 100V
Capacitor F 12.0j MT 100V
Capacitor 60 Uf 150v

Non Inductive Resistors:

Resistor 10w 4Rj
Resistor 10w 4Rj
Resistor 10w 25Rj

I-Core Inductor:

4.0mH

Back of Crossover:

Capacitors:

250 uf 100v

Non Inductive Resistors:

10w 30rj

I-Core Inductor:

0.73mH

Standard Inductor:

0.31mH
-----------------------------------------------------------

Tom,

I was thinking the same thing about the Dayton vs. Mills
Resistors, the Dayton resistors don't look any better than
the one's that are on the board now.
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#7 of 11 OFFLINE   Mark_E_Smith

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Posted January 23 2003 - 12:43 PM

My sugestion,

Leave all the resistors alone
Change the 4 mH iron inductor to a 14 gage air core same value, leave the 250 uF alone. these are in the woofer section an seem to cross at about 150HZ, cant be exact because I don't have the driver info.

Change the .73 mH and the .31 mH to 16 gage foil inductors, replace the 40 and 60 uF caps with metalized not high dollar film and foil, replace the 4.3 and the 12 uF with film and foil caps or replace the 12 uF with a 10 uf metalized and parallel a 2 uF film and foil cap with it. I would try one speaker and see how it sounds compared to the other.
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#8 of 11 OFFLINE   Scott Simonian

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Posted January 23 2003 - 01:37 PM

Quote:
You can also use a small uF "bypass" cap that will greatly improve a lower quality large cap.

That is one thing I could never understand. I see how they equal up in capacitance but how would using a bad one and a good one help? Most of the electricity would go to the big cheaper cap and just a small fraction in a good cap.

Can someone explain how bypassing works in depth?
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#9 of 11 OFFLINE   Brett DiMichele

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Posted January 23 2003 - 02:10 PM

Mark you were very close.. The Sub crosses at 100Hz Posted Image

I was thinking about doing what you suggested before you
even suggested it! I can't see upgrading the sub's cap
that's the big money cap.. All the other components are
affordably priced I could eliminate $90.00 just from not
replacing the sub's caps..
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#10 of 11 OFFLINE   Brett DiMichele

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Posted January 25 2003 - 06:42 AM

HOLY COW...

I just got both speakers done (no I didn't touch the XO's
yet..)

I just replaced all the wire with 12Awg OFC with 4% Soldered
connections.. I stuffed the Midrange Enclosures with 50%
Polly-Fill and did the same for the SubWoofer Enclosure..

I can NOT believe the difference.. The Bass is Tight I mean
REALLY tight now... Kick Drums smack you in the chest and
almost knock you over, but they don't boom.. They are fast
and clean sounding..

The mids, Brass Instruments are smoother (silky) vocals are
even better.. Better Instrument Seperation...

I am very skeptical about everything and I rarely hear any
differences.. But I sure as hell am now!

I think I am going to go ahead and order all the parts to
rework the HF side of the XO's with top quality Caps and
Alpha Foil Inductors..

*shakes head* Unbelieveable!
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#11 of 11 OFFLINE   Brian Fellmeth

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Posted January 25 2003 - 07:34 AM

Scott :

I'll take a stab at your question. With the 2 caps in parallel, the current has 2 options, 2 paths to choose from. At high frequencies, a cap is essentially a short. The concept is that the better cap is a more perfect short to let the current flow to the tweeter a little cleaner.