LR Xover maths !! Brain about to explode. - Page 2 - diyAudio
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Multi-Way Conventional loudspeakers with crossovers

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Old 24th September 2001, 03:39 AM   #11
paulb is offline paulb  Canada
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Tantalum caps are not recommended for audio. See http://www.capacitors.com/pickcap/pickcap.htm.
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Old 24th September 2001, 04:54 AM   #12
Wizard of Kelts
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Location: Connecticut, The Nutmeg State
Thanks Paul.

A dividend of the link you gave is the possible solution to a problem that has been bugging me for a while.

A signal generator I made using the Intersil 8038 chip works fine for awhile, then goes bad. Always wondered why. Now, the same article that showed that tantalums should not be used in crossovers also revealed that ceramic capacitors should not be used in audio at all.

Apparently, ceramics can create audible distorion even when used as bypasses in power supply circuits for audio equipment. And I had been using them for tuning circuits on the oscillator!!

Again, thanks.
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Old 24th September 2001, 06:05 PM   #13
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Kelticwizard,

1) I live in France, so I don't know where you can purchase surplus capacitors in your country, sorry.

2) Attenuation for filters is given far from nominal frequency, so a low-pass filter at 2000 Hz with 24 db/oct can have only 12 db attenuation at 1000 Hz, depending of structure. But it is possible to design a filter with zero transmission at 1000 Hz, more complex...

2) Don't think twice : tantalum capacitors are poor quality in audio equipements, especially in signal path.

Regards, P.Lacombe.
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Old 24th September 2001, 09:18 PM   #14
Simon is offline Simon  England
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P. Lacombe

Thankyou for pointing out the resonance issue.

There are (as far as I am aware) two practical ways that the tweeter resonance can be 'damped'.

1) Connect a resonance trap (LCR type bandcut filter)circuit directly across tweeter terminals. Thus using the R to damp the tweeter back EMF (presumably).

or

2) Connect the tweeter directly across the amplifier terminals and rely on the amp's output impedance (provided low enough) to effectively shunt the voicecoil.

Option 2 being what you are referring to I think.

Option 1 will work with both active & passive x-overs solutions.
Option 2 requires the active approach either with or without a very 'high' C value (and thus low impedance at resonant freq) series DC blocking cap.

Have I understood you correctly ??

If so, I have a couple of questions for you:

a) Option 1: Are there any rules of thumb for LCR values in a resonance trap.

b) Option 2: Just how far away from the Fr of the tweeter would you put the Fc of the DC blocking cap. I'm thinking in terms of
- any additional phase shift at pass frequencies caused by the DC block capacitor.
- What constitutes a low enough capacior impedance (relative to voice coil) to effectively 'shunt' it.

I have been up til now considering Scanspeaks D2905/95000 this has a relatively low Q (by the look of it) resonance at 550Hz. I would crossover at 2000 Hz which seems popular for this driver.

As an aside I have noted that many (otherwise well reputed) systems, fail to address the tweeter resonance issue in their crossover design. Why is this do you think ?

Cheers,
Simon C
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Old 24th September 2001, 10:40 PM   #15
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Simon,

Yes, it is possible to damp the foundamental resonnance of the tweeter with a series RLC reject filter across the terminals. The resistor must be the same value that the nominal impedance of the driver. To calculate values for L and C, you must know the Q factor of the foundamental resonnance, which is not publicated by the manufacturer...

But damping by the internal impedance of the amplifier seems to be better, because of damping of all frequencies, without tuning necessity.

Additional phase shift is phase advance, that is good thing for a tweeter, and can be taken eventually in account in the active filter design. (especially if the capacitor is not very large).

If the tweeter exhibit very low Q factor at the foudamental resonnance, all this precautions are useless. This is the case when tweeters are damped by ferrofluid. This precautions are also useless if the crossover frequency is high, 4 kHz and more.

Regards, P.Lacombe
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Old 26th September 2001, 03:50 AM   #16
Wizard of Kelts
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Simon:

Just did a quick review of Falcon Acoustic's price list for drivers.

You might want to consider two American companies:

http://www.madisound.com
http://www.speakercity.com

Both deliver internationally. Madisound seems to outright brag about it's international delivery. I leave it to you to call and find out about extra charges, etc.

I have dealt with Madisound-very satisfactory. Advice over the phone, etc.

Have sent for extensive info on drivers from Speaker City in the past, and gotten reams of info back-a mark of good customer service.

I would recommend both.

I strongly suggest you consult the price list of all three, and compare.
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