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Multi-Way Conventional loudspeakers with crossovers

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Old 15th July 2001, 03:30 PM   #1
Simon is offline Simon  England
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I am looking to build an active 3 way system.
Filters will be Linkwitz Riley 4th Order.
HF amp will be class A.
Would like to use the output capacitor of the HF amp to provide the 'last pole' of the HP filter.
Brain hurts from bashing the Laplace Transforms.
Any one done this before - Maths never was my strong point !!!
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Old 22nd September 2001, 04:17 AM   #2
Wizard of Kelts
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You are going to use a separate Class A amplifier for the tweeter, (I assume that is what you mean by an active system-one with separate amplifier and dividing system for each driver)?

If that is the case, why not just put the High Pass network BEFORE the output stage? For one thing, that way you will not have to worry about driver impedances and how they react with the crossover. Driver impedances vary quite a bit throughout their playing range, whatever their nominal rating.

By using the output capacitor as a crossover element, you are basically using a separate amplifier to drive the tweeter, but a using passive crossover on that amp. One of the best reasons for buying a separate amp for each driver is to be able to put the crossover network before the output stage.
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Old 22nd September 2001, 04:57 AM   #3
Wizard of Kelts
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If you decide to do it the way you describe anyway, and you want a simple calculation program, go to Steve Ekblad's Free Audio Software site at: http://www.wssh.net/~wattsup/audio/

Scroll almost all the way down until you see the TC Xover program. Download. Small program.

Very simple program, gives you the values based on resistive loads. From your post I gather you already know the configuration, just need the values.

Good luck.

[Edited by kelticwizard on 09-22-2001 at 12:02 AM]
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Old 22nd September 2001, 10:52 AM   #4
Simon is offline Simon  England
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Thanks for the reply.
I didn't explain myself very well in the original post.

The goal is an active three way system.
i.e: three active filters, three amplifiers, but with one 'protection' related caveat.

I am very nervous about connecting tweeters directly to the power amplifier output terminals. If an output semi goes down you can say good bye to the tweets as they go up in smoke. I don't like the idea of fuses (too slow to save a tweet anyway probably) or electronic protection. A single cap between the amp and the driver will block DC. My thinking was to provide 3 poles of the LR4 filter 'actively' (ie in electronic filter before the amp) and the last pole with the DC cap.

Simon
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Old 22nd September 2001, 07:13 PM   #5
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Simon,

Don't make things more complex than they are.

Make your active filter in the normal way, and connect in series with the tweeter a capacitor of sufficient capacity in order that his impedance is negligible, for instance 50 uF, and your driver will be DC protected.

If the output capacitor is to small, the fundamental resonnance of the tweeter may be incorrectly damped, especially if you use a class A no-feedback amplifier, the outut impedance of which is not very low.

Regards, P.Lacombe.
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Old 22nd September 2001, 10:16 PM   #6
Simon is offline Simon  England
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I could indeed use a high value cap. A 50uF item would do the job and probably have its 'pole' far enough away from the xover point to do no extra harm phase-wise.

I have read a lot about passive xover caps recently (some of it here). The usual recommendations being:
"dont skimp on xover caps in the signal path....
"the single most important component in the chain...

(While I don't necessarily agree with the last statement I can see there is some logic there)

However, a good quality 50uF cap would be pretty expensive I think (one of the reasons i've never bothered to consider 'standard' class A before - you need thousands of uF of DC blocking, the only option being electrolytics - yeuch!!!). Good (non 'audiophile' quality) polypropylene 10uF caps are 8.00 ea !! That puts a 50uF combo at 40.00 !!

I would still be interested in the solution I've outlined earlier. It seems more 'elegant' and should be a whole lot cheaper !!
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Old 22nd September 2001, 11:49 PM   #7
Wizard of Kelts
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Simon:

Not trying to tell you what to do. Just letting you know your options.

Solen audiophile capacitors, 50 microfarad, 8 pounds and change, each.

Falcon Acoustics. Page 7 of their price list.

http://www.falcon-acoustics.co.uk

[Edited by kelticwizard on 09-22-2001 at 06:52 PM]
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Old 23rd September 2001, 11:16 AM   #8
Simon is offline Simon  England
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kelticwizard:

Thanks for the tip on cap's from Falcon (Geoff just referred me to Falcon Acoustics for drivers also - will have to check them out).
My source was RS (not sure who the oem is) - 7.14 for a 10uF polyprop. Looks like I won't be buying from then.

Thanks all for your input.
Simon

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Old 24th September 2001, 12:07 AM   #9
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Simon,

Pardon me, but your solution is not elegant, because of, again, the problem with tweeter resonnance damping. Default of this nature is easily audible.

Polypropylene capacitors can be found in surplus market, since precision value is not required.

Regards, P.Lacombe.
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Old 24th September 2001, 01:47 AM   #10
Wizard of Kelts
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Lacombe, (or anybody really good with crossovers):

1) A link to a surplus house for capacitors is appreciated

2) Many tweeters have a resonance about 1,000 Hz, yet they recommend crossing over at 2,000 Hz or higher. Simon is crossing over at 24 db/octave. Counting his capacitor in series with the tweeter as a 6 db octave passive crossover, the tweeter's resonance point will be at least 24 db down at his lowest likely crossover point, (2,000 Hz). How important is it to have the tweeter's resonance damped when it will be so far below the level of the passband, (and removed from it by at least an octave)? By the way, I favor the fully active crossover approach just on general principle.

3) I have seen tantalum capacitors, and some called "tantalum electrolytic" capacitors. How good are the tantalums, and are the "tantalum electrolytics" any better than aluminum electrolytics for crossovers? They seem to have a high value of capacitance for the price.

Thank you.
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