LR Xover maths !! Brain about to explode.

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 !!!
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
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.
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
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]
 
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
 
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.
 
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 !!
 
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
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
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.
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
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.
 
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.
 
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
 
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
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
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.