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Old 15th October 2008, 02:56 AM   #1
felmont is offline felmont  United Kingdom
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Default Will it work? My second-order two-way system design

Hi guys. I'll preface this by letting you know that this is indeed my first loudspeaker project so any advice is greatly appreciated and I am keen to learn!

I have some old 2-way speakers (Technics SB-CS50 to be exact). They're a sealed enclosure, two way (woofer/tweeter) and have a bass reflex port. The woofer is 7ohm while the Tweeter is 4ohm. It has a small crossover which I can't work out yet, and I know nothing about the drivers themselves in terms of wattage and frequencies, but that shouldn't be important since I only wanted to use them for their enclosure design.

Anyhow, I've chosen to use them as a reference to make some similar ones, e.g. the enclosure will have almost the same dimensions as my SB-SC50's but the driver + crossover will be my choice and design. There seems to be so much choice as far as the enclosure goes so I'll stick with what I know works.


Now, as I'm in England I've chosen to order the majority of the parts from www.rapidonline.co.uk as they were recommended to me. My driver choices so far are:

This Tweeter - 60W @5kHz power rating, 8ohm, freq. range of 3.5kHz to 20kHz

This Mid/Low driver - 125W RMS power rating, 8ohm, freq. range of 65Hz to 5kHz, resonance of 76Hz.

Yes the drivers are fairly cheap, but as this is a first project I am trying to keep it fairly low. If it goes well, I'll make the big daddy set and beef them up nicely.

So next for designing the crossover... I've chosen second-order (butterworth) as it will give me the 12dB/octave dropoff which seems fairly standard - not too sharp but still sharp enough. The cutoff will be 4000Hz. After some quick "googling", formulae and sketching brought me to this design:
(I think the attached picture is at the bottom of thread)

Pretty simple, so now I've started sourcing some components for the crossover.
I'll need 2x 3.516uF capacitors and 2x 450 uH coil inductors. I'll buy the caps but make the coils myself - I hope I'm not being too ambitious here. To detail the coils quickly, I used a great site (here) to get the following dimensions: 26mm inner diameter x 25mm length x 40mm outer diameter. ~144 turns of 20SWG insulated copper wire @ 24 turns per layer and 6 layers. This would create a coil inductor with 0.451 mH inductance... right? Thats spot on for my crossover. This is the wire I will use.

Now as for the capacitors I don't know what voltage I should get them rated to... 100V seems standard, will that work? I've had a look and for C1 and C2 I'll have to use multiple caps in parallel to reach the required 3.516uF rating. So, two choices.

First choice = "Metallised Polyester Capacitor".:
2.2UF 250V 10% MET POLYESTER CAP
1UF 250V 10% MET POLYESTER CAP
0.33UF 250V 10% MET POLYESTER CAP
= 3.53uF

Second choice = This "Metallised Polyester Capacitor".:
2.2U 250V MKS4 POLYESTER CAP CASE H RC
1UF 100V MKS4 POLYESTER CAP CASE L RC
220NF 100V MKS4 POLYESTER CAP CASE J RC
100NF 100V MKS4 POLYESTER CAP CASE I RC <<< optional
= 3.42uF (3.52uF inc. optional cap)


By the way I'm grateful if you have read this far already! :P

So should I choose option 1 or option 2 for the capacitors, and am I right in thinking that wiring them in parallel will result in adding them to be 1x 3.53uF cap?

Also, do you think my crossover design and calculations are correct/will they work?
Should I perhaps use a first order crossover to maintain phase?
What do you think about my choice and combination of woofer/tweeter?
If I should change some stuff or if it's obviously flawed, how can I fix it?

Thankyou guys in advance for any help, it's greatly appreciated!

Cheers,
Craig
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File Type: jpg crossover.jpg (7.5 KB, 280 views)
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Old 15th October 2008, 03:31 AM   #2
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"I am keen to learn!" - good, I don't not want to put you off, but first some comments on your approach and choices -

"They're a sealed enclosure, two way (woofer/tweeter) and have a bass reflex port" - if they have a port they're NOT sealed...

Re: enclosure size - you choose the woofer first and then decide what size box is suitable, (use a freeware simulator such as Winisd) not the other way around; what works for one speaker is not usually suitable for another.

The woofer you've chose has too high an Fs for decent hi-fi bass;
the specs given on the site linked are inadequate to design a system.

I'm not sure how you've calculated the crossover parts values, but you need to know the ACTUAL impedance of the driver at the chosen xover frequency, not nominal....

Now, to stop being negative, here's some reading which should set you on the right track:
Getting started in Crossover Design

The basics (You've got to understand the rules before you can break them :-)
http://sound.westhost.com/lr-passive.htm
choosing xover point:
http://www.speakerbuilder.net/web_fi...xpointmain.htm
Baffle Step Compensation: http://sound.westhost.com/bafflestep.htm

Getting it done:
Jay has a page on designing xovers using manufacturers specs without measuring:
http://www.geocities.com/woove99/Spk...esigningXO.htm

Calculators:
http://ccs.exl.info/calc_cr.html#second
http://www.sengpielaudio.com/calculator-Lpad.htm

Measurement freeware:
Synrta - http://libinst.com/SynRTA.htm
ARTA http://www.fesb.hr/~mateljan/arta/download.htm
ARTA Jig - http://zobsky.blogspot.com/2008/01/s...t-jig-for.html

Examples from the designs of others can be quite instructive:
http://www.zaphaudio.com/
http://www.troelsgravesen.dk/Diy_Lou...r_Projects.htm
http://www.rjbaudio.com/projects.html
http://www.humblehomemadehifi.com/
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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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Old 15th October 2008, 07:48 AM   #3
felmont is offline felmont  United Kingdom
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Cheers Pete that is all very helpful!
You're right, thinking about it now does make a lot of sense to choose drivers first and then the enclosure size later!

"The woofer you've chose has too high an Fs for decent hi-fi bass" - yes I saw it was 65Hz but as I don't know what is the norm for a two-way system I couldn't compare it. I'll have a deeper look and choose a different woofer.

"you need to know the ACTUAL impedance of the driver at the chosen xover frequency, not nominal" - right, I'll either try to find some more detailed specs of the tweeter and new woofer and work from that; or find somewhere that sells the drivers and also the tech specs along with it. I don't really want to get into trying to measure the frequencies etc myself because it seems very complicated really. For a beginner like me at least!

Well, it looks like I've got quite a bit of reading to do then! Thanks for the heads up and all of the links mate, watch this space!!
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Old 15th October 2008, 10:22 AM   #4
tinitus is online now tinitus  Europe
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ALL you do cost money these days...and with time and effort spend...
I would say that there are ok designs where the drivers are cheap and dont cost that much, but would give you nice sound ...need I say more

From your choise of woofer I suspect we have to find a nice cheap, properly designed and well documented 2way with semi-pro drivers
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Old 15th October 2008, 11:57 AM   #5
sreten is offline sreten  United Kingdom
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http://www.rjbaudio.com/Audiofiles/FRDtools.html
http://www.geocities.com/woove99/Spk...esigningXO.htm

http://www.zaphaudio.com/
http://www.rjbaudio.com/projects.html
http://www.troelsgravesen.dk/Diy_Lou...r_Projects.htm
http://www.humblehomemadehifi.com/
http://htguide.com/forum/forumdisplay.php4?f=39

http://www.linkwitzlab.com/
http://www.geocities.com/kreskovs/John1.html

Hi,

Some useful links to learn about loudspeaker design / designs.

None of the drivers listed at rapid electronics are particularly good.

Click the image to open in full size.

The Technics look European made and fairly decent, if you want to
do better than those with new drivers it is not going to be cheap.
You should work out the crossover so you can find it has nothing
to do with textbook crossover values, like all the designs linked.

A far better approach would be simply better speakers bought used.

If you want to build some good loudspeakers it will take a lot
of time and effort and care not to do it too simply, i.e. wrong.
Building an existing design would be a good idea.

A good source for drivers :
http://www.bmm-electronics.com/Producten_aB.asp?code=3

What do you envisage the "big daddy set " to be ?

/sreten.
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Old 15th October 2008, 12:57 PM   #6
tinitus is online now tinitus  Europe
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This one from Intertechnik cost 115EUR...dont know if its any good, but surely better than what you can do yourself
Attached Images
File Type: jpg it_mystery-pa12.jpg (58.3 KB, 208 views)
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Old 15th October 2008, 05:09 PM   #7
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http://www.iol.ie/~waltonaudio/pione.html

You might want to read the link above. It uses the same woofer i think as the one you chose.

Godzilla
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Old 15th October 2008, 11:22 PM   #8
felmont is offline felmont  United Kingdom
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Okay excellent cheers for the replies so far guys they have all been helpful to me.

At the moment I've decided to scrap my idea of half-copying the Technics SB-CS50's that I already have...
I'll either try to find a good set of used speakers, all ready to go (easy way out, but not as much fun) or I'll build my own set but I'll allow a big budget (harder, but much more fun and rewarding). I have access to almost any kind of tool/equipment (I mean almost industrial grade stuff, also high precision).

I'm looking forward to completely designing my perfect speakers now, rather than just making them for the sake of it.

I guess I should start by looking at the layout of my room first... I'll draw a sketch tomorrow because I'm too tired tonight but basically it's a 10foot x 7foot rectangle... It's in the apex of the roof though so the roof is mainly at an angle.

... more calculations tommorow then!
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Old 27th October 2008, 05:48 AM   #9
GOWA is offline GOWA  Canada
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Is it just me or does that tweeter look like it has been punched in?
This Tweeter - 60W @5kHz power rating, 8ohm, freq. range of 3.5kHz to 20kHz
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Old 27th October 2008, 05:55 AM   #10
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Quote:
Originally posted by GOWA
Is it just me or does that tweeter look like it has been punched in?
This Tweeter - 60W @5kHz power rating, 8ohm, freq. range of 3.5kHz to 20kHz
Yeah, I was wondering why they couldn't find a better sample to photograph.
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