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Old 19th October 2004, 08:04 PM   #1
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Default Crossover question from newbie

Hi all,

I'm still fiddling with my surround speaker project, and I've been here to calculate the basic crossover components for a 4ohm tweeter, and an 8ohm woofer:

http://www.lalena.com/audio/calculator/xover/

But how do I know what impedance that will be showing to my amp?

I'd like to keep it at approx 6 to 8 ohms, but given my very basic knowledge of resistances in parallel, will this set-up be showing less than 4ohms?

I don't know the differences between 1st order, 2nd order etc, so as you can see, I've a lot to learn.

Any help appreciated

Gary.
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Old 19th October 2004, 08:09 PM   #2
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For frequencies passed by the filter to the tweeter it will see 4 ohms, for frequencies passed to the woofer it will see 8 ohms. You may want to look at sensitivity more than impedance. Chances are you will need to pad the tweeter anyway to compensate for increased sensitivity over the woofer.
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Old 19th October 2004, 08:48 PM   #3
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Hi tim,

Thanks for your reply.

Ah, Ok,

So the amp will see different impedances depending on what it is sending to the speakers?

How do I go about padding the tweeter to make it match the woofer? If the sensitiveity of the tweeter is say 3db higher, what is needed to compensate?

I feel I'm asking very very simple questions, but can't find the answers by searching, so I kinda feel like a dope.

Gary.
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Old 19th October 2004, 10:26 PM   #4
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The best way would probably be to use a selectable L-pad and once the tweeter is where you want it replace the L-pad with the same value resistor(s).
Some people use L-pads as a permanent installation though.
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Old 19th October 2004, 11:05 PM   #5
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Thanks Tim,

Good idea!

I can use the l-pad for other projects too which is handy.

What's the best way of measuring the sensitivity of the tweeter after adding the l-pad?

Will the resistance add to the overall impedance of the tweeter?

Gary
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Old 20th October 2004, 12:14 AM   #6
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What system will you use for measuring your speaker? I suppose you could do it by ear but you'd do best by getting some measurement software. There are many choices.
The padding resistors that replace the selectable L-pad are often referred to as "L-pads" because they shape an L in a schematic. In other words, you have a series resistor and a parallel resistor. This not only attenuates the tweeter but presents a steady load to the amp.
This may clarify it some.
http://ccs.exl.info/cust_cr.html
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Old 20th October 2004, 12:42 AM   #7
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Thanks again Tim,

Actualy, that was to be my next question!

I've seen some software which needs you to make up a box - Speaker Workshop, but will a simple microphone, test tone and recording software do a reasonable job?

The SW stuff looks very useful for ascertaining the T&S data which I'd like to try sometime, as I couldn't get any info on some Acoustic Energy speakers I have (I was going to use the drivers and crossover in a new box, but the new box I made is too big, so I need to see what to change, other than the internal volume).

What would you reccommend for a very green newbie like myself for basic testing?

TIA

Gary.
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Old 20th October 2004, 01:20 AM   #8
Grahamt is offline Grahamt  Canada
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Quote:
What would you reccommend for a very green newbie like myself for basic testing?
Having graduated to regular newbie status, I recommend you read The Loudspeaker Design Cookbook by Vance Dickason and other books by John Murphy and David Weems. You can find many useful speaker design or electrical design books at your local library.
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Old 20th October 2004, 06:36 AM   #9
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Yep, I second the David Weems book for getting started. In particular, "The Great Sound Stereo Speaker Manual". Just take his quarter wave theory with a "grain of salt" if you know what I mean.
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Old 20th October 2004, 09:52 AM   #10
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hi, does anyone know of any good books i could get to assist with my high and low pass filter settings ? i have 2 x 12" Lightning Audio Subs (300Watt RMS)with a Starsound 1500Watt Amp -

Ultra stable PWM (pulse width modualted) MOSFET power supply
2Ω stable operation with output power increase
Variable High pass filter (40Hz - 1KHz) & Low pass filter (40Hz - 250Hz)
Enhanced variable crossover for Super Bass OdB/ +6dB/ +12dB @50Hz
RCA line Input and line Out jack
Gold plated Power, Speaker terminal
Soft start delay turn-on circuit
Automatic remote ON/OFF switching
Bridgeable & Tri-Model operation
Full chromed finish design
Protection circuit against Thermal, Overload, Short circuit and DC Offset

i've only got the 2 subs connected to the amp ( not bridged ) and nothing else. the rest ( 6x9's and sub-standard 80Watt speakers to serve as mids ) all run off my head unit ( JVC shuttle controller - 40 x 4 ).
the amp sometimes clip - probably 'cos of my gains being set too high ????
when this happens, i turn down the gains but still get intermittent
clipping at high volume and bass on the head unit...

please please please - i will greatly appreciate any advice as i am only a novice and learning - please please please.....?
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