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Multi-Way Conventional loudspeakers with crossovers

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Old 22nd July 2008, 09:36 PM   #1
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Default First Project - some advice needed

Hey everyone,

I am getting into speaker design and I have a couple of design questions. I am not sure if this is the right place since this has to do more with powering the loudspeakers rather than the design of them. I am open to suggestions on everything.

I am building a pair of two way loudspeakers. The woofer and tweeter I am going with are:
Tweeter Info: http://www.madisound.com/catalog/pro...oducts_id=8354
Woofer Info: http://www.madisound.com/catalog/pro...roducts_id=554

Both speakers are rated 50W RMS and are 4 ohm. I am getting the WT3 impedence tester so I can mess around with different crossover networks.

Looking at the charts provided it looks like i will need an x'over at 2500-3000Hz. Could I get away with having a X'over frequency thats 1800-2500? I am not sure of the trade-offs of that. I have read that having a x'over 2.5x the Fs of the tweeter is recommended. The reason I think it should be lower is because the woofer starts acting a little unstable at the higher frequencies. What kind of network filter would help smooth out that?


I am also going to put together my own power supply and amplifier. This is where I am starting to get confused. Since I am doing a pair of loudspeakers I need 2 channels from 80-120W. I am not looking to drive these speakers to their full potential since they both have a sensitivity rating in the 90s.

Most amplifier designs I have seen are rated for 8 ohm or 4 ohm and I know that it is not wise to drive a lower impedance than what the amplifier is rated for. I was going to hook up the woofer/tweeter in parallel and have a high/low crossover along with zobel/L-pad/impedance matching to correct any inconsistencies in the output. Would the resulting parallel setup end up looking like 2 ohms to the amplifier? Will additional zobel/l-pad/impedance matching networking add a significant increase to the impedance of the woofer that I can get away with building an amplifier rated for 4 ohm loads at 80-120W? Should I run the speakers in series to avoid building an amplifier that has a 2 ohm load?

Finally, does anyone have a good DIY amplifier/power supply website I can reference? I have just been googling for a few days and searching these forums. Thanks for your time and I look forward to hearing from anyone!

Jacob
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Old 22nd July 2008, 09:50 PM   #2
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Location: Western Sydney
I'd cross over at around 2000, for the reasons you suggest, probably best to go for at least a 2nd order xover because you're close to the woofer's peaky region.

Because the xover splits the frequency range, the lowest impedance the amp sees is what ever the lowest impedance of either unit is, not both in parallel (hope that makes sense), so you don't need to worry, if both are 8 ohms, the amp will see 8 ohms.

Plenty of advice, construction articles etc at this site (both amps & crossover design):

http://sound.westhost.com/
__________________
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 22nd July 2008, 11:30 PM   #3
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Location: Wellington, New Zealand
I presume you are going to do a passive crossover. If a parallel crossover, I think your system impedance will be too low with 4 ohm drivers.

The tweeter's xmax is only 0.1mm. Unless you use really steep slopes at 2KHz - you might hit excursion early. You are also close to resonance. The 2.5x Fs is not a bad guideline.

David.
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Old 22nd July 2008, 11:30 PM   #4
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AH that makes perfect sense. So on that same thought pattern, do I only need 70-75W to the speakers since both speakers wont be active at the same time?
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Old 23rd July 2008, 07:42 PM   #5
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In a 2 way speaker, both drivers are always active - in the sense they complete the circuit. Sure - the different legs in a parallel crossover that service each driver present a very high impedance to prevent power from reaching the driver outside the passband, but the driver is still in circuit.

There are 2 types of power handling a driver has. Thermal limit and excursion limit. Thermal limit is the advertised power rating of the driver. In your case each driver is thermally "limited" to 50 watts. This does not mean you can send 50 watts to your tweeter at 10Hz. Below a certain frequency, a driver becomes excursion limited. This is where the power needed to drive the cone to full excursion becomes less than the thermal power.

Most of the power in a speaker system is used to drive bass frequencies and therefore bass drivers. Power demands as you go up the frequency spectrum decrease to produce the same SPL.

the reason excursion limited power ratings are not provided, is it depends on where and how steep you cross a driver over.

For an example, below is a crossover showing a 1300Hz point between woofer and tweeter. This crossover is ok at lower power, but at high power, you can see the tweeter will hit excursion before the woofer (although the graph doesn't include the low frequency response). There is only 14dB of headroom over a nominal 1 watt drive level - therefore, the system will reach its excursion limits with 40 watts (2 ^ (14/3). Even though each driver is rated at 150 watts (woofer) and 90 watts (tweeter). See how excursion limits depending on crossover point and slope mean the power handling of a speaker can be substantially less than the thermal rating of both drivers combined?

The dotted lines show the excursion limit (slope) turning into the thermal limit (flat) for each driver

Cheers,
David.
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Old 23rd July 2008, 10:56 PM   #6
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What would it take to test the excursion limit after deciding where and how steep the crossover network is? Are there any guideline/rules to keep in mind? Does the excursion limit get better or worse with a steep crossover? Would you recommend moving the X'over point back to 2300-2500 to help avoid excursion?

As for doing your calculations. Are you getting the 14dB of head room based on the tweeter's sensitivty of 91 and the chart saying the tweeter's thermal limit is ~105dB? How does that correspond to excursion happening at 40Watts? 2^(14/3) is 25.4.

So in this examples instance, what power would you recommend amplifying the signal with?

Just as a question not involving the excursion limit. If I want each speaker to have 30W RMS power available. Does that mean my amplifier has to have 60W available? or just 30W?

Thanks a ton for your posts, it is really helping me understand.

Jacob
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Old 23rd July 2008, 11:52 PM   #7
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Hi JAcob,

Yes you are correct - my maths is on holiday. Excursion reached at 25 watts.

Excursion for a given SPL is related to bunch of factors, but halving frequency requires 4 times the excursion to produce the same SPL.

I'll attach a spreadsheet I created which models excursion for your drivers when I get home tonight. Essentially you plug in some T/S params, it shows you excursion on a graph which you can export raw data so you can import into your modeling package etc....

As you've worked out, increasing SPL by 3dB requires double the amplifier power. Increasing SPL by 10dB requires approx 10 times the power.

Depending on your nominal listening level, most people would feel 10dB headroom is sufficient to handle transients. Classical seems to have a higher peak to average ratio than rock music. You could probably get away with less headroom for rock.

For my speaker modelled - a 50 watt amp would be sufficient for the type of music I listen to (mainly rock/pop). More power is generally better - since low powered amps when overdriven will create nasty high frequency harmonics that will pass happily through your tweeter filter yet overload and blow your tweeter.

Of course, I haven't mentioned harmonic distortion of the drivers themselves. Of course a driver might be producing a nice flat amplitude response, but could be producing audible harmonic distortion. There's debate about what level of HD is audible. -40dB down from the fundamental being played = 1% distortion, -80dB down is 0.1% distortion. I couldn't comment on what is audible to me... many of the speakers I design have other problems that probably swamp distortion effects.

Generally odd order harmonics are more offensive than even order, since even order are a multiple of the fundamental tone. For some examples of measured driver HD - see Zaph's site. (www.zaphaudio.com) - driver tests section.

Cheers,
David.
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Old 24th July 2008, 01:21 AM   #8
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Hi Jacob,

I probably should have mentioned the following from the start....

Stage 1 - If this is your first speaker DIY - I strongly recommend you building a proven design. A forum search will give you a range of options.

Stage 2 - If you really want to design something new, visit Jay Kim's web pages or Roman Bednarek's and follow their instructions on how to simulate a crossover without taking your own measurements.

Stage 3 - Be able to take your own measurements and model a crossover using your actual driver amplitude, impedance responses in the enclosure / baffle you intend

Stage 4 - The last stage is to actually understand what and why you make the design choices you do and can anticipate the result without building lots of prototypes.

(I'm barely at stage 3 and the more I learn, the more I realise I don't understand).

If you can't at least do stage 2 for this project, be prepared to spend ages fiddling with some hit and miss solutions to get the speaker sounding right - which you may not do. It is also possible these cheap drivers may not exceed the expectations you have (compared to whatever your reference speakers are).

Not trying to discourage you, but just to point out the possible frustrations you might face.

Cheers,
David.
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Old 24th July 2008, 02:34 AM   #9
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haha, yeah I understand. I put together a subwoofer a few years ago but never took measurements on it. I am doing this out of interest and trying to apply the EE crap school has been feeding me so I guess that is about level 2.
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Old 24th July 2008, 06:09 AM   #10
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Location: Wellington, New Zealand
If you want to get into this... it doesn't have to cost you a lot of money.

Getting a panasonic mic capsule, powering it with a 9v battery a custom made mic-preamp (which it sounds like you could make) and creating an impedance jig for your computer to use with free Speaker Workshop should be cheap as.

David.
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