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Jeroth 12th June 2002 09:29 AM

Speaker Selection?
I have recently purchased a 2 Channel Stereo Amp, which outputs 120W RMS per channel. I have decided to build myself a pair of 2 way speakers. I havent decided on any brands yet because there are a few things I am confused about.
As the Amp is 120W per channel, does this mean that the Woofer and Tweeter have to have a combined wattage of 120W(60 W ea) or does each one have to be 120W?. I will be using a 2 way crossover. And another thing, what do you reccomend the frequency response should be for the Woofer and also the Tweeter. They will most likely be in a .25 Cubic Metre Sealed Box, and will be about 6.5".
As you have proberbly realised I am new to this whole thing, I am therefore on a budget and I just want to build a pair of reasonable quality speakers. Any other suggestions are welcome.


AudioFreak 12th June 2002 10:54 AM

In most musical situations when using a 2-way crossover even if the amp is near to clipping, assuming relatively standard power distribution and of course depending on crossover points and the type of music you listen to etc. about 40% of the output voltage from the amp will appear at the tweeter and the remainder of the voltage will appear at the woofer. This has the following effect.....

Of course, all figures below are generic and many assuptions have been mad but it should give you a general idea as to how the whole thing functions.

Now if your amp is 120W RMS into 8ohms and the speakers are also 8ohms, that is about 31Vrms output from the amp.

If 40% of that voltage (about 12.4Vrms) appears at the tweeter, the tweeter will receive about 19W and the woofer would receive about 43W. So, it can be seen that when using passive crossovers, not only does the music suffer because of the crossover but you also usually (more on this soon) lose about 50% of the output power. This allows commercial loadspeaker manufacturers to use drivers with a lower power rating. The exception to this is that if almost all of the input voltage is contained within a single passband. In this case, almost all of the amplifiers rated power can be delivered to a single driver frying it in the process.


Jeroth 12th June 2002 11:12 AM

Ok, Thanks
Thanks for your informative response!

So does what you have just said basically mean I sould get a 19W Tweeter and a 43W Woofer.I wouldnt get these exact amounts, proberbly about a 25W Tweeter and a 50W Woofer.Is this the right thing to do?


Jeroth 12th June 2002 01:41 PM

I made a mistake in my first post by saying that the box will be .25 CU METRES but this should be .25 Cubic ft.


AudioFreak 12th June 2002 01:43 PM

Well it depends on what type of music you like to listen to... generally, i'd probably go for a about a 60W - 75W woofer just to err on the safe side because some music is very bassy and so more than 60% of the total musical power (voltage) may go on the woofer. A 25W tweeter should be adequate as not much music has more than 40% of the musical power (voltage) located in the treble but a slightly higher rating wouldnt hurt.

Jeroth 12th June 2002 02:28 PM

That has helped me tramendously.
When you say a 60W Woofer, is that the RMS or the MAX.Would you recommend getting woofers that are 50W RMS/75W MAX or should they be 60W RMS. And also, what frequency range would u reccomend for the woofer and tweeter.
Sorry for the amount of questions but I dont want to get the wrong thing and blow them.

Thanks Again!

griff 12th June 2002 03:09 PM

The wattage isnt all that important, although it does play a part, another thing to look out for is the sensitivity. Most DIY speakers are rated between 87-92dB, though for a 120 watt amp, less sensitivity will be fine.

For a simply 2-way, we really do need to get some exact budget figures.

As for the frequency responses, it depends on the drivers, lets start with a budget and work from there.

AudioFreak 12th June 2002 03:34 PM

All values I quoted should ideally be RMS values. 50Wrms woofer and 25Wrms tweeter would be the minimum requirements in my opinion.

Yes as Griff says, efficiency is important and so is your budget for this project.

Please post your budget.

Jeroth 12th June 2002 03:58 PM

My Budget would proberbly be about AUD$200 for the speakers, although i would prefer to have a bit of money left over,and then once I have those I will save for boxes, crossovers etc. Do you guys know any Online stores either within Oz or ones that offer shipment to Oz via money or cheque order, as i dont have a credit card.Thanks!


AudioFreak 12th June 2002 04:13 PM

ok well that's about US$110 for the pair. There are a number of suppliers in australia for good drivers.... Scanspeak, Vifa, Peerless, Audax, Adire Audio and a few others are all available in the country.

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