Chipamps; Design Considerations for bridging and paralleling.
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 28th January 2004, 02:48 PM #1 PHilgeman   diyAudio Member     Join Date: Dec 2003 Location: Arlington Heights, IL Chipamps; Design Considerations for bridging and paralleling. For an 8 ohm 80dB sensative woofer (After Baffle-Step-Losses of course) to hit 100dB you need 100 Watts. P=(V^2/R)/1.414 P=(I^2*R)/1.414 (ported box) At Fb impedence is a nice and easy 8 ohms, at say Fb+30Hz there is an impedence peak of say 30 ohms. Lets figure this out: At Fb: P=100 R=8 Solving for V and I we get: V=33 volts peak I=4.2 amps peak At Fb+30Hz: P=100 R=30 V= 92 Volts peak I = 2.17 amps peak For an 4 ohm load we'll give the benefit of the doubt and say that they are paralleled woofers with greater sensativity of 86dB. Fb: P=25W R=4 V=11.89 volts peak I= 2.972 amps peak Fb+30Hz: P=25W R=20 V=26 V peak I=1.32 A peak Because of these things, dosnt it really make a lot more sense to do designs with higher sensativity, lower impedence drivers, using amplifiers with high current output ability? I really dont think that many amplifiers ever will be able to drive many woofers to 100W at one of the impedence peaks. It seems that the lower impedence speaker is easier to drive in every respect because of the better efficency and overall lower power requirement. -Paul Hilgeman
GregGC
diyAudio Member

Join Date: Oct 2003
Location: Toronto
Re: Chipamps; Design Considerations for bridging and paralleling.

Quote:
 Originally posted by PHilgeman dosnt it really make a lot more sense to do designs with higher sensativity, lower impedence drivers, using amplifiers with high current output ability? I really dont think that many amplifiers ever will be able to drive many woofers to 100W at one of the impedence peaks. It seems that the lower impedence speaker is easier to drive in every respect because of the better efficency and overall lower power requirement. -Paul Hilgeman

I think the power requirement is connected only to the sensitivity of the speaker and if you need more voltage because the impedence is higher, that's another story. I think there is an optimum of how low your speacker impedance should be so that you don't have to build monster amps to be able to drive 0.5 ohm speackers.

 28th January 2004, 04:23 PM #3 PHilgeman   diyAudio Member     Join Date: Dec 2003 Location: Arlington Heights, IL I think the thing that people forget is that as impedence goes down sensativity *generally* goes up, reducing the overall power requirement. -Paul Hilgeman
macboy
diyAudio Member

Join Date: Oct 2003
Location: Ottawa, Canada
Quote:
 I really dont think that many amplifiers ever will be able to drive many woofers to 100W at one of the impedence peaks
You don't need to drive the woofer at 100W at an impedance peak. The reason is simple: impedance peaks correspond to resonances. At a resonance, the woofer delivers more SPL with less power. The efficiency rating (SPL @ 1W/1m) is nominal, which means kind of an average over the usual frequency range. The story is very different at resonance frequencies. Also note that when they say "SPL 1 Watt, at 1 metre" they really mean "2.83 Volts" not "1 Watt". The actual power doesn't matter, just the input voltage of 2.83 Volts (which equates to 1 watt into a nominal 8 ohm impedance but of course much less power at a impedance peak).

Also, don't forget about room gain. Below a certain frequency, you will experience a rise of (I think) 6 dB/octave due to room gain. Basically, when the wavelength of the sound is longer than the dimensions of the room, the sound no longer radiates from the woofer as a wave; the woofer pressurizes the room. So you probably will end up needing less than 100 W to achieve 100 dB.

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