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Discussion Starter · #1 ·
"Contingency Plan" got me thinking, one of the replies cost someone $10, this one cost (got me) a case of brew. We had a hot shot know-it-all apprentice at the shop awhile back, who needed to be taken down a few pegs. Audio Stuff.

I have a single amplifier (2 channel - 150W per), that is stable to a 4 Ohm load. On a per channel basis, there are 4) 4 Ohm loudspeakers connected in parallel to each channel (tweet, mid, mid-bass, and Sub). There are no "active" or fancy electronics involved, just capacitors and inductors. The amplifier "sees" a 4 Ohm Load, and is not overheated. How or why is this possible?


(real system, in my '64 BTW)
 

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Ok, I'll play (beats working) but it's been several years since I opened the audio encyclopedia. 2 series, 2 parallel produce 4 ohms.
 

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The speakers are not in parallel if you are using crossovers (caps and inductors); they will have these components in series with them. The speakers are only going to see the frequency range the crossover is designed for, so you could say that the amp will see the impedance of the tweeter at frequencies above 10,000 hz, and the impedance of the midrange is only seen from 5000hz to 10000 hz, etc.
 

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Gene's got it. Because of the crossover, each driver only loads the amplifier over it's crossover operating frequency band. Then, the impedance of that particular driver-crossover combination increases over the rest of the audio band. The crossover becomes a high resistance outside of it's operating range, which increases the resistance "seen" by the amp.

As a simple example, at 50hz the amp only see's the sub as a load and at 10khz the amp only sees the tweeter as a load.

BTW, speakers are driven with AC signals and they present a complex load to the amp, not just a simple 4 ohm resistance. I'd also bet that there actually won't be very many points where the amp load actually equals 4 ohms. There will also be ranges where 2 drivers each present a higher impedance load to the amp.

Peter
 

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Discussion Starter · #7 ·
Gene gets a cookie!


He's correct. The load is frequency specific. With the proper crossover networks the amplifier will only see a 4 Ohm load at any given time.

Since music is a "dynamic" load there will be instances (xover point) where the load will become nearly a dead short, but they are so short in duration it's not a factor.

I had the HARDEST time explaining this to our self-appointed "super-tech" until I wired it up and used an adjustable freq. generator to prove that only 1 loudspeaker was playing at a given time at at given frequency.
 

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Damn! Didn't even get a chance to play!
 

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Would you mind providing the values of the caps used and are they in series or parallel with each speaker? I'm guessing they would be in series.

Also can you answer a question or two?

If a speaker is rated at 4 ohms impedance at what frequency is it 4 ohms, is there a standard frequency at which all speakers are rated or is it an average over the audio frequency range?
 

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Discussion Starter · #11 ·
The cap or inductor values vary depending on the frequency you're trying to pass or block and the nominal impedance of the driver. I'll dig around in my stuff and come up with a simple crossover 101 chart or something and post it in the future.

A loudspeaker's impedance is infinitely variable in operation, but measured sitting static the reading will be in the neighborhood of 4, 8, 16, or 40 Ohms (common values). The spec. sheet on "pro" stuff will give exact measurements and the conditions tested.
 

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Discussion Starter · #13 ·
At any given time there will be a moment in time (Ms) where the signal (music) is going to hit the point where one xover stops and another takes over. In other words the transitional period between highs and mids, or mids & bass, etc. This moment is so short in time that it isn't noticed audibly, but electrically it is an overload (two drivers "working" at the same time).

The slope (depth) of the x-over (6,12,18,24 db/octave) and the "Q" (width) will dictate how audible or in-audible (smoothness) these transitions will be.
 

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Ok, so it's not a short. If the crossover is properly designed it will be attenuating both drivers at the same time so that both present a load higher than 4 ohms. This means they won't present any more load then at any other point in the frequency spectrum.

The crossover isn't a sudden switch between drivers but a gradual transistion that could occur over 100's of hz or more.

Peter
 

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The reference frequency for speakers is 1khz unless it is too inefficiate at that frequency such as a sub or a tweeter. the generally accepted method is to read the impedance at frequency in the center of its active band. In ac circuits power is calculated by cosine of the phase angle in relation to the voltage and current. This is why a door bell transformer consumes no power until the door bell on the secondary side of the coil is rung. Ideally a crossover will be down 3db or half the power at the crossover frquency. effectively the two speakers should add together to alleveiate any noticible difference but no one could notice that anyway. Your answer is right as long as you use a coil in your crossover to keep the highs out of the lower drivers.
 
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