speaker loads

[priapus]

I was reading through the posts the other day, I believe in the general q&a section, when I came across some people flaming each other back and forth about impedance values and effect on wattage or some such thing, but it brought to mind a question and I've had a hard time finding an answer.

So... here we go. Please keep in mind that I dont own any hi-end gear at this time, and that most of this is based on my understanding of things, which may be way off. Say your speakers are running at the pretty standard 8 ohm and you want to run at 4 ohm to increase your available wattage to stop clipping at peaks or whatever how do you go about doing this? Is this the same as bi wiring or am I way off? Maybe a setting on the amp, or do you change a resistor from the speaker itself (with a pot or physically)? I also believe that someone said that heritage speakers are best matched to the amps at 2.8 ohms. True/False?

On a completely unrelated topic, I recently saw an install of the most hideous baffles for walls that I have ever seen. They overlap each other, stick out several inches from the wall, and are varying depths and sizes. I understand the reasoning, and I know the math is beyond me, but wouldn't it be nearly as effective (and much more pleasing to the eye) to use carpet or heavy drapes or maybe put those Styrofoam egg crate things on?

[dodger]

For an example, a nominally rated 8 ohm speaker does not present an 8 ohm load constantly.

It depends on frequency, the ohm rating for each speaker or horn in the whole unit, plus the quality of crossover can be a factor.

Most of what I have seen in changing output on the Amplifiers is with units that have individual 16, 8, 4 ohm taps that you can hook up to. There are other ways, but this scenario is usually what is being flamed, discussed, etc.

Some amps can be bridged, thus in part offering a 4 ohm output with higher wattage output.

But the 8 ohm speaker can present 4, 5, 6, 7, 8, up to 12 ohms at different frequencies.

[michael hurd]

For starters, most tube amplifiers require that the user select the correct "tap" or output that best matches the speaker impedance. With solid state, this is not necessary, with certain exceptions ( macintosh ss with autoformers ).

It was noted that the RF-7 dipped to a low of 2.8 ohms impedance, and the driving amplifier should be up to the task of driving a difficult load. They will certainly work with almost any amplifier, even modestly inexpesive receivers, but will produce better results with an amplifier that can drive a low impedance load well.

Regarding room acoustics, there is more to it than just adding a couple of egg crates to the wall. While some common materials might help, most will work over a small range of frequencies, rather than being "broadband", or affecting all frequencies equally.

I encourage you to read up a little on the subject on how it can affect the quality of reproduced sound. Most people would sooner spend hundreds, or thousands on equipment to try and "upgrade" the sound, without considering the effect of the room itself on the sound.

[michael hurd]

As Dodger noted, speakers are never truly " 8 ohms ", or their nominal rating. The impedance ( different from simple resistance as measured with a volt-ohm meter ) is constantly and wildly changing with frequency.

A nominal rating is simply a rating where the speaker is most often similar to that load.

By wiring like speakers to a single amplifier channel, this changes the " apparent " load that the amplifier sees. If we revist our science classes, when lightbulbs are hooked in series, each one becomes dimmer, because the resistance doubles and current is choked off. If we turn around and hook them up in parallel, you can add several bulbs, and they will all be as bright as a single bulb.

By wiring speakers in parallel, ( tying + to +, and - to - ), then hooking that to a single amplifier channel, you are halving the load as seen by the amplifier. For example, if you had 2, 8 ohm speakers, by wiring them in parallel, you would end up with a 4 ohm load. The same two speakers wired in series would get you a 16 ohm load.

To find the wattage that a speaker receives, take the amplifier voltage ( AC )at the speaker output terminals, then square it ( times itself ), then divide by the speaker impedance.

If we measured 10 volts AC at the speaker output terminals, then 10*10=100. Dividing 100 by 8 ( ohms ) would give us 12.5 watts. At the same voltage ( 10 volts ), dividing by 4 would give us 25 watts.

[michael hurd]

To continue ( don't want to have you burned out yet ), an amplifier will try to put out more wattage as the load is decreased again. For that same voltage at the speaker terminals, say we connected (4) 8 ohm speakers in parallel. We now have a 2 ohm load presented to the amplifer. For the same 10 volts at the speaker terminals, 100/2 = 50. We now have 50 watts that the amplifier is putting out.

However that 50 watts is being shared among 4 speakers, so each speaker is still receiving 12.5 watts, the same as a single speaker would.

Most amplifiers have a rating on then as to how low an impedance they can drive safely. Some inexpensive units have limited current capacity output devices, and are limited to an 8 or 6 ohm load ( most receivers ).

Better units are rated to safely drive 4 ohms, have more robust output devices, and better cooling. Some pro-sound units can drive loads at or below 2 ohms.

Note: even though an amplifier may be rated to drive a low impedance load, this is not an indicator of quality, nor is the price tag.

[michael hurd]

Say you were to hook a 2 ohm load to an amplifer, rather than a safer 8 ohm load, the current that the output stage must put out will quadruple. Instead of 1.25 amps of current at our 10 volt drive level ( at the amp output terminals ), it must put out 5 amps of current instead.

Due to added thermal increase and extra resistance, we may not actually get 50 watts, as the voltage may drop sligthly.

[dodger]

On a scale of 1 - 10 - 10 being the best, I will give Michael a 9.9 for his replies. (I only give 10s when paid LOL)

Hopefully between what has been noted, your questions are answered.

Great job, Michael!!

[wuzzzer]

Yow, that's a bit of techno-savvy if I say so myself! Gotta love it. This is truly an awesome forum!

So, can someone give me a quick answer to this question: My SS receiver has a 4ohm/8ohm switch. Will it make any difference to my RF-25s what position it's in? I know that in the past when I've hooked up speakers that were labeled 4ohm and had the switch in the 4ohm position the lights on the front panel of the receiver would dim slightly with deep bass when I had the volume cranked up.

[michael hurd]

A receiver that has a 4/8 ohm switch has seperate taps on the power transformer, when you move that switch, you are changing the tap on the power transformer.

This allows the unit to put out more current at the lower impedance setting, than it would be at the 8 ohm setting. My gut feeling is to try it on the 4 ohm tap, and see for yourself.

If it seems to work better there, than leave it in that position. I won't tell you what to listen for, that would be me giving you an impression. Be sure to switch only with the unit in the off position, never attempt this while the amplifier is operating.

[wuzzzer]

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On 7/5/2005 10:39:29 PM michael hurd wrote:

Be sure to switch only with the unit in the off position, never attempt this while the amplifier is operating.

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Wow, thanks for the warning! I've switched the 4/8 ohm before while the receiver was on, but only if there was no sound or very low sound playing. What damage could result from this?

[priapus]

thanks for all the info, i'll sleep better at night knowing after all that, ill never be able to pose an unaswerable question here.

much appreciated, and i will definately look into the room treatments.