8 ohms vs. 4 ohms

[m00n]

I have seen on some aps where you will have say 100 watts at 8 ohms, or if you bridge you will have say 50 watts at 4 ohms...

Does decreasing ohms have an affect on sound quality?

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[lynnm]

Just picking a nit here but I think you've got it bassackwards there m00n. Typically one would expect the amplifier to be rated as having a higher power output into 4 ohms than into 8.

As to whether amps sound better driving a high impedance load than a low impedance load I don't think it makes any difference whatever,(assuming of course that the amplifier is well designed and that the 4 ohm and 8 ohm speakers being compared are otherwise very similar).

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[j-malotky]

Moon wrote:

Does decreasing ohms have an affect on sound quality?

The answer is NO.

There are quality 4 ohm speaker and quality 8 ohm speakers. The key is to have an amp that can drive the ohms your speakers are.

Lynnm is correct about power / ohms ratio.

Here is an example. Take two 8 ohm speakers and connect them in parrellel. (meaning connect the two positive together and the two negitives together)and connect it to your amp. The combinination you now have is a 4 ohm speaker. Is this speaker more effecient or sound a lot better - Probably not. Since the power to sent to both speakers equally, it requires twice the power to drive both speakers.

JM

[boa12]

& applying this to what i think you're thinking of doing - connecting 2 sets of 8ohmer surrounds to the same terminals - that means that it will draw more power from the amp to be able to do so. so for a specific channel rated 100W @ 8ohms, driving into a 4ohm load you may get say for example 150W out of the amp. keep in mind though these are total output figures & not what you'll usually be pushing the amp to.

but this is for 2 speakers. iow, you have to have an amp w/ the power reserves & capability to do it. & obviously each speaker won't get the same amount of amp power available as to a single 8ohm speak.

if theoritically you want the same 100W power as before to 2 speakers on that same channel, you need something rated at 100W into 8ohms & 200W into 4ohms. which would tend to be a pretty pricey amp.

but your hk receiver may be fine driving 4ohms on each surround channel w/ efficient speaks. depends on its power capability & how much you're going to drain it. don't think hk publish or release a 4ohm rating, though you could still try it & see if it clips.

may help to just get a good 2-channel amp for the fronts & let the hk drive the rest. like i said you may even be able to delegate the hk to run rear center speaks since its then not driving the fronts. if your hk will do this like the denons, then it'll also have speaker terminals for the rear surround/effects channels.

sorry if the above is not electronically precise as far as voltage & current & such, but hopefully it gets across the general concept w/o too much error in assumptions.

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This message has been edited by boa12 on 07-06-2002 at 03:21 PM

[WMcD]

Yeah, the math is wrong and there are some other problems. "Bridging" is another issue.

The below are gross simplifications.

Generally, the power drawn by the speaker is

Power = Volt * Volts / Resistance (ohms).

If you want 1 watt into an 8 ohm load, the voltage must be 2.82 volts. 2.82 volts x 2.82 volts / 8 ohms = 1 watt.

The following is the electrical connection across the 8 ohms assuming one amp and one speaker. It corresponds to hooking up the speakers to the amp output with red and black terminals.

(+) 2.82 volts -- <8 ohms> -- (0) volts (ground).

If you have two amps and two speakers, you get 2 watts. They add. No problem, the amps and speakers could be on different planets.

Also, generally, if you hook two 8 ohm speakers in parallel, the result is 4 ohms; and the single amp output is 2 watts or 4 watts total if you have two. (And some speakers have a nominal 4 ohm resistance to begin with.)

This looks very nice because if you do the math, there is twice the power delivered into 4 ohms. Or 2 watts from each amp. If you have two 4 ohm speakers hooked up.

But looking at the amp specification. You'll see something like 100 watts into 8 ohms, and probably 180 (or so) watts into 4 ohms. This is just an expression that the amp can't provide the same voltage into a lower resistance. The power supply runs out of steam from lack of current capacity.

Is the frequency response degraded? Maybe not. However, if there is too low a resistance presented, the amp's performance is harmed. Perhaps a protection circuit trips out first.

"Bridging" is not the same as hooking speakers in parallel. But it relies on the same principle and causes the same problem of the amp running out of current, or steam.

When two amps are "bridged", they are arranged so that their outputs are 180 degrees out of phase, or polarity reversed. They are used to drive ONE speaker. It is more than I can explain here.

The result is that single speaker is "bridged" across the "plus" outputs of the two amps. One output is going positive 2.82 volts while the other is going negative 2.82 volts.

You have to consider that this "bridge" is making a single 8 ohm speaker look like two 4 ohm speakers in series.

You have to step back a moment and realize that two 4 ohm resistors in SERIES is equivalent to an 8 ohm resistor. Or, rather, an 8 ohm resistor is equivalent to two 4 ohm resistors. Same concepts and interchangable. Now the circuit looks like:

(+)2.82 v. -- <4 ohms --(0 v.)-- 4 ohms>--(-)2.82 v.

It takes some looking at. The speaker terminals at the left and right are going to plus and minus 2.82 volts respectively. Also, we've replaced the 8 ohms with two 4 ohms in series in the < >. Importantly, where they are in series, the center voltage is zero. This is the same as "ground" and could be called a "virtual ground."

So, look carefully. We have an 8 ohm speaker hooked to amps driven out of phase. To either amp, it looks like a 4 ohm load. At least it is 4 ohms to ground voltage (zero "0" volts) in the center.

The bottom line is that by bridging a single speaker across two amps, each amp sees half the nominal value of the resistance of the single speaker. Therefore, each amp is delivering twice the power. But again, this assumes each amp has a hefty power supply.

Gil