Speaker Ohm question - AVS Forum
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post #1 of 17 Old 03-16-2014, 12:23 PM - Thread Starter
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I have a set of Bose outdoor speakers (model 251) rated 4 - 8 ohms.  I have been buying fairly cheap bookkshelf stereos over the years to put in the backyard for music and have to replace them every couple but the speakers still work beautifully.  I just purchased a Sony bookshelf mainly because I can plug in my iphone 5 and play all my music through the phone and icloud.  But, in opening the system I see that the speakers that came with the system are rated at 3 ohms and the place on the back where you plug in the speakers also say use 3 ohm speakers.  Can I use the Bose speakers or what kind of trouble will there be?

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post #2 of 17 Old 03-16-2014, 01:21 PM
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Originally Posted by chuckebabe View Post

I have a set of Bose outdoor speakers (model 251) rated 4 - 8 ohms.  I have been buying fairly cheap bookkshelf stereos over the years to put in the backyard for music and have to replace them every couple but the speakers still work beautifully.  I just purchased a Sony bookshelf mainly because I can plug in my iphone 5 and play all my music through the phone and icloud.  But, in opening the system I see that the speakers that came with the system are rated at 3 ohms and the place on the back where you plug in the speakers also say use 3 ohm speakers.  Can I use the Bose speakers or what kind of trouble will there be?

When an amp is designed to drive low impedance (3 ohms) speakers, and is used to drive slightly higher impedance (4-8 ohms) speakers nothing bad happens.
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post #3 of 17 Old 03-16-2014, 01:25 PM - Thread Starter
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Thank you very much for the info.  I'm going into the backyard right now to hook em up and start playing some music!

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post #4 of 17 Old 03-17-2014, 11:23 AM
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One issue you may have is limited volume as the higher impedance speaker will draw less current (and therefore less power) at a given amplifier voltage - but that usually isn't an issue for outdoor / background music. One big advantage is that you could probably run two sets of speakers in parallel (if they were both 8 Ohm) and still be within the designed impedance of your amplifier.

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post #5 of 17 Old 03-17-2014, 11:51 AM
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output will of course depend on the individual speakers' sensitivities. Typically, lort of, lower impedance speakers are less sensitive (that's why they're often rated at the voltage that yield 1 watt at 8 ohms - - so that the unknowing will not realize they need to double the power (ie X dB at 2 watts instead of 1 watt) for a 4 ohm speaker.

The lower sensitivity/ lower impedance speaker will get less loud for any given power, but you have to turn down the volume control to get, say, a 4 ohm and 8 ohm speaker to hit the same power level. IOW at any given volume control setting (non-calibrated) the lower impedance speaker will be louder because it's getting more power. BUt you can just turn up to get more power to the higher impedance speaker. Almost all amps will supply more clean power (ie higher power rating) into the higher impedance (within reason, say 2 vs 4 vs 8 ohms) so that in the end the higher impedance speakers can get louder assuming a person ever gets close to clipping their amp . . .. Otherwise its just about where the volume control (or how the gain staging) is set, and is just not terribly important . . .
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post #6 of 17 Old 03-17-2014, 12:08 PM
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Originally Posted by mtn-tech View Post

One issue you may have is limited volume as the higher impedance speaker will draw less current (and therefore less power) at a given amplifier voltage - but that usually isn't an issue for outdoor / background music. One big advantage is that you could probably run two sets of speakers in parallel (if they were both 8 Ohm) and still be within the designed impedance of your amplifier.

I don't know how much we should worry about the difference between speakers rated at 3 ohms and speakers rated at 4-8 ohms. I know for sure that some manufacturers take speakers that should be rated at 3 ohms, and call them 4-8 ohms and vice-versa. And efficiency ratings if they are even given, can vary as much between the spec sheet and the real world.
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post #7 of 17 Old 03-18-2014, 08:55 AM
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I experimented with my stereo receiver Pioneer XR-P170C rated from 16 to 4 Ohms with a generic 25 W RMS per channel rating at, I believe, the proprietary speaker impedance of 4 Ohms. I tested with a 500-2500Hz -20dBFS pink noise on the left channel of a stereo track with mute on the right channel and if I connect only one of the speakers with the positive to the left output and the negative to the right output of the receiver, I lose 6dB at the same volume knob level. Shouldn't bridging add power, thus sound louder at same knob level? What's happening in my case?
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post #8 of 17 Old 03-18-2014, 10:12 AM
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Originally Posted by luca_frontino View Post

I experimented with my stereo receiver Pioneer XR-P170C rated from 16 to 4 Ohms with a generic 25 W RMS per channel rating at, I believe, the proprietary speaker impedance of 4 Ohms. I tested with a 500-2500Hz -20dBFS pink noise on the left channel of a stereo track with mute on the right channel and if I connect only one of the speakers with the positive to the left output and the negative to the right output of the receiver, I lose 6dB at the same volume knob level. Shouldn't bridging add power, thus sound louder at same knob level? What's happening in my case?

you are not describing bridging. You are simply describing, I think, either connecting one 4 ohm speaker or two 4 ohm speakers in parallel. Bridging uses both "sides" of a stereo amp to increase power, and unless the amp is built to do it is a wonderful way to turn an amplifier into a boat anchor.

When you double the speaker area at the same power you get a 3 dB theoretical gain. When you double the power without changing the speaker area you get a 3 dB theoretical gain. When you connect two speakers in parallel, you halve the impedance and, because Ohm's law is actually a physical law, you double the power at any given volume control (voltage gain) setting. So you doubled power and doubled the speaker area, and as it happens 3 dB gain plus 3 dB gain is 6 dB gain (or loss).
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post #9 of 17 Old 03-18-2014, 10:26 AM
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Quote:
Originally Posted by luca_frontino View Post

I experimented with my stereo receiver Pioneer XR-P170C rated from 16 to 4 Ohms with a generic 25 W RMS per channel rating at, I believe, the proprietary speaker impedance of 4 Ohms. I tested with a 500-2500Hz -20dBFS pink noise on the left channel of a stereo track with mute on the right channel and if I connect only one of the speakers with the positive to the left output and the negative to the right output of the receiver, I lose 6dB at the same volume knob level. Shouldn't bridging add power, thus sound louder at same knob level? What's happening in my case?

Bridging involves driving the two amplifiers with reversed polarity. It doesn't seem to me that you are doing this. It would involve feeding a mono signal to a polarity inverter and driving one channel with the direct signal, and the other channel with the inverted signal.

Fully muting one channel is essentially the same thing as making its output be the same as ground. What you've created is called an active ground.
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post #10 of 17 Old 03-18-2014, 10:59 AM
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I shall explain myself better: the pink noise is in the left channel of a stereo audio track with the right channel of the stereo track mute. Only one speaker is connected to both amplifier's channels the following way: left positive to speaker's positive, left negative empty, right negative to speaker's negative, right negative empty. Why am I having a 6dB loss? When I play the pink noise on both channels to the aforementioned single speaker connected that way, the gain is still 6dB over 1 channel, thus how could ground being lowering output of side channels?. Is the impendance being doubled instead of halved?
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post #11 of 17 Old 03-18-2014, 12:34 PM
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when you really Bridge an amplifier, 1 channel becomes the " push" and the other channel becomes the "pull" of a push pull amplification device. Absent some adjustment to the amplifier self, simply connecting thespeaker ground too a separate amplifier does not magically create bridgett application. It does, potentially, create dead amplifiers.
Perhaps more technically, as arny notes, it's necessary to reverse the polarity of one of the amplifying channels. Nothing you have done reverses the polarity of a channel. Accordingly, you cannot be bridging, whether you think you are or not.
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post #12 of 17 Old 03-18-2014, 12:37 PM
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Originally Posted by luca_frontino View Post

I shall explain myself better: the pink noise is in the left channel of a stereo audio track with the right channel of the stereo track mute. Only one speaker is connected to both amplifier's channels the following way: left positive to speaker's positive, left negative empty, right negative to speaker's negative, right negative empty. Why am I having a 6dB loss? When I play the pink noise on both channels to the aforementioned single speaker connected that way, the gain is still 6dB over 1 channel, thus how could ground being lowering output of side channels?. Is the impendance being doubled instead of halved?

Asked and answered. This looks to me like an excellent example of why people should study up on what they are doing before they try to do it, and if that is too time consuming or hard for them, they should bag it.

Here's a fishing lesson: http://en.wikipedia.org/wiki/Bridged_and_paralleled_amplifiers
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post #13 of 17 Old 03-18-2014, 01:03 PM
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to put it very shortly, if the amp does not have a switch on the back that says "stereo" in one direction and "bridged" in the other (or something similar) then IT I NOT SAFE to try to run it in bridged mode. It requires a lot more engineering change than simply mis-connecting the speaker to actually create bridged mode. If all that is meaningless to you, OP, STOP NOW. At best you'll accomplish nothing, at worst you'll burn down your house. In between, your amp will become a boat anchor.

The connections you describe seem, to me no different than me connecting the positive side of one channel of my receiver to the + of the speaker, and the _ of the speaker to the minus side of one of my guitar amps. They are two separate, separated amps. Not connected. Not set up, in any way, to act as a single bridged amp. Don't go there . . .

AFAIK, how impedance and speaker area affect output are already covered and require no further explanation, unless you can identify your particular difficulty in understanding it. (no shame in such difficulty - I continue to learn, and have probably at best a tenuous grasp on the issues). But as a person who drives tube amps well into distortion (way way past 10 percent) several times a week, I have to get a bit of a grasp on how things work or risk trying to put a microphone on my stratocaster at some gig after I've effed everything up. Trust me, the mic will not work at even coffee shop levels, with a solid body guitar,let alone at midnight at a bar gig. Ever heard an electric guitar direct through the PA without any EQ/preamp? Unless you're playing jazz, and can just roll off the highs, it's not a pretty thing. . .
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post #14 of 17 Old 03-19-2014, 11:50 AM
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What if both grounds are shared in the receiver? I'm asking this because I played some movies in their entirety at -52dB from maximum volume and a pressure close to Dolby's reference level at 40-50cm from the speaker and nothing bad happened. It even survived the infamous 51' minute of The Dark Knight Rises with no harm. Am I just using a volume level too low to cause an overloading?
Is there another safer wiring? Would using only the negatives or only the positives be better?
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post #15 of 17 Old 03-19-2014, 02:25 PM
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Quote:
Originally Posted by luca_frontino View Post

What if both grounds are shared in the receiver? I'm asking this because I played some movies in their entirety at -52dB from maximum volume and a pressure close to Dolby's reference level at 40-50cm from the speaker and nothing bad happened. It even survived the infamous 51' minute of The Dark Knight Rises with no harm. Am I just using a volume level too low to cause an overloading?
Is there another safer wiring? Would using only the negatives or only the positives be better?

In home audio the grounds for all output channels are typically connected together. IOW, the outputs aren't bridged by default.

Bridged amplifiers are widely used for automotive audio, in order to obtain enough voltage while running directly off of the car battery. However, many modern car audio amps have switchmode power supplies and are therefore not bridged.
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post #16 of 17 Old 03-19-2014, 02:32 PM
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so the OP would be better off simply using the amp the way it is designed. connect the speaker(s) for each channel to one channel of the amp, rather than using the ground from a different power amp section of the particular amplifying device. Because if all the grounds are the same, using the "wrong" ground is exactly, precisely, the same as using the correct ground. He gains nothing and is still using the transistors in (say) the left channel to give him every iota of gain he's getting from the amplifier. The channel that's not connected isn't doing anything.
Connecting positive to positive sounds like "how to blow up my amp" to me. The relatively minor impedance of the speaker(s) makes this not significantly different from simply stringing a piece of wire from positive to positive and waiting to see if a fuse blows before something starts smoking . . .
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post #17 of 17 Old 03-19-2014, 02:59 PM
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so the OP would be better off simply using the amp the way it is designed.

That has been true all along.
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connect the speaker(s) for each channel to one channel of the amp, rather than using the ground from a different power amp section of the particular amplifying device. Because if all the grounds are the same, using the "wrong" ground is exactly, precisely, the same as using the correct ground. He gains nothing and is still using the transistors in (say) the left channel to give him every iota of gain he's getting from the amplifier. The channel that's not connected isn't doing anything.
Connecting positive to positive sounds like "how to blow up my amp" to me. The relatively minor impedance of the speaker(s) makes this not significantly different from simply stringing a piece of wire from positive to positive and waiting to see if a fuse blows before something starts smoking . . .

Agreed. I think we have run out of ways to tell the OP that he's risking far more than any possible return.

I'm thinking that there may be an (presumably unconscious) desire to fry the old amp and buy a new one, even though the old one seems perfectly adequate. Its biggest fault maybe that it is too durable to please its owner. ;-)
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