I know the bi-amping Q must be a tired old horse...

[HippieDave]

...but I've got a real practical Q, and I don't have the expertise to answer it.
I'm getting a pair of B&W 603 S2s to set up in my den/office. Their power rating is: 25 - 120W into 8 ohms. The B&Ws have dual terminals. I have a Harmon/Kardon PA2000 Bridgeable Multichannel Amplifier with the following rating:

Power Output
Normal Mode 4 x 45 watts @ 8 ohms, 20Hz – 20kHz, <0.07% THD, all channels driven
Bridged Mode 2 x 100 watts @ 8 ohms, 20Hz – 20kHz, <0.07% THD, all channels driven

I want to know whether I'm better off 1) bridging the amp, and using the speaker terminal jumpers to run the speakers with 100W/channel; or
2) unbridge the amp and bi-amp the speakers, running one 45W per channel amp into the tweeter and a separate 45W per channel amp into the woofer/mid-range drivers.

Any help here?

 

[markw]

Personally, I'd think that since bass requires more power than mids and highs that bridging them for more power and thereby giving the bass section access to as much power as possible would seem logical to me.

But, since you have everythng already, why not spend an afternoon and try both ways and see which sounds best to you. And, if you really can't find a clear winner, just pick one.

I'm assuming thatthose speakers aren't 4 ohms. If they are, ignore this post.

 

[HippieDave]

nope..8 ohmers.
But it is a question of buying and rigging another pair of cables...so I thought I'd start with finding out if there was a clear favorite alternative here.

 

[Pyrrho]

Assuming that there are no nasty dips in the impedance of the speakers, you will want to do it as markw suggested, and bridge the amp. There is a reason, though, that he neglected to tell you. To properly bi-amp the speakers, you would want to use an electronic crossover so that the different channels of the amplifier are not all trying to amplify all of the audio spectrum.

Now, if there are nasty dips in the impedance, bridging amplifiers makes them less capable of dealing with that. Generally speaking, amplifiers that are bridged have a minimum recommended impedance that is double the minimum recommended impedance for the amplifier unbridged. You might want to look at your manual for the amplifier, or download it (assuming it is available online) if you don't have a printed copy.

 

[markw]

I corrected that for ya.

. Generally speaking, amplifiers that are bridged have a minimum recommended impedance that is one half the minimum recommended impedance for the amplifier unbridged

 

[HippieDave]

The quote in my OP is from the users manual on the amp and gives it a rating of 100W into 8 ohms bridged-- is this amp different than Pyrro was thinking, or am I still not understanding this?

 

[markw]

The quote in my OP is from the users manual on the amp and gives it a rating of 100W into 8 ohms bridged-- is this amp different than Pyrro was thinking, or am I still not understanding this?

All it's saying is that when you run it in bridged mode, you must use 8 ohm speakers. It doesn't come out and say it, but you cannot use 4 ohm speakers without running the risk of damaging your amplifier.

Aside from that one typo I corrected, Phyrro gave you the reason.

nope..8 ohmers.
But it is a question of buying and rigging another pair of cables...so I thought I'd start with finding out if there was a clear favorite alternative here.

There is no generally accepted favorite here. Most of us don't bother with either. As I said earlier, why not try both ways for yourself and then decide for yourself.

 

[Pyrrho]

I corrected that for ya.

Your "correction" is wrong. What I stated originally is correct:

... Generally speaking, amplifiers that are bridged have a minimum recommended impedance that is double the minimum recommended impedance for the amplifier unbridged. ...

Consider an amplifier that is rated for 4 ohms minimum in normal use (unbridged). Most likely, it will be rated for 8 ohms minimum when bridged. Thus:

Generally speaking, amplifiers that are bridged have a minimum recommended impedance [8 ohms] that is double the minimum recommended impedance for the amplifier unbridged [4 ohms].
​

8 is double 4, not half of 4.

I happen to own a bridgable amplifier that is rated for 2 ohms minimum normally, and 4 ohms minimum when bridged. So what the minimum is will vary according to the amplifier, just as is the case with unbridgable amplifiers.

 

[markw]

Sorry, P, no.

You can spin it any way you want, but a bridged amp seem the load as one half (not double) it's rated impedance. I.e., a bridged amp will see an 8 ohm speaker as a four ohm load. Don't try to confuse the newbies with word games.

You can do your own homework on this one.

 

[Pyrrho]

The quote in my OP is from the users manual on the amp and gives it a rating of 100W into 8 ohms bridged-- is this amp different than Pyrro was thinking, or am I still not understanding this?

A power rating into a particular impedance does not tell you the minimum recommended impedance. However, if it is fine bridged into 8 ohms, it will be fine into 4 ohms unbridged.

My first comments in this thread also includes the idea of speakers not being one impedance (because impedance typically varies with frequency), and considering what the minimum impedance of the speaker really is. Many manufacturers basically lie about the impedance when they give a "nominal" rating, and so one should take care about what the impedance of a speaker really is when matching an amplifier to it. This means looking for professional reviews in which the speaker is actually measured for impedance, or basing one's matching on the minimum impedance of the speaker rather than the claimed nominal impedance. Some manufacturers are honest in their impedance ratings, and so the nominal impedance that they list is correct, but one can still use the minimum impedance for proper matching. You can read the article at Wikipedia about this:

Electrical characteristics of dynamic loudspeakers - Wikipedia, the free encyclopedia

The nominal impedance is defined as 1.15 times the minimum impedance, though, evidently, "nominal impedance" has no legal meaning, given the fact that so many manufacturers just pull numbers out of their asses when stating the "nominal impedance." Most likely, this is due to them making a speaker with a low impedance, but they wish to sell it to people who do not have amplifiers appropriate for them and would not willfully do an improper match. And thus when the poor sucker, their customer, destroys his or her amplifier, that is just too bad for the poor sucker, as the speaker maker is not going to pay for the damage that they caused by lying about the impedance.

In the case of bridging an amplifier, one is more limited in the speakers that are recommended for it, as the minimum acceptable nomimal impedance (see formula at link above) tends to be double the minimum acceptable nominal impedance when unbridged. (Higher impedances typically do not harm amplifiers, but typically one can get less power from the amplifier at higher impedances.)

 

[Pyrrho]

Sorry, P, no.

You can spin it any way you want, but a bridged amp seem the load as one half (not double) it's rated impedance. I.e., a bridged amp will see an 8 ohm speaker as a four ohm load. Don't try to confuse the newbies with word games.

You can do your own homework on this one.

I did not write anything about what load the amplifier "sees;" I wrote about what minimum nominal impedance of speaker is recommended to be hooked up to the amplifier. With an amplifier that is normally (i.e., unbridged) recommended to have a speaker with a minimum nominal impedance of 4 ohms, it will typically be recommended to have a speaker with a minimum nominal impedance of 8 ohms when bridged. Thus, the minimum nominal impedance of the speaker to be hooked up to the amplifier DOUBLES when one goes from unbridged to bridged. This is the information that one needs to know, not what the amplifier "sees." It is you who are confusing the issue.

 

[markw]

I did not write anything about what load the amplifier "sees;" I wrote about what minimum nominal impedance of speaker is recommended to be hooked up to the amplifier. With an amplifier that is normally (i.e., unbridged) recommended to have a speaker with a minimum nominal impedance of 4 ohms, it will typically be recommended to have a speaker with a minimum nominal impedance of 8 ohms when bridged. Thus, the minimum nominal impedance of the speaker to be hooked up to the amplifier DOUBLES when one goes from unbridged to bridged. This is the information that one needs to know, not what the amplifier "sees." It is you who are confusing the issue.

No, you tend to use too many words to convey too little information of importance in order to appear more intelligent than the one asking the question.

All one "needs" to know on this is that when bridging two amp channels, it will treat the speaker as one half of its rated impedance and that could be fatal to many an amplifier when pushed.

 

[slipperybidness]

No, you tend to use too many words to convey too little information of importance in order to appear more intelligent than the one asking the question.

All one "needs" to know on this is that when bridging two amp channels, it will treat the speaker as one half of its rated impedance and that could be fatal to many an amplifier when pushed.

Yup. So if the amp is rated as 4ohm unbridged, then putting a 4ohm on it bridged is asking for trouble. When bridged, you would want no lower than 8ohm load as the amp will see a 4ohm load.

I think you got a little confused, or if not, then the way you stated it in your first post was confusing.

 

[zhimbo]

No, you tend to use too many words to convey too little information of importance in order to appear more intelligent than the one asking the question.

Actually, his original post was very concise and also perfectly clear (and correct) as originally written.

When you write:
"it will treat the speaker as one half of its rated impedance" you are AGREEING with Pyrrho's original post.

 

[markw]

Actually, his original post was very concise and also perfectly clear (and correct) as originally written.

When you write:
"it will treat the speaker as one half of its rated impedance" you are AGREEING with Pyrrho's original post.

Concise? No. Clear? No.

Obfuscation of a simple answer to a simple question from a newbie who could probably care less about the inner workings of electrical theory? Yes

For all intents and purposes, this whole matter could have been cleared up with one sentence, which I did in posts 2, 7, 9, and 12.

Of course, that leaves no room for showboating. The closest he comes to that simple answer is here, which is essentially what I've said from the beginning:

In the case of bridging an amplifier, one is more limited in the speakers that are recommended for it, as the minimum acceptable nomimal impedance (see formula at link above) tends to be double the minimum acceptable nominal impedance when unbridged. (Higher impedances typically do not harm amplifiers, but typically one can get less power from the amplifier at higher impedances.)

Now, which is clearer? Berevity and clarity combined is a good thing.

But, it may be a moot point. I think the newbie was intimidated by the implied complexity and ran screaming to another forum. Jolly good show!

 

[zhimbo]

Pyrrho said "what [you've] said from the beginning"? So, according to you, he was correct, in his first post in this thread, in what you call "closest to that simple answer". I really don't understand why this discussion even took place.