Any "Down-Side" To Bridging? - AVS Forum
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post #1 of 8 Old 03-10-2012, 03:38 PM - Thread Starter
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Can any of you EE-types, tell me if there is any issue...or "down-side" to bridging an amp, "made" to operate in stereo?

Please...no lectures about how all SS amps sound the same, or that it's only function is to do this or that. I'm just wondering...using Parasound amps for example: the A23 bridges to 400wpc at 8ohms...and you can get a pair for $1900. The A21 is "only" rated at 250wpc at 8ohms, and it runs $2300 (all these prices, of course are full retail...and akin to paying sticker for a new car...lol; but for the sake of comparison). Along those same lines...the A21 can be bridged, and is rated at 750wpc into 8ohms...and a pair runs $4600; whereas the JC1 mono is 400wpc into 8ohms, and costs a cool $9k for a pair.

Why would anyone do it...unless there was something more to account for, than just these power numbers? I know current delivery has something to do with it; and the bigger units can probably deliver more, etc. But is there something you could likely hear, that is a trade-off with bridging?

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post #2 of 8 Old 03-10-2012, 05:54 PM
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The only warning I know of certain receivers had bridging capabilities and warned do not use if speakers are 4ohm or less thats about it if there is no warning relating to the impedance then it should work ok.
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post #3 of 8 Old 03-11-2012, 01:36 PM
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Quote:
Originally Posted by CDLehner View Post

Can any of you EE-types, tell me if there is any issue...or "down-side" to bridging an amp, "made" to operate in stereo?

1. No (assuming the amplifier is stable into half the impedance of the speakers you plan to use with it). The damping factor drops by a factor of 2 but this isn't enough to be audible (and with a low output impedance can be insignificant compared to the speaker cable resistance).

2. On any sort of musical signal the peak SPLs you can get into an 8 Ohm speaker will be as if you had an amplifier 4X the unbridged 8 ohm rating and not double.

This is an an artifact of FTC stereo and mono consumer amplifier testing where output power ratings into low impedence loads are limited by the waste heat that results from running the amplifier at 1/3 rated output power for an hour.

3. This all ignores whether you need the added SPL (draw-down rates in real rooms are about 3dB/distance doubling which is a lot better than the text book 6dB/distance doubling for point sources which only exists in the real world outdoors and in anechoic chambers) and whether more amplifier power will actually produce it (speakers with low enough efficiency to require such power in domestic environments are generally inefficient due to small enclosures and the drivers which go with them and are likely to run into their mechanical limits if you force the issue). More efficient speakers are probably a better idea (Paul Klipsch once noted that "what the world needs is a good five Watt amplifier") - you have as much headroom with a 100W amplifier and 95dB speaker as a 1000W amplifier and 85dB speaker.

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But is there something you could likely hear, that is a trade-off with bridging?

No.
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post #4 of 8 Old 03-11-2012, 02:54 PM
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Most amps that are "bridgeable" are strictly limited to 8u when bridged.

As a rule I recommend against bridging, if you want more power get a bigger one. Bridging will sum the noise and distortion of 2 amps into one, and the amp is essentially running wide open all the time. IOW, no amp will sound as clean when bridged.

If I'm looking for ~400WPC, it'll be from an older Sunfire model, probably the Cinema Grand line. There are stereo models at 300 and 600 WPC, and 5-ch models at 200 and 400 WPC. Cost is more than reasonable, though still not cheap.
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post #5 of 8 Old 03-12-2012, 11:28 AM
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Quote:
Originally Posted by rdgrimes View Post

Most amps that are "bridgeable" are strictly limited to 8u when bridged.

No although they're often missing an FTC power rating because it might look bad.

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As a rule I recommend against bridging, if you want more power get a bigger one. Bridging will sum the noise and distortion of 2 amps into one,

On paper you might loose 3dB of signal to noise ratio but having a -107dB noise floor instead of -110dB still means you're not going to hear anything until you get within inches of a driver. Likewise distortion remains inaudible when it increases from 0.005% to 0.010%.

Quote:


amp is essentially running wide open all the time.

With musical signals it's just idling along and not doing much of anything the vast majority of the time regardless of whether you've chosen a bridged configuration.

My bedroom system runs each midrange driver off a bridged pair of LM3886 chip amps on 35V (unloaded) power supply rails and 4 degree C heat sinks (about match box size). They don't run any hotter than the single chip on each tweeter or get close to thermal shutdown.

Quote:


If I'm looking for ~400WPC, it'll be from an older Sunfire model, probably the Cinema Grand line. There are stereo models at 300 and 600 WPC, and 5-ch models at 200 and 400 WPC. Cost is more than reasonable, though still not cheap.

Bridging is a fine idea when you want more output (it gives you a lot more latitude picking output transistors than the traditional half-bridge configuration because the voltage drop across them is half what it would need to be) although if SAF is an issue there are usually better options than one or more 35 pound amplifiers per speaker.

Usually these discussions are more about audiophile prejudices than hard science.

FWIW, the Jeff Rowland Model 10 (MSRP $7500) uses 6 LM3886s per channel in a bridged/parallel configuration.
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post #6 of 8 Old 03-18-2012, 12:47 AM
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Interesting. I have a six channel bridgeable amp using the LM3886 and the manufacturer says not to bridge it into loads below 8 ohms. Should I take this as them being ultra-conservative? Likewise, I have seen some amps using the latest Icepower chip recommending against bridging into 4 ohm loads, whereas others come as bridged and state they are quite fine with the same loads.

I also have a 12 channel 60w Class A/B amp which I would love to be able to bridge for into my 4 ohm bass driver load in my active dipoles but the specs don't mention a rating. Will I most likely be fine?
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post #7 of 8 Old 03-18-2012, 09:30 AM
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Quote:
Originally Posted by Drew Eckhardt View Post

On paper you might loose 3dB of signal to noise ratio . . .

Actually, I believe you will GAIN 3 dB of signal to noise ratio when bridging.
This is because when you sum noise sources that are not coherent, the net increase in noise level is only 3 dB, whereas summing coherent signals will raise the level by 6 dB.

However, the main disadvantage is that there will still be a net 3 dB increase in noise coming from the speaker, even when no input signal is applied. This means high efficiency full range speakers WILL produce more audible hiss when listened to up close.
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post #8 of 8 Old 03-18-2012, 01:52 PM
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There's no downside if the speakers can handle the extra input power (400W is quite a lot), and won't present an impedance that's dangerous to the amplifier in question. So the reason you get "8ohm minimum" values on most ampilfiers is because each "channel" is seeing 4 ohms - hooking up a 4 ohm or 2 ohm speaker presents the amplifier with something around 1-2 ohms (at least this is my understanding); similar to running speakers in parallel. That can be a death-blow for some amplifiers, others can do it just fine (there are some Accuphase amplifiers, for example, that will run BTL into 2 ohms just dandy (non-bridged they're spec'd down to 1 ohm); nevermind that they cost something like $40,000 a pair). Generally speaking 2 ohm speakers are quite rare though, and depending on how hard you want to ride with 4 ohm speakers, it may or may not be the end of the world; I'd be wary though (it'll probably make a lot of heat).

I also would not bridge any amplifier that doesn't explicitly say it's designed to be bridged, as it's possible to short it out. When in doubt, contact the manufacturer (not a sales drone, the manufacturer). In the case of these Parasound units, the only advantage I see for the JC1 is that it probably handles much lower impedance loads. Isn't it also one of their units that does "Auto Class A" up until a certain point? That would surely add into the cost and complexity of the device. As an aside, I think the "JC" components are also "special" for some reason (limited edition or some promotional thing, because they've got John Curl stamped all over them - it probably explains part of the price as well; I'm not saying they're a ripoff, I'm just saying the price hike is probably more to do with other features (aesthetic or otherwise) that that amplifier offers, not just the power output abilities).

Finally, I agree with what Drew said - figure out if you need the extra 3 dB (which isn't hardly anything), and if it's worth double the price. Generally speaking it isn't; if it were 10 dB that'd be another story (and as many of these threads conclude: can your speakers even survive that much power?). More sensitive speakers are probably a better idea if you need more SPLs.

Audible artefacts wise - hrm. I'd suspect if the speaker's impedance were too low you could hear some clipping right before it blew apart, but beyond that, probably not - maybe a bit of white noise up close, maybe not. Your environment's noise floor and the speakers themselves will play into that (and if you have very sensitive speakers, why do you need 400W? round and round it goes).
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