Bridge or not to bridge

Someone please help!!! Do I buy a Parasound 3500 at 350 watts/Adcom GFA 565 at 300 watts or do I bridge a Parasound 1500A, Carver 500xTHX, Adcom 5500, or acurus a200 to drive a pair of CItation THX subs. Pros and cons please...thank you in advance!!!
Bridging can offer you the benefits of mono amplifier (total separation) and higher power. However, the downside is that most (not all) amps when bridged do not perform as well at low imedances. In this case it is important to match the speaker and amplifier being bridged carefully. If your speaker performs at high imedances, then it is unlikely you will experience any of the cons typically associated with bridging. However, if your speaker goes into low impedances, then you need to watch your amp specs very carefully. For example, in non bridged mode does the amp double it's power from 8 to 4 ohms, and then again down to 2 ohms? If so, does it maintain that "doubling" property when it is bridged? Of the amps you mentioned--I don't know the answer, but perhaps others do. In the end if your speakers are low impedance, and the amp does not maintain it's characteristic watts/ohms from unbridged to bridged you are likely sacrificing quality for quantity (watts). If your speakers are high impedance (throughout the frequency response--not just the nominal), it is unlikely even if the amp does change characteristic watt/ohms that you will experience any appreciable degredation from the bridging.
Generally speaking bridging is not the best way to go. (see reasons mentioned above) I tried it for a while with a Classe six-channel amp but eventually abandoned the idea, opting for a traditional higher powered five channel amp.
You can't bridge any of the Acurus amps. Their design philosophy was way too minimalist to allow that sort of option.
as a rule, bridging significantly degardes damping factor.
Listening tests are in order to determine possible effects in your own rig.
I've questioned bridging myself. I have 2 Krell KAV-250a power amps bridged output 1000 watts per channel driving a pair of Martin Logan Monoliths bi-wired. Should I bi-amp the speakers feeding 250 watts to each of the 2 woofers and the 2 panels? I'd go from 2000 watts to 1000 watts total, but would I gain detail and control? What do you think? Henry
Personally, i would recommend against bridging and just shoot for a bigger amp if you really need that much muscle. Most of the reasons have been previously stated.

As to Trollmuse, if you can't do it with 1000 wpc, you're not going to do it with 2000 wpc. I would always take quality over quantity and you should have a very fair measure of both running it at 1000 wpc. Sean
I had the same questions when it came to my pair of Jeff Rowland Model 1's (bi-amp or bridge). I posed the question to Jeff R., and in this situation, he recommended that bridging them would be much better. His reasoning being that the amps were designed from the ground up as mono (hence the 'bridged' switch on the back of the amps). I am currently running them in bridged mode (& bi-wired) to drive my Snell Type A/IIIi's. I did (per Jeff R.'s recommendations) have to bump up the internal fuses as the amps are seeing a 2 ohm load (Snell's are 4 ohm speakers). I guess it all depends on the amps. -John R.
I'm in a similar situation, and am purchasing a Bryston 7B to run the one sub I have now, and possibly a better unit or two in the near future. My research agrees with the advice above. I had considered buying the 4B and running two subs from the stereo channels with the option to bridge, but a Bryston tech told me that in his opinion the 7B would be better because of it's ability to handle low impedances and deliver high current.
Trollmuse: I have the Monolith IIs bi-amped. They are incredibly better bi-amped because you don't have the low impedance of the panel competing against the high power requirement of the woofer. In your case there is no question--take the amps out of bridged mode, buy an active cross-over (you have to do it this way with the monoliths--as the bi-amping poles completely bypass the crossover), and enjoy the music. You will be amazed at how much better your system sounds.
Rives Audio: You make some good points, but I think that there are is a misconception about power amplifiers that should be cleared up. Despite tons of advertising claims, no amplifier doubles its power when the load impedance is halved. I know, I know -- we read it all the time in advertising copy, and some reviewers parrot this line, but it's simply not possible.

To double power when impedance is halved would require that have a damping factor of infinity. in other words, a lossless amplifier capable of putting out full voltage when the current doubles.

To get that, you must design an amplifier with absolutely no losses anywhere -- no losses in the emitter resistors (by definition not possible), none in the output transistors, zero ohms in the power supply wiring, lossless rectifiers, zero losses in the power transformer and zero ohm AC Mains wiring. But that doesn't happen, so the amplifier's power supply "rails" sag under load and the amount of voltage required to get that doubled power is no longer available.

So what's a manufacturer to do when he knows that the customers want to hear that his amp "doubles"? They understate the amplifier's power into the higher load impedances, so that it appears to double when the impedance decreases. For example:

Real-world Amplifier Measurements
100W - 8 ohms
190W - 4 ohms
360W - 2 ohms

The voltage rails sag under increased loading, so you raise the voltage so that you end up with more power into 8 ohms, e.g.,

110W - 8 ohm
210W - 4 ohms
400W - 2 ohms

Then write your specs and ad copy to say:

100W - 8 ohms
200W - 4 ohms
400W - 2 ohms

Finally - wait for the review: the measurements of the amplifier will reveal that "the manufacturer's claim of 100W into 8 ohms is very conservative. We measured an easy 110W in our tests!"

Another approach is to purposely limit the amount of power available into higher impedances by designing the amp so that its rails are voltage-regulated. This is not lossless either: the voltages still sag, but prior to the regulator.

The only other point about bridging that I'd like to make is that, like anything else, how it sounds and performs depends more on implementation, and how the amp sounds, than on some hard and fast rule. For example, some amps have picked up a bad reputation about how they sound when bridged because the the additional circuitry required to flip one channel out of phase with the other (required to have a bridged amp) sounds bad.

My experience with the bridge designs I've done are that, when done right, there is increased bass authority, greater dynamics (duh), and a lower sense of distortion. But if the amplifier does't sound good to start with, you will hear a decrease in sound quality: after all, a speaker being driven by a bridge amp is a speaker driven by two amps (one pushing, one pulling, and two crummy sounding amps will never sound better than one.
Michael, not to challenge your findings or experience, but i have a copy of test review for an amp that basically ( give or take by a very small amount ) does "double down" as impedance is halved. I am not talking about rated power but actual power at clipping. After all, that is what matters most and tells the true tale of how much current the power supply and output devices are capable of sustaining. The amp under review was tested down to 2 ohms and was pulling appr 20 amps of current at that impedance.

Other than a select few amps like the one above ( which act as a true voltage source ), Michael's statements are right on the money. I would even go so far as to say that most "normal" amps don't produce as big of a difference between 8 and 4 ohms as Michael states ( almost doubling ) and the differences get even slimmer between 4 and 2 ohms.

As to "good" and "bad" sounding bridged units, that will obviously vary from design to design and how that design is implimented when going into bridged mode. There are obviously more than a few ways to build a piece of gear with the designer's personal preference ( or is it the "bean counters" that matters most ??? ) coming into play. Sean
Sean, so it effectively has infinite damping factor? Cool -- I'd love to see a copy of that review -- is there any way you could scan it and e-mail it? I had a sneaking suspicion that my blanket statement would bring forth the odd example or two of amps that were designed to meet a certain spec -- such as "doubling."

There's always someone who notices that the market has latched onto some spec (like TIM, or slewing-induced distortion, or bandwidth, or some spec du jour) and, without regard for anything else, designs a product specifically to perform really well in that one area. Makes for great ad copy and a good "story" at the retail level. But usually (to avoid another blanket statement) that kind of single-pointed approach rarely results in a great-sounding product.

Lots of feedback won't prevent your voltage supplies from sagging when heavily loaded. I mentioned regulating the output device rails, a method that actually limits the amplifier's output into higher impedances -- it doesn't increase power into low impedances. There's one other trick I know of: let the amplifier voltage clip in a stage prior to the output stage. If you don't want the amp's power to rise above spec when lightly-loaded, but not run out of gas when driving a low impedance, you make sure the output stage has more volts than it needs, but limit the maxiumum voltage swing somewhere prior to the output stage. Like run the entire front end of the amp at lower voltage than the output stage. That also works.

But a true voltage source? I'm suspicious. Since power supplies are not lossless, somewhere in the amp voltages are sagging -- we're just not letting the output stage see it. Any word in the review about how they did it? A circuit description?

Citation web page says that one can bridge Citation amps to drive their Subs. I guess they have enough confidence in their amp design. Get a four channel Citation 5.1 or 7.1 and you'll be happy.
Question for Michael_elliott:

Since most amps employ feedback for stability, is it wishful thinking that the same stability is maintained in bridged mode without adding additional circuitry/feedback?
Hi Wywhcan, I better come clean right from the get-go: I hardly ever use feedback. The only power amps that I designed that use much feedback were the "Solid" amps(Solid-1, Solid-2, etc.) designed for home theater and for those dealers that insisted that Counterpoint have transistor amps. They used clever circuitry, but suffered from the same limitations of transistor amps, namely quite a lot of distortion, so feedback was used to linearize them, and to get those low-low damping factors so favored by folks that think you can predict the quality of bass by looking at a specification.

So I'm not an expert on high-feedback amps. There is, however, no reason to expect that a power amp is gonna need a big change to its feedback compensation when it is driving a speaker that has a second amplifier on its other side, running out of phase, instead of a ground. Expect maybe at very high frequencies, where the counter-driven amplifier might not sink current as well, but I can't image how that could be any problem.

That said, guys who design high-feedback amps are welcome to chime in here to help me before I sink under the weight of my ignorance.
OK. Here's the latest fromm Trollmuse. I've talked to techs at Krell and at Martin Logan. Krell, the Kav 250a was designed to work in the bridged mode hince the switch and instructions on the back of the amp. Martin Logan chooses bi-wireing over bi-amping on their speakers to avoid a hole around the x-over point between mid-range and woofer that bi-amping might cause. My ML Moniolths, bi-wired using KAV 250a in a bridged mode, sound excellent, so on to another STEREONUT project......Trollmuse
Do not buy an Adcom GFA565. I had a pair a few years back and it was crap. I used them on my pair of subs at that time.Anemic is the word. Adcom GFA 5500 is a much better amp or a 5800 or 5802. Tom