Bridge or not to bridge

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
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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?

As3411,

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?