Bridge or not to bridge

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?

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.