Bridge or not to bridge

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
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