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Old 7th August 2009, 09:17 PM   #1
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Default Bridged vs Conventional Amps

There have been some interesting discussions about the benefits of bridged amps in other threads lately, such as the Bridged Capacitor PSU Thread. Various forms of bridging seem increasingly popular in high-end designs.

Historically I think bridging has mostly been used as a way to either save money in high power amps or to increase the flexibility of multi-channel amplifiers (i.e. allow a 2 channel amp to be used as a higher power monoblock). But the trend in many flagship high-end power amps is towards more "balanced"--i.e.bridged--output stages. Lots of companies are doing it now including Bryston, Outlaw, Emotiva and some, like Crown, have long used it for their flagship products.

If this has already been well discussed in another thread, let me know. But many of the bridged amps on the market are relatively new designs. And I haven't seen bridging presented as a cost-no-object ultimate amplifier topology by anyone. I also searched AES and the last paper on the topic was from 1984 by Sansui and it was largely in reference to using bridging instead of a transformer.

The marketing hype implies bridging is done to reduce distortion. Bryston, for example, says of their 7b and 14b: "employs a balanced-output design that reduces THD and IMD to unprecedented low values." But, from what I know, a bridged amp can only cancel even order distortion products--and that's not the dominant source of distortion. Crossover distortion typically dominates and it's mainly odd order.

A bridged amp could also, in theory at least, have a better slew rate. But slew rate, in practice, rarely causes real world distortion in modern amps. I suppose slew induced distortion *might* show up with full power sine wave testing at 20 Khz of an extremely powerful amp but even that doesn't seem very likely until you get up to obscene power levels.

In general, I would expect a bridged amp to have *more* distortion, not less. Generally distortion rises into lower impedance loads due to things like beta droop. And I would expect some of the non-linearities to be additive between the two halves of a bridged amp--i.e. perhaps thermal related distortion, phase distortions and/or mismatches between the sides.

So am I missing something as to why more and more flagship amps are bridged besides they're cheaper to build that way? Some of them, like the $8000 Bryston 14b and flagship Crowns, are supposed to be "no compromise" cost-of-little-object designs.

Perhaps once you get past a certain power level, the other benefits of a bridged design (like the broader choice of output transistors operating at half the voltage) allow it to genuinely outperform a non-bridged design? Or are the manufactures still trying to save money or just want to be different for marketing reasons?

Does anyone have any conclusive data on a bridged amp offering overall better performance than a similar conventional single-ended design?
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Old 7th August 2009, 10:03 PM   #2
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In addition, I should add that most of these high-end amps, like the Brystons, likely end up connected to similarly high-end speakers that often present relatively difficult low impedance loads from lots of drivers and/or complex crossover networks, etc. To me, this makes a bridged design seem even less ideal. So why would a well respected amp manufacture like Bryston choose a bridged design for an $8000 amp?
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Old 7th August 2009, 10:28 PM   #3
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There have been hi end bridged amps around since at least as far back as the mid 70s. Properly done, they are NOT cheaper. In fact, they can cost almost twice as much to build.

Some of us believe the extra expense is worth the performance gain achievable. Others do not.
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Old 7th August 2009, 10:45 PM   #4
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Quote:
Originally posted by Steve Dunlap
There have been hi end bridged amps around since at least as far back as the mid 70s. Properly done, they are NOT cheaper. In fact, they can cost almost twice as much to build.

Some of us believe the extra expense is worth the performance gain achievable. Others do not.
Yeah, the bridged Crown's for example go way back. But still, for the reasons I listed above, any sort of bridged amp would seem to be at a disadvantage. So what I'm after is how can a bridged amp outperform a single ended design? Where does the "performance gain" come from?
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Old 7th August 2009, 10:50 PM   #5
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one could build an amp with more dynamic headroom within a given supply voltage.
regards
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Old 7th August 2009, 11:14 PM   #6
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Quote:
Originally posted by Juergen Knoop
headroom?
Hmmm... That's an interesting point. In a real world design, because a bridged amp works the power supply twice as hard, there would likely be more DC voltage "droop" with sustained output. That would likely give a bridged amp more measurable headroom compared to its sustained output. But power supply droop generally isn't considered a good thing in a high-end amplifier.

And, in some designs, a bridged amp is more likely to trigger its current limiting and/or suffer other losses into low impedance loads because of the effective load impedance being half of whatever is connected. So, as the load impedance drops, I would expect the single ended design to eventually have the headroom advantage all other things being equal.
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Old 7th August 2009, 11:23 PM   #7
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I did say properly done. That is why it can cost twice as much.
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Old 7th August 2009, 11:30 PM   #8
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Quote:
sustained output
this is probably an overrated feature. I would prefer more appreciation of dynamic headroom/music power.
regards
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Old 7th August 2009, 11:30 PM   #9
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Quote:
Originally posted by Steve Dunlap
I did say properly done. That is why it can cost twice as much.
But how? No matter how high quality the halves of the bridge are, it seems to me the halves (or in the case of a grounded bridge design the "high side") will individually outperform the bridged version using the same design.

So even with an unlimited budget, if you set out to design the best amplifier in the world to drive a pair of real world speakers, why would you choose a bridged output over a single ended one?
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Old 8th August 2009, 12:15 AM   #10
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2nd breakdown limited SOA in older bipolar output Q really limits I as Vsupply goes up - more than linearly so bridged @ 1/2 the supply V the safe Ipeak could be much larger and use fewer devices in total

MOSFET or thermal limited Bipolar SOA devices come out about the same in total number of output Q for given output power ( bridged can cost a little more in fixed Vdrop for the 2X bias components)

the extra small signal and driver parts usually are a minor production cost compared to mounting/heatsinking the output Qs
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