Bridged audio amplifiers

[bibios]

Hello to everybody

I bought two bridgeable stereo audio amplifiers to drive two KEF R700 speakers, because the dealer told me that the mono amplifiers (each for any speaker) were more expensive and the bridgeable would do the same work. I think that I made a bad deal, because I was not told that the amplified signal is degraded as it carries bigger distortion. I understand that bridging is to use only in PA systems and never in HiFi. I would appreciate a lot, if someone could teach me a bit of theory.

 

[TLS Guy]

Hello to everybody

I bought two bridgeable stereo audio amplifiers to drive two KEF R700 speakers, because the dealer told me that the mono amplifiers (each for any speaker) were more expensive and the bridgeable would do the same work. I think that I made a bad deal, because I was not told that the amplified signal is degraded as it carries bigger distortion. I understand that bridging is to use only in PA systems and never in HiFi. I would appreciate a lot, if someone could teach me a bit of theory.

That is not quite true. The key is having both halves of the amp absolutely identical. Yes, distortion does rise unless the system is carefully set up with instrument control.

However, the biggest problem is that the load has to be double the impedance. So, if an amp is rated stable to 4 ohms, then bridged it is only stable to 8 ohms. Most speakers are in fact 4 ohms, no matter what the manufacturer says. So for safe bridging you need an amp that will drive 2 ohm loads. That is expensive.

In real practical terms there is no significant advantage of a multichannel amp over a mono block, power for power and corresponding quality.

 

[lsiberian]

1. Always convert your watts to dBW(The actual gain of the amp) for every comparison between amplifiers you ever make. It will save you thousands of dollars in some cases.

P(dBW) = 10 · log10( P(W) / 1W) is the formula to convert your watts into the proper format for a legit comparison.

2. Bridging two amps together only creates 3 dBW that's not certainly not enough to risk bridging them together in my book.

 

[Schurkey]

2. Bridging two amps together only creates 3 dBW that's not certainly not enough to risk bridging them together in my book.

Doesn't it depend on the amp(s) in question? Theoretically, you could bridge a stereo amp and get 4x the watts. More realistically, the increase is rarely greater than 3x, and sometimes not that much. Who doubles the impedance of their speakers when switching to a bridged amplifier?

Example 1: Thousands of years ago, Carver provided an amplifier that was easily field-bridgeable: The M1.0 was rated for 200 watts (23 dBW)/channel into 8 ohms, they claimed 1000 watts (30 dBW) into 8 ohms, bridged, but with distortion not specified.

Example 2: Adcom GFA-555se is shown on the Adcom web site as being 200 watts (23 dBW)/channel/8 ohm; or 600 (27.8 dBW) watts into 8 ohm bridged.

PROPERLY bridged, all common-mode distortion decreases, and voltage rise-time doubles. The trouble begins with a pair of amps that aren't that good to begin with--not actually identical; and especially when a weak output section is fed with a weak power supply. The resulting bridged amplifer wets the bed when presented with a low-impedance speaker load, and promotes the idea that a bridged amp cannot have reasonable sound quality. As an aside, in the whole history of Hi-Fi, there've never been so many 4-ohm (and less) speakers for sale.

In short, there's little wrong--and a great deal right--with PROPERLY bridged amplifiers of suitable quality; provided they'll drive the speaker load you intend to use them with.