Amps bridged to mono with 4 Ohm Magnepans?

Hi all,

My current amp is rated at 200/300 wpc @ 8/4 ohms and bridged mono at 600/850 w @ 8/4ohms.

I am considering getting a second amp as a mono block for my Maggie 3.6s,(at this time I am not considering active or passive bi-amping), and I have a 3-part question;

Most amp manufactures only list their 8 Ohm bridged output and not their 4 Ohm. I assume they don't recommend bridging to 4 ohms because;

1. The S/N spec would be through the roof. Correct?
2. The power draw from the wall would be too high (Energy Star and such). Correct?
3. Related to No 2. I suspect, the amp would get too hot and go into protection mode. Correct?

The bottom line is that I don't want to damage my Maggies or amp. Should I just stay away from bridging, or could I consider it for a manufacturer that lists their 4 Ohm specs?

I have never been a big fan of Bridging 2 amps. To much dynamic power can damage your speakers not to mention the distortion levels are always a lot higher when you Bridge most amps. Consider some quality mono blocks or a high power stereo amp. I would consider a tube Preamp with a powerful solid state amplifier.This would give you the best of both worlds for your maggies.
I agree with Matt that in general distortion performance and sonics will tend to not be as good in bridged mode as in stereo mode.

The reason that 4 ohm bridged mode power ratings are not specified for most bridgeable amps is that in bridged mode the output stages of the amp will "see" a load impedance equal to 1/2 of the speaker impedance, at any given frequency. So a 4 ohm speaker would be "seen" by the output stages of the amp as if it were a 2 ohm load, which is too low for many amps to handle with good results, or even at all. And the impedance of many "4 ohm" speakers will often be considerably less than 4 ohms at some frequencies (although that is not generally true in the case of Maggies). So in those cases, at those frequencies, the amp may see a load equivalent to 1.5 ohms or even less.

That halving of the effective load impedance is a major reason for the degradation in distortion performance that tends to occur in bridged mode.

The reason that halving occurs is that in bridged mode equal and opposite voltages are applied to the two speaker terminals, rather than one terminal being grounded (i.e., at zero volts). So from the perspective of the output circuit that drives one output terminal of the amp, a given output voltage will result in twice as much current being drawn as in stereo mode. Per Ohm's Law, that doubled amount of current is the same as what would be drawn from that terminal in stereo mode, for the same output voltage, by a halved load impedance.

Regarding the question of how much power the Maggies can safely handle, I have no idea.

-- Al

You can also say the damping factor is halved (output impedance is doubled) with a bridged amp, so your bass performace (control over the bass driver)is not as good, even though you have more watts available.

I've never been a fan of bridging, PA systems use it, that's where it should stay.

Cheers George
I have a set of Magnepan IIIas and tried bridging my Carver amps with them. Though I liked the additional sound stage improvement, I didn't like the additional distortion I could hear. Instead, I went with an active crossover and currently use two Carver amps to power my IIIas. I would say this would be the best way to go with your 3.6s. Actively biamping your speakers like mine, gives a marked improvement to the sound stage and dynamics to the music without you worrying if you are going to fry the speakers. But as you know, Magnepans love power and I am using one Carver amp rated at 400 watts into 4 ohms while the other is rated at 550 watts into 4 ohms in my biamped system.
Most amps which can be bridged mention NOT to use them bridged for a 4 ohm load.
If you must have more power, buy two mono amos, or one bigger map and sell your current amp.
Trying to use it bridged to a 4 ohm load is asking for trouble. Then the fact you will be buying another one anyway.. Not a good idea. Buy one bigger stereo amp. or a pair of true monoblocks. Sell your current amp.