[ancient hippy]

i own one of each 4b nrb and 4bst and was considering mono bridging. was wondering what issues i might run into.

[srb]

When you bridge a bridgeable stereo amp it sees half the impedance of the connected speaker load.  So bridging is generally recommended for 8 ohm speakers, as the bridged amplifier would see them as a 4 ohm load.  With 4 ohm speakers, the amp would see a 2 ohm load, which is not generally a good thing and will cause it to run hotter.
 
That is a simplistic view, and a lot depends on the actual impedance curve of the speaker that shows the impedance at different frequencies rather than a generalized 8 ohm or 4 ohm "nominal" or "average" impedance spec.
 
From a technical or purist view, bridged amps generally have a bit more distortion and lower damping factor, which may or may not be audible to you.  Most important is the specific impedance characteristics of your particular speaker and whether that will cause the amp to run too hot or possibly even shutdown.
 
Steve

[ancient hippy]

thanks steve. i have been told that mono bridging the 4b nrb into a 4 ohm load is okay as it is 2 ohm capable not that i would ever do it.  i was wondering if the gains on the amps are the same or at least similar.

[srb]

Well again, it depends on the specific 4 ohm speaker, and maybe Bryston is just "playing it safe" but the manuals and rear panels of the amplifiers themselves both say 8 ohm minimum for bridging.
 
Neither Bryston manual for those amplifiers lists specs, so although I would expect the gain to be the same, that would be a question for James or one of his technical people.
 
Steve

[ancient hippy]

i never noticed that ill have a look at the back of the nrb. i always wondered if the increase in distortion occurred across the whole power spectrum or only after the 250 watts available in the unbridged format

[srb]

James replied in another thread about bridging a 3BST with a 4 ohm load and said to go ahead but monitor the heat.  So it can certainly be done, it's just that one 4 ohm speaker is not necessarily the same as another.  Some 4 ohm speakers can dip as low as 2 ohms and even lower, so my "official" answer in the manuals and rear panels would probably also say 8 ohms.
 
I had thought that the increase in distortion was an artifact of bridging, not just above a certain power level.  Generally distortion rises with lower impedances and bridging makes the load a lower impedance.  On the other hand many people are happy with bridging, particularly those with their Class D NCores and Spectrons, so who knows?
 
Steve
 
 

[ancient hippy]

my only other concern is that  polk rti 12 will be able to handle 800 watts.(they are rated for 500 watts) what will the warning signs be if there in trouble. i.e. will i hear the speakers distort before they fry.

[MarvinTheMartian]

Hippie
You are probably better off bi-amping the Polk RTi12's. The older NRB on the bass and the ST for the top. That way you will have consistent left right sound.
Shawn