I have to ask this, it's embarassing, and you guys know this for sure.

[Zut8083]

Well,

If you look at the back of a Gallien-Kruger 800RB you will see a high frequency, biamp output and a full frequency/low frequency biamp pair of outputs.  One is cautioned NEVER to use less than 8 ohms resistance in a cabinet connected to the high frequency output, and to not dip below 4 ohms resistance for a cabinet connected to the lower/full output ("4 ohms total, minimum", or did I read that wrong); the extension to two cabinets is unclear, 4 ohms resistance, total or per each cabinet.  Thus, what cabinet resistance would one select for use if there are cabinets connected to both of the low/full outputs, 8 or 4 ohms, and would that change per application?  Would the choice in choosing a 4 ohm cabinet only be to increase to maximum volume produced by the cabinet, regardless of whether one or two enclosures are connected (200W at 8 ohms vs. 300W at 4 ohms).  Is the 8 ohms cabinet a safer choice for each output, rather than 4 ohms?

Lastly, I think there has to be a broader, wiser lesson here about pairing a cabinet, cabinets, or a wall of sound with an output stage, in general.  This seems especially true if there is not one simple amp involved, but several sound modules, eqs, a preamp, power amps, and then speakers.  What is the rule of thumb/much broader context of how one sets up a sound system with variable voltages, currents, or resistance for the given parts of the signal path to the speaker?  Any help or hard won insight would be most welcome, and I hope this makes sense.  I got stuck with this for about 90 minutes, so the question may be sort of chewed through at this point.  Thank you.

Cheers,
Zut

[Zut8083]

Whoops,

Please see the attached back panel of a Gallien Kruger 800RB courtesy of Talkbass.com.  Thanks.

-Zut

[adriaan]

Pretty clearly marked - each output needs at least an 8 ohm load, so for the low end the combined load (as the outputs are in parallel) is no lower than 4 ohm. And when bi-amping, the 8 ohm speaker receiving the highs must be able to handle 100 watts RMS, and there will be 300 watts RMS going to the speaker(s) receiving the lows. That should keep you safe.

[Zut8083]

I do realize it's an asinine question based on the strictest reading of the rear panel; the whole confusion was the term "total minimum" which created a snag because I had forgotten the relevant physics and was hung up on the simplest interpretation.  I found the equations, I think, and you are right: 8 ohms, each, for two enclosures connected to the full/low outputs is what one should stick to for the bottom outputs, in addition to the prescribed 8 ohms for the high frequency output when bi-amping.

What was forgotten by me is that series and parallel circuits are supremely different and have distinct equations describing them, as the total resistance for the parallel circuit is inversely additive, and not linearly so.  So, using two 4 ohm cabinets is not a good option, as 1/(R min)=1/(R enclosure 1) + 1/(R enclosure 2)=1/(4 ohm)=1/(8 ohm) + (1/8 ohm).  4 ohms resistance in one cabinet works only when one output is used, not when both are, nor with unmatched cabinets with a value less than 8 ohms a-piece, as that is less than the total minimum of 4 ohms "total minimum".  Cabinets with a resistance less than that value, and the output current will be higher than what is acceptable for the amp to handle, as the parallel outputs maintain voltage and variable current in response to the inverse sum of the impedance of the enclosures, and you will ruin something.  After sitting and unpacking your explanation with the forgotten physics and running some numbers, I think I get it now.

Thanks very much for the help, and sorry for the "boy who cried wolf" routine.

[pauldo]

No such thing as a silly question... better to ask foolishly than to act foolishly.

I burned up an output transformer on a Sunn head by not heeding impedance.

My 800RB has not seen use in many years, but when it did I never used both Lo outputs.  Mostly bi-amped with a Mesa Boogie 1 x 15 and a Hartke 2 x 10.

[Zut8083]

Thanks, Pauldo.  I appreciate the input.