Wow! I'd like to find a way to replace my 7700 with a 7900. Maybe the Outlaws will cut me a special break since one of the channels in my 7700 failed (fuse looks okay) and I'm going to have to send it back anyway.

A few questions for any knowledgeable person:
(1) How does the 7900 select between the unbalanced and balanced inputs? The 7700 has a small switch for each channel to select which input is connected to the amp. But it looks like the 7900 has no switches. Does it automatically select the connected input based on signal present? Or based on cable/shield ground present? If it is an auto-selector, I guess the only downside is for anyone who is connecting two sources, one balanced and one unbalanced, and using the switches to select between them.

(2) How well will a 7900 work with both plugs on a single 20A circuit? What is the real maximum power input possible combined between the two inputs? If I plug both inputs into a single 20A circuit that could provide 2400W.

The 7900 has 7 * 300W = 2100W of power output capability but the input is somewhat higher due to inefficiency. The 7700 output 7 * 200W = 1400W with a spec max input of 1800W giving an efficiency of 78%. This same efficiency applied to the 7900 would yield a max power input of 2692W.

So it seems like the 7900 power input could exceed a single 20A circuit under absolute max loading, but not by much. It needs about 22.4A peak, if my math is right.

I'll probably use 5 of the 7 channels at this point so I think that would need 16A or 1923W, fitting well within a 20A circuit. Anyone think I'll ever exceed a 20A circuit or experience any sound degradation in this configuration?

Comments and math corrections appreciated!

Thanks!
-Dan

p.s. - the height dimensions is messed up on the webpage. Does it intend to say 9.5" high? This sucker is so huge I better make sure I have enough space!

Last time I checked, we hadn't gotten a manual yet, so we don't know how it handles balanced/unbalanced selection.

A 7x200W amp plugged into a 20A circuit is not likely to ever be able to output a full 2100W of power to its speakers simply because of the circuit's limitations. A breaker will actually trip before delivering its full rated current (if I remember the EE's at work correctly, it's around 75% to 80% by design) - for a 20A breaker, that means you will get 15A or 16A at most. That equates to 1800W or 1900W of input power. Class AB amps are not exceptionally efficient, which is part of the reason for all that weight - they need to reject the waste heat, which is most effectively (and quietly) done with big heat sinks. With 1800W of input, you're probably not going to get more than 1000W out (although I'm just guessing there, as there's almost no hard evidence available available). This will also hold true for the 7900. With two 20A circuits, there will be close to 30A of total power available to the two power supplies, or roughly 3600W of input power. With a single 20A circuit shared by both power supplies, the maximum input current falls to around 15A or so before the breaker trips. What that means is that a couple channels could still do a full 300W of output, but only if the other channels weren't being placed under too great a demand.

Oh, and yeah, it's 9.5 inches tall (not 95). You'll want to check on space and on supports, because in addition to being huge it's also mighty heavy.

Anyone have 7.1 system where the surrounds can handle a continuous 300 watts?

What about headroom? Let’s assume for a moment that even if you’re running your system hard, you’re still going to allow 6dB of headroom for spikes. If 300 watts continuous is 0dB, then –6dB means about 75 watts continuous. When the spikes come, they’ll be short lived.

While the average circuit breaker may trip when a continuous current reaches 80% of the breaker’s rating, the average circuit breaker will not trip for short bursts that mildly exceed the breaker’s rating.

I’m not going to buck the manufacturer’s recommendations, but I do know that on a single 15A circuit I can run two power-hungry ‘cube’ subs and a 770 into seven 6-ohm speakers (about 250 wpc max), driving them with raucous music such that my SPL meter reads nearly 110dB without the subs and over 120dB with the subs (not a large room), and still the breaker doesn’t trip. “Your experience may differ.”

Bestbang4thebuck makes an excellent point about the real-world conditions that our systems experience. My system includes a Model 7500, two Model 200's, an LFM-1, 32" direct-view CRT HDTV, seven speakers (all in the moderate to low efficiency range), and the usual array of source components, but even at very high volume playback the ampmeter on my power conditioner rarely gets up to more than about 5A.

Voice of skepticism here: The difference between the specified power outputs of the 7900 and the 7700 is

I run all my 7.1 system off the 770 and the sub, TV, etc from a 15A circuit and the current meter on my panamax never goes above 10A, even when listening in mono to drive all channels the same.

When I did concerts in college and the first time we rented a generator to run the set up, we spec'd the generator based on the max load, but when we cranked pink noise thru all the channels/sub for the 40KW sound system we never got above 35% of the max we spec'd with our current clamp. The lights on the other hand, they drew near their rated power. We subsequently rented smaller generators using 50% max for the sound later and it always worked. These were all Crown Macrotech amps, which were awesome amps, could drive down to 1ohm loads easily.

Point of fact: The difference between the specified power outputs of the 7900 and the 7700 is (300/200 = 1.5 = ) 1.8 DECIBELS. I know it's an article of faith in American culture (yes, I was born here too) that More Is Always Better and capitalism couldn't run without it and all that, but now, apparently, reason counts for nothing.

I'd better shut up or I will be personna non grata here and y'all will lose the benefits of my wisdom.

Don't worry about chiming in on this topic, psyprof1 - there is no harm in pointing out that a 7x300W amp is a monstrously oversized beast that is likely to yield no audible difference for most systems. At almost ten inches tall, 125 pounds, and $3.5k it is clearly a "not for everybody" kind of amp.

There may be a few cases where the 7900 might actually serve a beneficial purpose (large rooms that open on to other large areas, combined with some of those really nasty speaker loads like the Polk RTi15 or RTi25) - I can think of one or two posters who were actually able to push a Model 770 to its outer limits in this sort of manner. In most cases, of course, the 7900 is going to be overkill (albeit impressive, physically imposing, and very gravity-obedient overkill).

dgilley,

The Model 7900 does not require a switch between XLR and RCA inputs as it will automatically determine which input is being used.

Best,

Scott