I don't think the wattage has anything to do with it. There are different technologies involved in volume control, and I wouldn't expect the volume control of 1 manufacturer to correspond to another, and I am surprised that your 30 year old accomplished is as close as it is to the Marantz
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Wattage refers to the limit of output, more related to Volts than watts really, but it refers to maximum output before clipping.
The issue is that the gain, the ratio of Vout/Vin but we measure it in DB's so 20log ( Vout/Vin ). By convention amplifiers offer ~ 23 dB of gain. Receivers aren't under the same convention, and it IS just a convention, so nothing really wrong with an amp offering more ore less.
One other thing. Any amplifier requires a certain voltage to achieve unity power. The 555 can do 200 wpc, but it requires around 1.7 volts on its input rca jacks to do this. At least the current 555SE needs 1.7 volts, perhaps the old 555 needs a different spec.
So the question is, what is the max voltage of the pre outs on the Marantz? Most mid level AVR's do 1 volt max, which will not let the Adcom get to 200 wpc.
I believe that many multi channel amp manufacturers take this into account and achieve unity power at 1 volt, not sure about the GFA 7500.
I found an old review of the 555 II. It needed 1.75 volts at the inputs to achieve full power.
Another old review of the 7500 says it needed 1.14 volts to hit 150 wpc, so that should be fine for the Marantz.
The pertinent information is buried in these links, hope this helps.
I now have the GFA-555 running my LR Fronts and the GFA-7500 running my Center, SLR, and SBLR.
I ran Audyssey again and then adjusted with a sound meter. Look at the gain difference between the Adcom amps. I’m thinking of moving the GFA-555 to the Surround Back LRs.
My Fronts and Center are identical NHT M7s. The Center is set at +2.0 db and the Front LRs are +7.5 db.
Maybe I’m just OCD and this is not an issue?
Thanks to everyone responding!!!
My knowledge on this is by no way exhaustive, but it’s a little concerning to me that Audyssey is having to run the left and right so hot to compensate for the input sensitivity of the 555.
it might be better to have the 555 on the rear channels where less demand will be put on the marantz preout section. Others may have additional thoughts.
I don't know about the Marantz, but most receivers have one main shared power supply (for both the amp section and the preamp section). The preamp power is just regulated from the main power supply section.
Putting an amp on the rear channels doesn't reduce the demand on the preamp/preout. The Marantz preamp section will still have to drive an amplifier (whether it's built into the receiver or external).
I would use the receiver speaker connections for surround channels because the volume/energy on the surrounds are actually very small. The fronit left/right speaker along with the center require an enormous amount of power. It's still best to use an external amp for these.
The only caveat being that the rear channels have less information than the front channels, and may be closer to the seating position. Both items would serve to reduce demand on those rear channel outputs over having the 555 amp driving front channels.
But perhaps the difference would be too small to worry about.