amplifier specifications and speaker balance

[bjmsam]

In my home theater, the L+R speakers are Acoustic Research (Teledyne) AR9 driven by an Adcom GFA-585 amplifier while the C speaker effectively is the top section of an AR9 driven by the bridged channel of an Adcom GFA-2535 crossed over at 200Hz. Audyssey XT32 on my Denon AVR-X4000 balances acoustic output by boosting the C channel significantly, and though it sounds right, I would like to understand why. The seemingly relevant specifications published for both amplifiers are compared below (my speakers are 4Ω but the 8Ω specs are more complete), but I don't have a good handle on what those figures represent functionally or if volume and output are linear. Can someone help shed some light on this for me?

	Adcom GFA-585	Adcom GFA-2535 (bridged)
Power Rating	250 watts continuous average power into 8Ω at any frequency between 20Hz and 20kHz at less than 0.02% THD	200 watts continuous average power (bridged) into 8Ω at any frequency between 20Hz and 20kHz at less than 0.09% THD
Signal to Noise Ratio, "A" Weighted	250 watts into 8Ω >= 110dB	60 watts into 8Ω >= 100dB (bridged spec not listed)
Gain	27dB	27dB
Input Impedance	50kΩ	22kΩ
Input Sensitivity	250 watts into 8Ω = 2V RMS 1 watt into 8Ω = 130mV RMS	200 watts into 8Ω = 1.75V RMS 1 watt into 8Ω = 130mV RMS

 

[DonH56]

Missing required data are the sensitivity (in dB/W/m) and listening distance to the speakers.

 

[bjmsam]

The MLP is 15 feet (4.5 meters) from the midrange driver of all three speakers.

According to the AR9 specifications:
"1 watt will produce 87dB SPL on axis at one meter."

The sensitivity also is provided in German DIN standard format:
"8 watts are necessary to produce a sound pressure level of 96dB at the measuring microphone, the watts being calculated from Watts=V²/R, where V is applied voltage and R is numerically equal to the stated impedance" of 4Ω.

The C speaker has only the top portion of the crossover:

 

[DonH56]

Does the center have the same attenuator switches, and are they all at 0 dB?

 

[bjmsam]

C does not have attenuation switches, but L and R switches are at 0dB.

 

[bjmsam]

Another data point: when powered by one channel of a GFA-5802, the C speaker plays 2dB louder than when powered by the bridged channel of the GFA-2535. Below is the table updated with its specs and XT32 adjustments for LCR balance. I used my SPL meter to verify that the delta remains constant regardless of amplitude, at least!

	Adcom GFA-585	Adcom GFA-5802	Adcom GFA-2535 (bridged)
Power Rating	250 watts continuous average power into 8Ω at any frequency between 20Hz and 20kHz at less than 0.02% THD	300 watts continuous average power into 8Ω at any frequency between 20Hz and 20kHz at less than 0.18% THD	200 watts continuous average power (bridged) into 8Ω at any frequency between 20Hz and 20kHz at less than 0.09% THD
Signal to Noise Ratio, "A" Weighted	250 watts into 8Ω >= 110dB	300 watts into 8Ω >= 105dB	60 watts into 8Ω >= 100dB (bridged spec not listed)
Gain	27dB	29dB	27dB
Input Impedance	50kΩ	105kΩ	22kΩ
Input Sensitivity	250 watts into 8Ω = 2V RMS 1 watt into 8Ω = 130mV RMS	300 watts into 8Ω = 1.7V RMS 1 watt into 8Ω = 100mV RMS	200 watts into 8Ω = 1.75V RMS 1 watt into 8Ω = 130mV RMS
XT32 Gain	L & R: +2.5dB	C: +7dB	C: +9dB

Since the AR9 is considered a 4Ω nominal speaker, each parallel circuit depicted in the crossover schematic above presumably is 8Ω nominal:

total impedance = 1 / [ (1 / top section impedance) + (1 / bottom section impedance) ]
4Ω = 1 / [ (1 / 8Ω) + (1 / 8Ω) ]

C (top section only) therefore must be drawing less power than L or R, but does that explain the lower volume considering C (top section only) is doing less work than L or R?