Hi, I need some assistance. I'm trying to figure out what would be a good SET match for my ZOB speakers.
I have a Rega Apollo CD player which I've read has 2.175 volt RMS output.
http://www.stereophile.com/content/rega-apollo-cd-player-measurements#YZbZaGDrU5...The amp I use is rated at 20WPC into 8ohms maximum, and has an input sensitivity of (I think) 600mV. With my preamp set at -15db on the volume control, I can drive the speakers to about 86db average with 94db peaks (based on a free iPhone SPL meter app), which is quite a bit louder than my average listening level.
Is it correct to calculate the amplifier's voltage gain in this way:
http://www.rapidtables.com/calc/electric/watt-volt-amp-calculator.htmpower in Watts = output in volts^2/speaker nominal resistance in ohms
ZOBs with FRX2 drivers are rated 8ohms, and
Sugden A21ap amp is rated 20WPC into 8ohms, so at 600mV input:
20 = V^2/8
160 = V^2
V = 12.649V
And, using this:
http://eaw.com/docs/6_Technical_Information/StudyHall_and_TechNotes/dB_calculati...My preamp is attenuating the Rega from 2.175V to an amplifier input voltage of:
Volts(input) = 10^(dB/20) x volts@0dBu (0.775)
V(i) = 10^(-15/20) x 0.775
V(i) = 10^(-0.75) x 0.775
V(i) = 0.137817 or 137 mV
If the amp's gain is linear, then I am using 137/600mV to hit a quite loud listening volume. I know an amp needs some healthy headroom, but does this mean that I am effectively "using":
output voltage at very high listening levels = 137/600 * 12.649V
V(o) = 2.888V
Say we give that value a healthy 100% headroom:
2.888*2 = 5.776 V(o)
and therefore a suitable SET amp power rating would be:
W(set) = V^2/R
W(set) = 5.776^2/8
W(set) = 4.17
So, a ~4 watt SET would be enough to drive these speakers to absolutely ridiculous volume? Does that all add up? That seems way too low, but if it's right, then I could choose say an 801A or 300B output tube with an appropriate driver, and stay out of 845/GM70 territory, which would be extreme overkill.
Right, or wrong?