the amplifier bias is not affected by the speaker impedance.
ClassA bias for a push pull output stage is set to half the peak output ClassA current.
If you want 1Apk of ClassA current then total output bias is set to 500mA.
If you want 2.4Apk of ClassA current then total output bias is set to 1.2A.
Note changing the load impedance does not influence the bias setting.
I got sent down this path after reading a paper by Nelson Pass about a DIY class A amp build. His check out steps goes over bias adjustments. He indicates 175ma for 8 ohms and 220ma for 4 ohms. So perhaps only in this particular project was the impedeance a factor.
I'm getting that Nelson Pass designed the Threashold line of amps, so I assumed that the basic bais concept was consistent.
thats funny. no some. the 1 ohm resister on each of the 24 output transisters are not even warm. the 10 ohm board resister is not even warm. heat sink is hot to hold but not to touch. It sounds better the higher I go. the adjust ments can take the board resister bias well past 1 amp. I think 1.2 amps puts it at max wall outlet draw. unless I can find something or somone that can rationalize going high than .7 board 70ma individual transister , going to go with .7.
thanks for all the assistance.
Why is this amp called class A when it obviously isn't?
Its fairly typical in the marketing world to call Class AB class A.
(In my book optimally biased Class B is class aB).
For pure class A load impedance affects the required bias current.
For Class AB load impedance affects the A to B transition point.
Biasing of AB is simple, the more current the better, here
presumably limited by the apparent heatsink temperature.
Just apply some common sense to the maximum current.
So.... let me make sure i know what i think i know.
"For pure class A load impedance affects the required bias current."
as the impedance drops the total current cannot go over the total being supplyed to the output section....right?
and with no signal applyed to the amp, and its sitting there dissapating 100w in heat....as signal is applyed the wattage being dissapated as heat is less due to the diffrence being output as audio correct?
I have an usher amp which is a clone of the Threshold Stasis 2 Class A Power Amp. Anyone know what the bias current should be?