Class A bias, speaker sensitivity, watts...?

@tomcy6

I’ll tell you what I think I know and someone can correct me. Class A watts approximately halve from 8 ohms into 4 ohms, so you have between 5 and 10 watts of class A power.

You invited a correction, so here goes.  (If you were talking about the point where a class AB amplifier starts to move from class A to class B and starts to exhibit cross-over distortion, forgive me!)

Most amplifiers provide voltage gain - the output voltage should be a multiple of the input voltage. When the amplifier is connected to a speaker, current flows which makes the amplifier work harder to keep the voltage the right multiple.

The amount of current flow depends on the impedance of the speaker. It turns out that the amount of power required doubles every time the impedance is halved. For example, my venerable Class A Krell KSA-80 is rated at 80-Watts into 8-Ohms, 160-Watts into 4-Ohms and 320-Watts into 2-Ohms.

Marketers sometimes use this to misinform, by publishing an amplifier’s power into 6-Ohms, or even 4-Ohms. The latter doubles the boast!

Almost all speakers have different impendences at different frequencies, and many have big dips at crossover frequencies where two drivers have to work in parallel. That’s where an amplifier with big current reserves scores. A quick way to check an amplifier’s current reserves is to compare its power into 8- and 4-Ohms. Ideally it will double, but rarely does

@richardbrand

Yes, I agree with what you’re saying.  I was trying to address a different matter.

I was told by an amp manufacturer whom I called (don’t remember which one) that the class A watts are specified at 8 ohms and would approximately halve into 4 ohms. In my case it was 25 watts in class A into 8 ohms, but that dropped to 15 watts in class A into 4 ohms. I assumed other amps operated the same in that matter. Is that your understanding?

@tomcy6

Not my understanding at all! The relationship that requires double the power when impedance is halved applies to all amplifiers of all classes.

Solid state amplifiers exhibit an anomaly at the crossover point from +ve to -ve output voltage. Class A amplifiers avoid this anomaly by keeping entirely on one side of it, let’s say the +ve side. To do this, the input voltage is DC offset (biased) to ensure the output never goes -ve.

There are a couple of consequences - a lot of output power is wasted as heat, and the input power consumed is constant whatever the audio output power. My Krell consumes about 700-Watts all the time and has huge fins to get rid of the excess heat.

A class AB amplifier on the other hand does allow the output to go -ve, at a point determined by the bias voltage applied. The idea is that the cross-over anomaly occurs when there is some musical content, and won’t be as obvious as when playing near silence.

Your amplifier manufacturer may be saying that his class AB amplifiers go into the crossover zone at much lower power when feeding 4-Ohms, compared with 8-Ohms. In this case, class A power would be defined as the most power the amplifier can deliver before reaching the crossover zone.

Seems to me that his AB amplifiers do not really have enough current delivery capacity in reserve, at least on paper! It would be interesting to ask what AB power they can deliver into 8-Ohms, and into 4-Ohms. Ideally it would be double, even if the pure class A numbers take a nosedive

@richardbrand

Yes it would be interesting to hear from some high bias Class A/B amp builders whether their amps switch over to Class B power sooner as speaker impedance drops. Maybe I just misunderstood.

@tomcy6

I've been trying to think of the physics that may be responsible.  When an AB amplifier is in Class A. the bias voltage is sufficient to accommodate the signal swings.  If the impedance of the speaker drops, more current is needed to keep the output voltage at the right level, but the current also tends to make the voltage drop.  I'll surmise that if the amplifier cannot keep up with the current demand, the bias voltage could drop sufficiently to actually reduce the Class A power.

According to Stereophile, the OP's Vincent Audio SV236MK measured 150-Watts into 8-Ohms and 231 in 4-Ohms.  It is specified at 10-Watts Class A into 8-Ohms but not specified into 4-Ohms.  Pity!