CLASS A AMPLIFIERS

Unsound,

The Krell FPB "Class A" series maintain Class A operation
from 8 ohms to 2 ohms.

They have well regulated power supplies that have the current
capacity to deliver the current that low load impedances
require.

Given the well regulated nature of the power supply [i.e.
the ability to put out the rated voltage independent of load ]
and the current capacity to back it up - that's WHY a
Class A Krell will double down.

Dr. Gregory Greenman
Physicist

Unsound,

You can also check out the reviews by Stereophile, such as
Martin Collums April 1997 review of the then current FPB-600.

http://stereophile.com/amplificationreviews/829/

In the Measurements segment, Martin addresses the power that
the amp can deliver into low impedances:

http://stereophile.com/amplificationreviews/829/index8.html

"I wasn't able to run my long-term continuous testing at
2 ohms, but compromised with five-second bursts-long by
peak-measurement practice (eg, 20ms). The FPB 600 could
sustain a 29.3dBW level into this load, corresponding to
3.4kWpc-an extraordinary figure."

Dr. Gregory Greenman
Physicist

Spear11,

It depends on the amplifier; the regulation of its voltage and its current output capacity.

We've been discussing amps that "double down" - those that will double their power
if the load impedance is halved. If we let V be the RMS voltage; then we can use
equations for power that look like the DC equations - but the use of the RMS values
make them applicable for AC power.

For a voltage source, the power P = V^2 / R; or P*R = V^2 = a constant if V is strictly
regulated. If the amp is rated at 200 watts at 8 ohms, then V^2 = 1600 volt^2. For
your 3 ohm speakers, P * (3 ohms) = 1600 volt^2.

Therefore, P = 533.3 watts.

Now this also presupposes that the amp has the current reserves to fulfill this. For
200 watts at 8 ohms; the current is given by P=(I^2)R; so that the current I = 5 amps.
For 533.3 watts at 3 ohms, the current will be 13.33 amps.

IF [ big IF ] the amp has the current reservers to output 13.3 amps, and the voltage is regulated to be the same irrespective of load impedance; then this hypothetical amp
can put 533.3 watts into the 3 ohm load.

Dr. Gregory Greenman
Physicist

Unsound,

The circuit topology of the Krell FPB series is Class A.

Class A amps don't automatically resort to A/B when they
are asked to output large amounts of power.

No - that is a characteristic of Class A/B amps that are
designed to maintain Class A at low levels - and then they
revert to what they really are Class A/B.

However, a true Class A like the Krells doesn't automatically
revert to A/B at high levels.

Dr. Gregory Greenman
Physicist

Atmasphere,

Careful here. The ability of the ss amp to "double down" is
really a function of the regulation of the power supply in
conjuction with its power reservers. This is not really
negative feedback.

Additionally, it's the feedback on the gain stages for the
signal that can have a deleterious effect on the sound
quality.

One can design a zero-feedback gain stage that is fed by
a well-regulated power supply that has, in essence feedback.
However, the sound of such an amp won't be compromised.

The only drawback is that the amount of headroom in such
an amp is not dynamic. As you pointed out, it doesn't
increase in power for short transients.

However, I'd wouldn't count on the dynamic headroom in any
case - I'd size the amp so that the continuous power level
meets my needs.

Dr. Gregory Greenman
Physicist