Why KSA100 can delivery 400W at 2ohm loading

[emb51]

Dear All,

The output stage of KSA100 use 3 pair Power Transistor, but it can deliver 100W@8ohm, 200W@4ohm, 400W@2ohm, that means the output impendance is very low, why 3pair transistor can do that?
My Pass Aleph2 is use 8pair MOSFET to output 100W@8ohm, 196W@4ohm, 75W@2.6ohm, so I don't understand why KSA100 use 3pair transistor only but it can deliver 400W@2ohm

[AndrewT]

Hi,
Krell uses pull pull for the output stage.
bias current is set to half peak ClassA output current + a small margin.

For 100W into 8 ohms Krell needs 40Vpk and 5Apk to the 8r load.
the bias is set to 2.6A (5/2 + 0.1).

The power supply can push out about 6A continuous and achieve peak currents of three to four times that in the shorter term.

The bias current only takes 43% of the PSU continuous current potential.
The supply rails do not sag (there is significant ripple just as single ended and ClassAB) if the output power is about the same as the classA limit i.e.100W into 8ohms but the amp can push to about 130W into 8ohms just before clipping.
However into 4ohms the ClassA current is still limited to 5A and so the ClassA power falls to 50W but the rails are still at the quiescent voltage and this allows the amp to go on to deliver about 200W of ClassAB power into the 4r load.
Similarly the ClassA power into 2r is about 25W but maximum ClassAB power is about 200W into 2r.
Note however that the maximum power does not double each time the load is halved. The voltage delivered to the load does drop as load value decreases just like any other ClassAB amplifier, but due the high reserve of power in the PSU and the 4pair output stage, it drops a lot less than most other amplifiers (that do not have regulated power rails).

Your comparison figures for the Pass designs are I think single ended and the way power is delivered is somewhat different. There is no ClassAB just the HARD ClassA limit when the ClassA current is reached and the active device cuts off.

BTW that 4pair are 250W devices rated to 200degC totalling 2kW on very large heat sinks.

Hope this helps.

[AndrewT]

Hi,
There is a mistake in my last post.
ClassAB power:- 200W into 2r0 should read 400W into 2r0.

[junglejuice]

Originally posted by emb51
Dear All,

The output stage of KSA100 use 3 pair Power Transistor, but it can deliver 100W@8ohm, 200W@4ohm, 400W@2ohm, that means the output impendance is very low, why 3pair transistor can do that?
My Pass Aleph2 is use 8pair MOSFET to output 100W@8ohm, 196W@4ohm, 75W@2.6ohm, so I don't understand why KSA100 use 3pair transistor only but it can deliver 400W@2ohm

Does anybody have a schematic for this KSA100 amplifier?
My email is junglejuice72@hotmail.com

[AndrewT]

Krell KSA 100mkII Clone
Krell KSA 100 type amplifier
KSA100 as a Starting Point for a New Design
Krell KSA100
Krell Ksa100
Bias adjustment for Krell KSA 100

Try looking through these for a start.