Aleph Power Class

[vdi_nenna]

Question:

When the Aleph goes beyond it's "Class A" bias point, does it move to "Class A" push-pull, or is it "Class A/B".

Or, does it go from Single-Ended to "Class A" to "Class A/B"?

Vince

 

[Nelson Pass]

When the amplifier exceeds its bias on the positive,
it will clip, operating in Class A until it does. On the
negative going wave, when the positive current source
shuts off, the gain stage continues to supply current in
what I would consider Class B. The distortion goes up
at this point, but it continues to deliver more power.

The peak point at which the current source shuts off is
approximately twice the 8 ohm power rating of the
channel.

 

[roddyama]

Another Related Question

WRT the reply from Mr. Pass, does that mean that the current draw from the positive leg of the power supply is relitively constant (held so by the current source circuit), while the current in the negitive leg varies with the signal to the speaker?

Wouldn't this present a very asymmetric load on the power supply?

Would this be one of the sources of the asymmetric distortion of SE amps that is more akin to the "real nature of sound" discussed in your article on SE amplifiers?

Buhler... Anyone... ?

Thanks,
Rodd Yamashita

 

[GRollins]

Once you start clipping an amp, asymmetric draw on the power supply is the least of your worries. There are far worse demons running loose in your system. Moral of the story--don't clip.
In the ideal sense, a power supply should accept any punishment you might care to dish out, whether symmetrical or not. In the real world, it doesn't always work out quite as nicely, which why it's important to have a fairly beefy power supply for your amp. The more current capability in the transformer and the more capacitance in the main bank, the more your real power supply will behave like an ideal one.
If you think about it, music is asymmetrical. Hit a drum, and the power will swing violently to one side, but the return isn't nearly so strong. Some topologies, such as differentials, tend to cancel out the swing, but generally there's a stage or two in a circuit that will go ahead and react along with the music. The power supply should be there when the circuit needs it.

Grey

 

[roddyama]

Hi Grey

Thanks for your reply.

I agree with what your saying, and I guess I got off into the whole "real world sound" thing (which I've always agreed with), but we're missing the point. If it's true that the current draw on the positive leg of the power supply is (during normal operation) steady-state and on the negative leg, the current is dynamic, doesn't it make sense to optimize each leg of the power supply for its operating conditions. The Aleph (I'm eager to get all the parts together for my Aleph 3/5) has an asymmetric design, and an asymmetric mode of operation, but we feed it with a symmetric power supply.

For example, if we take the volts / current in both the positive and negative legs, the positive leg will be a steady load to the power supply so there will be little tendency for it to be drawn down from its steady state level. In the negative leg, however, V/i is dynamic with the signal so unless the negative leg of the power supply is extra "stiff", the tendency will be for its voltage to start to drop as the load increases (depending on the dynamics of the signal). So, during high dynamic current draw through the negative leg, the Aleph 5, for example, will have a relatively steady +34V on the current source side and some voltage less then -34V on the signal side as the rail is drawn down.

I'm I off base, or are my thought in line with what is happening in the Aleph amps?

Regards,
Rodd Yamashita

 

[GRollins]

Actually the current source modulates, via the signal taken from the resistor array at the output.

Grey

 

[Nelson Pass]

And so the current varies through both rails, but
averages constant.