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rtarbell 6th February 2006 03:28 PM

SMALA Amplifier concept
As mentioned in the thread regarding class-H, I've heard about a SMALA amp (switch-mode assisted linear amplifier), but I've never been able to figure out how it "worked"...

I know that a class-AB amp is put inparallel with a class-D amp, but they are not exactly in parallel, they are put in parallel via a wheatstone bridge kind of device.

==> One of my major questinos is this: I think both the class-AB and class-D amps can be considered as voltage output amps (right?). In school I learned that one can not put voltage amps in parallel because the output current from one voltage amp can "backwash" into the output of the other, possibly destroying them both.

How does SMALA work so that the output current add in such a way as to cancel out the distortion from the class-D amp? (see link below to a student's thesis on his SMALA amp).

darkfenriz 6th February 2006 08:16 PM

I think you are right. The outputs are virtually shorted, the paper looks generally weird to me. Linear amp with pwm may work in 'modulated supply' or 'error feedforward' configurations but never like this.

Eva 7th February 2006 09:29 AM

This is a paralell modulated supply, while the conventional approaches are series connected. I think that the major pitfall of the prototype analysed in the PDF is actually the high swiching frequency and the low bandwidth of the linear amplifier, because it is asked to compensate for the current ripple present in the output (converted into a high voltage due to the high impedance of the load at high frequencies), but it can't and the result is the input stage working in a switching fashion rather than in a linear way.

Anyway, the idea seems very interesting to me. I think that there is some room for improvement.

Ouroboros 7th February 2006 09:43 AM

Although the design presented in the paper doesn't look very good, it is theoretically possible to use a small high-speed Class-A amplifier to error-correct a larger Class-D amplifier in the same manner that a Quad 405 'current dumping' amplifier works.

I believe that Peter Walker of Quad presented this as a concept not long after the 405 was introduced, but I've never seen one built.

darkfenriz 7th February 2006 07:26 PM


This is a paralell modulated supply, while the conventional approaches are series connected.
Maybe I wrote too fast, the paper deserves another look... sorry

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