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Kees 9th October 2003 07:05 PM

Self regulating Class A
Class A sounds good but I don't like a 700W electric heater in my room, nor the electricity bill.

The solution is a quiescent current, dependant upon the output level of the amp.
I'm thinking of rectifying the amp output, low pass filtering and than clamping.
The output of the clamping circuit can drive the Vbe multiplier.
The Vbe transistor can be a photo transistor or an opto coupler.

I.e. there will be a loop that regulates the quiescent current in the output stages.

Are there any people who tried this?

tbla 9th October 2003 07:08 PM


Kees 9th October 2003 07:10 PM

I know.
I believe there are more companies doing it.

What about DIY'ers?

Nelson Pass 9th October 2003 07:11 PM

Lots of people, including Krell.

The original Aleph 1 had an automatic idling mode to reduce
dissipation when not in use (after 10 or 20 minutes) but
we immediately took it out because everybody complained.

Kees 9th October 2003 07:15 PM


I'm just wandering if there are any more detailed circuit idea's?
I just don't want to invent the wheel again.
Making a quick drawing is never a problem. It's always the detail design that takes 80% of the time.
I cannot imagine that there are no DIY'ers who did this before.

Richard C 9th October 2003 08:08 PM

Another variation is a dual voltage power-suppy. The amplifier switches between supply voltages depending on signal level e.g Marantz PM-64 and similar "Quarter A" amplifiers.

Kees 9th October 2003 08:48 PM

another option is PWM modulating power supply. Modulation by the music.
In this case the supply voltage on the ouput stage transistors is just a little higher than the signal level.

Very efficient but complicated.

Richard C 9th October 2003 09:20 PM

There appear to be two fundamental approaches; A system that tracks the signal level and makes a continuous adjustment or one that switches between two states dependent on some threshold.

But which one to choose...?


Kees 9th October 2003 09:39 PM

A PWM modulating power supply is too complicated. However it may be possible to build an amp that allmost has the efficiency of class D and the sound quality of class A.

My idea was to make a simple loop that regulates the quiescent current.

tinears02 9th October 2003 11:13 PM

sliding class A
You may want to check out
Electronics World (formerly Wireless World) June 2001
issue where the Circuit Ideas section has a
class A circuit that is biased dynamically by the
input signal. Class A in this case means that the
amplifying transistor never switches off, but idles
at low current.
Basically, it uses a small resistor in the collector of the
output emitter follower and a feedback loop using an
opamp tracks the voltage drop across the resistor. The
emitter follower is biased by a transistor current source.
The output of the opamp then biases the current
source transistor dynamically by amplifying the differential
signal of the input to the follower and the resistor.
Unfortunately I do not have a scanner, or I could send
a scan.

Hope this helps.

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