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Old 3rd November 2009, 05:53 PM   #1
Mooly is offline Mooly  United Kingdom
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Default New idea for low dissipation Class A amplifier.

A new idea for lower dissipation Class A... now all the usual arguments sliding bias etc are not Class A... that's been flogged to death and Class A to my way of thinking means the "old" definition, that the output devices remain in conduction for the full cycle.

So a different approach. What if you mostly listen at lowish volume, but want the full Class A performance at high levels for serious listening. In that case the volume control setting has determined the max power that can be output to the load. Put simply, apply say 2 volts RMS from a CD player (max possible output) and the max output of the whole system has been defined. If the bias could be set to match, then the amp would always be in Class A at that volume setting no matter what the music did.
Turn the volume up and the bias is low, Class AB, so some way of doing that automatically would have to be devised.

Two approaches came to mind... the electrically easy one, have another gang on the volume control to vary the bias as the volume is altered.

That's messy.

Another idea. We all love our remote volume controls (I couldn't live without one now anyway) so lets say you have a high quality motorised Alps pot and all.

Idea... the motor signal when it appears enables an oscillator (say 50 khz... way above audio anyway) that injects a low level sine of this frequency into the audio before the volume control. That signal is monitored at the output of the amp. Lets set the levels say that volume on full gives 3 or 4 volts output, so no tweeter damage etc.
That signal is picked off wih a filter and rectified and a peak value derived that then is used to set the bias on the amp.
Result, an amp whose bias is always correct for Class A but that doesn't generate excess power loss at low levels. Interfacing say a 0 to 10 volt DC voltage to a corresponding bias current would be a challenge... perhaps opto isolators could be used.

You could even apply the same idea to a manual non motorised pot and just have a push button to enable the osc and inject the HF signal manually on demand to set the value that way.

So there we are, just one of those ideas we all get from time to time.

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Old 3rd November 2009, 06:13 PM   #2
AndrewT is online now AndrewT  Scotland
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Hi,
as you change the bias current, the Vbe of the devices will change. If output offset is to be avoided then the change in Vbe of the +ve half must EXACTLY match the change in Vbe of the -ve half. If this cannot be guaranteed then the output offset will vary with bias current.
A DC servo could attenuate this variable offset to manageable levels.
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Old 3rd November 2009, 07:28 PM   #3
Mooly is offline Mooly  United Kingdom
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Hi Andrew,
Yes a lot depends on the actual amp of course. I hadn't got to thinking that far ahead. You know I currently prefer the single ended input topology which makes a DC servo mandatory of course for a DC coupled amp.
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Old 3rd November 2009, 07:37 PM   #4
Magura is offline Magura  Denmark
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Have you considered a way around, that mostly the distortion rises, as the bias goes down?
I did actually have a bit of the same idea, a few years ago. I used a basic Pass SE design as base, and made a few switchable bias settings. It was very short lived, to say the least.
All the low settings, which actually are the ones that I wanted to have, sounded downright bad, compared to the higher bias settings.

As I recall, I had some 5 settings, ranging from 200mA (maybe 100mA, can't remember that for sure) to 3A bias. I seem to recall, things went better around 1A though.


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Old 3rd November 2009, 07:48 PM   #5
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Magura View Post

As I recall, I had some 5 settings, ranging from 200mA (maybe 100mA, can't remember that for sure) to 3A bias. I seem to recall, things went better around 1A though.


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That kind of makes sense if the amp used HEXFETS.
It would be intresting to try with something like a JLH topology design.
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Old 3rd November 2009, 08:01 PM   #6
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Quote:
Originally Posted by Mooly View Post
the usual arguments sliding bias etc are not Class A... that's been flogged to death and Class A to my way of thinking means the "old" definition, that the output devices remain in conduction for the full cycle.
Unfortunately, this definition includes "sliding bias etc."

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Old 3rd November 2009, 08:05 PM   #7
Magura is offline Magura  Denmark
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Old 3rd November 2009, 08:12 PM   #8
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similar idea I had some years ago. But I prefer to change the supply voltage instead of the quiescent current/idle current (bias) for the output stage. But then one needs separate power supplies for the 0V offset adjustment potentiometer (and for the voltage gain stages and driver stages, if such are present).

In a "Parvel Macura SE buffer I realized this idea. I chose 3 different DC voltages for the output stage: 8V, 16V and 32V (+ /-4V, + /-8V and + /-16V) and for all voltages 3.5 A quiescent current through the single ended CFP output. The repositioned code plug changed voltage (auto transformer primary windings) and also the resistors for the each matching offset voltage. Depending on how loud music is heard, the user must appropriate repositioned code plug.

If you like remote control, you must develop a control unit, that operates with relais, but I cannot say, how it is to avoid overshooting effects.

I would be prefer a switch over by triacs between the auto trafo windings while operating. But therefore I have too low experience, because there are inductive load switching and the risk of damaged triac.

In general, it is always the royal way to work with bi- or multi amping. Pure Class-A is to use only at higher frequencies, e. g. as tweeter-amp (if I use an active crossover, I need only around 4 watts at arround 20W power dissipation). The other speakers can be drived with good class AB amplifiers.
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Old 4th November 2009, 05:34 AM   #9
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Class A (sliding bias / push-pull / pure class A / any kind of quasi Class A / Super A) and low dissipation ideas are like two parallel lines that will never cross path anywhere.
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Old 4th November 2009, 07:07 AM   #10
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by megasat16 View Post
Class A (sliding bias / push-pull / pure class A / any kind of quasi Class A / Super A) and low dissipation ideas are like two parallel lines that will never cross path anywhere.
I am talking about pure Class A in the accepted sense of the definition.
The bias current does not vary with the music as in sliding bias etc, it is determined by the max possible output for a given volume setting.
A class A amplifier rated at 100mw or 100watts correctly meets the definition... the bias is fixed.
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