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atiq19 8th January 2009 09:05 PM

Technics Class A Biasing
2 Attachment(s)
Hi there,

Can some one explain how this bias circuit works?


PRE Drive Q3 & Q4
Output stage Q5 & Q6
Bias Diode QD1
Bias Transistor Q7
Stabistor pack DP1
Overload det Transistor Q8

atiq19 9th January 2009 10:27 AM

Does anyone know how the bias scheme avoids complete non switching of output transistors?

AndrewT 9th January 2009 10:41 AM

does it matter if the non operation device passes near constant current or is actually switched off?

A single ended ClassA amplifier never switches off it's output device. The device remains in control of the output current.
If the output current nears or exceeds the bias current then the stage overloads/clips.
Similarly if the output device passes zero current the amp is no longer in ClassA.

A push-pull ClassA stage has an upper and lower pair of devices.
Both these devices remain in control of the output current.
If one or other of the devices drops out of control then the stage reverts to ClassAB operation to supply the extra current above the ~ 2times bias current.

If one device passes a constant (low) current and the other device is passing more than double the bias current to the load then the stage is operating in ClassAB.
It matters not that neither device has switched off. It is limited to ~twice bias current for ClassA operation.

atiq19 9th January 2009 10:47 AM

I was mistaken, I thought that Technics chip internals as shown is doing class-AB. Thought, the sabistor + resistors are stopping the cross over dist in Class-AB.

Anyway, thanks for your clarifications.

AndrewT 9th January 2009 10:51 AM

there are a lot of designers and builders that disagree with me, notably Prof. Leach where he describes the operation of his drivers in LO Tim as staying in ClassA simply because he has arranged that they never switch off. I contend that he is wrong.

You have to make up your own mind.

My "clarification" is not fact. It is my opinion.

atiq19 9th January 2009 11:02 AM

I appreciate it Andrew. But you must have some reasoning before taking any view.

I was looking for class-A performance but still was not ready to accept the inefficiency of it. However, I thought if the cross-over distortion, a drawback of proper class-B, can somehow be eliminated then it would be a good trade-off in between class-A and non-switching class-B.

I thought technics chip was doing that.

I really didn't want to start a philosophical debate about class conflicts and its subtleties.


homemodder 9th January 2009 11:07 AM

Its not really class a but it does help with crossover distortion, have a look at the krill amp thread, another inovative +new class a+ amp.

atiq19 9th January 2009 11:13 AM


Going back to my early post, I would like to know how this x-over minimisation mechanism works.

Krill amp utilise few diodes to achieve nonlinear relationship. Are those diodes not producing switching distortion?


homemodder 9th January 2009 11:39 AM

Think of it this way, these schemes help because the outputs are kept at a bias level and never go through a point where the devices switches completly off, theres more to the theory but Im really bad at explaining these things, andrewt or andyc would be much better at this. Asked them nicely, Im sure they will help you out. There are some explanations in the krill amp thread and the thread connected with it, also read the theory behind the cambridge azur840 amp, theres a white paper on the azur range.

AndrewT 9th January 2009 11:45 AM

D.Self's Cambridge work is very similar to the conversion of opamp output stages to single ended ClassA using a current source to one supply.

All this does is increase the level of low current ClassA available before the amp reverts to ClassAB for the higher current outputs.
I wonder if the asymmetrical nature of this ClassA to ClassAB handover is audible?

The opamps ClassA current is determined in conjunction with the load to never handover to ClassAB. Cambridge cannot achieve this trick with their power amp.

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