If I've done my math right, a power transistor with 10 MHz ft and conducting just 1A will have a hybrid-pi capacitance of about 0.7uF.
The amount of suck-out current available limits the available rate-of-change of the transistor's current. At the same time, a fast signal going through the crossover region can have a fairly high current slewing rate. I think I touched on this aspect in Chapter 10 on output stages in my book.
Cheers,
Bob
Thanks for your kind words, Tom. And you are definitely right - there is a lot that can be improved, expanded, or added. That is my challenge for the Second Edition. I hope McGraw-Hill will allow me to add about 100 pages (but they were very kind to give me 600 pages in the first place, when the original plan was for only 500).
Cheers,
Bob
Unfortunately all my original images are lost to time, but here is a thread:
http://www.diyaudio.com/forums/soli...-allison-based-output-stages.html#post1764485
Bob, please take time out in your new edition to cover CFA's in some depth. Your unbiased, un-opinionated and practical writing style is a breath of fresh air. I for certain will buy the second edition to sit alongside my 1st.
Yes, the scheme looks like a nice solution to the problem, so when you did not publish it I wondered if you had found a difficulty and dropped it.
But now I am reassured, with Damir and Ovidiu as advocates and you too
I think I will use it, so thanks for the reply.
Best wishes
David
I am still wrestling along with design of
inverting voltage to current converting power amp
objectives
lead lag compensation
no current sources no current mirrors
"musical" harmonics' spectrum
the relations of harmonics should stay constant over power and frequency
thd absolute of minor concern
dynamic thermal compensation of bias
idea:
with current driven speakers one can easily compensate the motion equation B*l*i=...
with a set of three integrators and one adder and does not need to know the speaker's
TS parameters.
All that is required is a sine burst a good microphone and a two channel scope.
Then adjust the output fro minimum over- and undershoot of the bursts. ( of course that will be a compromise as the burst adjustments must be accomplished over a range of frequencies )
"Market": with the new but widely used program sources as "the net" and CDs LPs...are a niche market I think the days of passive crossovers are over. The future belongs to active speakers.
inverting voltage to current converting power amp
objectives
lead lag compensation
no current sources no current mirrors
"musical" harmonics' spectrum
the relations of harmonics should stay constant over power and frequency
thd absolute of minor concern
dynamic thermal compensation of bias
idea:
with current driven speakers one can easily compensate the motion equation B*l*i=...
with a set of three integrators and one adder and does not need to know the speaker's
TS parameters.
All that is required is a sine burst a good microphone and a two channel scope.
Then adjust the output fro minimum over- and undershoot of the bursts. ( of course that will be a compromise as the burst adjustments must be accomplished over a range of frequencies )
"Market": with the new but widely used program sources as "the net" and CDs LPs...are a niche market I think the days of passive crossovers are over. The future belongs to active speakers.
From a quick look at the start of the thread this seems rather different, a class A bias scheme.
In a Class B+ amp there will only be about 50mV across the combined output emitter resistors. That sure won't set the bias!
Perhaps somewhere in the 600+ posts is a scheme similar to Bob's but it doesn't jump out to me.
Best wishes
David.
no you are not
I support you nonverbally
another objective adjustable compensation of voice coil resistance preferably
with respect to its temperature dependent value....
probably their temperature is increased because of my mental support; just the induction in a coil of weak electromagnetic field from my brain
The Allison circuit is being used to drive the output tranistors; the outputs are biased in class AB, the drivers are biased in class A with an Allison configuration.
Even if they don't technically qualify as being an Allison configuration (who decides that?) they are both stabilized by the same rules, so I thought it would be helpful to have some reference material.
Bob, whatever you do, don't waste your time covering the deeply excreble so-called "CFAs". There are no reasons that can be advanced for their use in audio.
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Belt up mate, there are also no reasons for not using CFA's in audio. There's more in audio than the 2nd harmonic cancellation in current mirror loaded long tail pair.
Think cancellation of all EVEN order harmonics.
Moreover, so-called "CFAs" are generally single gain stage designs generating needlessely insufficient forward path gain and, therefore, insufficient major loop gain. Not good!
I second that. I was reading it again yesterday and came away feeling again informed. Most importantly, it was nice to have information as opposed to just a series of opinions. I would also welcome some more info on common source output stages and more importantly, a more thorough practical exposition on the optimization of input LTPs. Thank you.
My Dear Lord, so much trolling here . . .
At least I have built both types. And listened to them and can comment from a position of working knowledge. We build audio stuff to enjoy the music, right?
CFA Amplifers
http://hifisonix.com/wordpress/wp-content/uploads/2013/01/The-nx-Amplifer.pdf
http://hifisonix.com/wordpress/wp-content/uploads/2012/10/The-sx-Amplifier.pdf
VFA Amplifiers:
http://hifisonix.com/wordpress/wp-content/uploads/2011/03/The_e-Amp_V2.03.pdf
http://hifisonix.com/wordpress/wp-content/uploads/2010/10/Ovation-250.pdf
The CFA designs were the last ones I did out of curiosity more than anything. You can remain biased against CFA toplogy amps (but why?), but the practical results tell a very different story: they are easily a match for VFA designs.
At least I have built both types. And listened to them and can comment from a position of working knowledge. We build audio stuff to enjoy the music, right?
CFA Amplifers
http://hifisonix.com/wordpress/wp-content/uploads/2013/01/The-nx-Amplifer.pdf
http://hifisonix.com/wordpress/wp-content/uploads/2012/10/The-sx-Amplifier.pdf
VFA Amplifiers:
http://hifisonix.com/wordpress/wp-content/uploads/2011/03/The_e-Amp_V2.03.pdf
http://hifisonix.com/wordpress/wp-content/uploads/2010/10/Ovation-250.pdf
The CFA designs were the last ones I did out of curiosity more than anything. You can remain biased against CFA toplogy amps (but why?), but the practical results tell a very different story: they are easily a match for VFA designs.
The few monolithic so-called "CFAs" that use two stages of gain also use minor loop compensation which loses the much vaunted "speed".
I fail to appreciate how and why the shortcomings of these miserable concoctions can be improved for audio.
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