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Old 13th August 2006, 01:18 AM   #21
ingrast is offline ingrast  Uruguay
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Quote:
Originally posted by djk
....
I built an amp like this (without your current limiting) in 1997. You have omitted a couple of caps for clarity I assume.
....
I did not omit caps though I used at some time Miller compensation on T7 (T10) for stability. It turned out not necessary later and choose to keep bandwidth as high as possible.
Probably the fact I selected on purpose 2N3904/06 with this in mind is why I could do away with compensation.

Rodolfo
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Old 13th August 2006, 01:25 AM   #22
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djk:
"There is another even earlier example of your bias scheme in Wireless World..."

this?
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Old 13th August 2006, 01:59 AM   #23
ingrast is offline ingrast  Uruguay
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Looks I did come out with nothing really new, as was to be expected after all.

Interestingly, note this early example from WW addresses the quiescent bias current through a differential amplifier (I rather like to see it this way rather than as current mirror) to gather enough gain, but does nothing to prevent Vgs to drop to 0 in the reverse cycle.

This is a big issue since the usually high gate capacitance implies the driver must supply suddenly a large charging current at crossover.

This happened with my early prototypes which followed a conventional CFP scheme much like the Tiger series of amplifiers did.

The crossover transient was absolutely unacceptable and led me to look for better schemes ending in the current posted one.

Rodolfo
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Old 13th August 2006, 03:13 AM   #24
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I mean this Blomley

It is using current mirror with gain, so the final transistors never turn off.
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Old 13th August 2006, 03:27 AM   #25
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Quote:
Originally posted by ingrast
Looks I did come out with nothing really new, as was to be expected after all.

Interestingly, note this early example from WW addresses the quiescent bias current through a differential amplifier (I rather like to see it this way rather than as current mirror) to gather enough gain, but does nothing to prevent Vgs to drop to 0 in the reverse cycle.

This is a big issue since the usually high gate capacitance implies the driver must supply suddenly a large charging current at crossover.

This happened with my early prototypes which followed a conventional CFP scheme much like the Tiger series of amplifiers did.

The crossover transient was absolutely unacceptable and led me to look for better schemes ending in the current posted one.

Rodolfo
Rodolfo,

YES and I agree with all
But there are only a few people who try to solve the crossover problem more elegant than with class hot... thats what count!...
...my opinion!
Beside the Blomley (but only the "Original" from 1970!) there exists some other solutions one I used since 20 years in many of my (non audio) designs ....Im curios to find it one time here
Chance is small, because folks like simple so You excuse yourself "it may look complex" my circuit (for source follower) need still more parts!

Rodolfo what You hide in the big yellow/black box?
Is this the jail for the residual distortion?

Regards
Heinz!
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Old 13th August 2006, 03:59 AM   #26
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Quote:
Originally posted by lumanauw
I mean this Blomley

It is using current mirror with gain, so the final transistors never turn off.
...yes, but now another problem exist in front of T8/10!
Blomley use a transconductance amp instead a opamp...why?

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Heinz!
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Old 13th August 2006, 09:13 AM   #27
djk is offline djk
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"powerbecker djk:
"There is another even earlier example of your bias scheme in Wireless World..."

this? "

No, I haven't seen that one before. Do you remember what year it was?

I'm trying to find the one I mean. The technical library at work is closing, I'm trying to find out what they will do with all their back magazine issues. First they moved the library into too small of a space, cut their hours by 3/4, made it almost impossible to use, and then when usage went down they say it's no longer needed!

IIRC, the circuit was an opamp driving a pair of common base level shift transistors with the HexFet pair biased with the clamp on the gates to the rails.

As part of another project around 2000, we had a modulation amp (a 1983 design) driven this way (without the bias clamp) and were required to remove the thermal compensation resistors from the bias circuit. I re-designed the amp and it was stable over temperature and had a clean spectrum. It put out about 250W peak with 40V rails. I was required to use an AD842 opamp (which is not unity gain stable).
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Old 13th August 2006, 09:38 AM   #28
AKSA is offline AKSA  Australia
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The Class AB Emitter Follower II circuit described by Self is very good sonically. Crossover is trivial if you use a good cap tied across the driver emitters.

The CFP has two essential problems; very tetchy bias adjustment with corresponding thermal stability issues, and instability caused by the CE phase shift/high fb factor. To eliminate this problem, you must apply lots of miller capacitance across base/collector of the driver; this chemo poisons the host and destroys the good sonics. Damned if you do, damned if you don't....

The actual issue to my mind is the considerable impedance variation as the signal traverses the crossover dead zone; this variation has profound impact on the high impedance collector of the VAS.

Fix this problem and crossover all but disappears.........

Cheers,

Hugh
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Old 13th August 2006, 09:56 AM   #29
AndrewT is offline AndrewT  Scotland
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Hi,
re circuit in post24.
Why are the output Qs collectors fed from r22 & r32?
56R here seems odd!
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Old 13th August 2006, 12:01 PM   #30
ingrast is offline ingrast  Uruguay
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Quote:
Originally posted by powerbecker


....
Rodolfo what You hide in the big yellow/black box?
Is this the jail for the residual distortion?

.....

Heinz:

Right, but I hope to disclose that soon, may be in the form of a paper in a relevant place.


Rodolfo
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