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Old 23rd May 2008, 09:38 PM   #1
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Default Class A biasing for the AD797?

Hi!
It has probably been discussed before, but if so, I couldn't find it.
Is it possible to bias the output stage in class A by referencing a current to the output node? If so, V+ or V-? Is it worth it sonically? Is the output stage class AB, and if so, which is the class A area currentwise, so a (very) light load resistor might work instead?
thank you,
Rüdiger
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Old 23rd May 2008, 09:58 PM   #2
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Default Re: Class A biasing for the AD797?

Quote:
Originally posted by Onvinyl
Hi!
Is the output stage class AB,

Quiescent current 8.2mA and +/-13v in 600ohm. Does it sound like class A?

What load do you have in mind?
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Old 23rd May 2008, 10:16 PM   #3
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Hi,
with light load I meant the use of an external buffer stage. Say 50k.
Rüdiger
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Old 23rd May 2008, 10:16 PM   #4
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Default Re: Class A biasing for the AD797?

Quote:
Originally posted by Onvinyl
Hi!
It has probably been discussed before, but if so, I couldn't find it.
Is it possible to bias the output stage in class A by referencing a current to the output node? If so, V+ or V-? Is it worth it sonically? Is the output stage class AB, and if so, which is the class A area currentwise, so a (very) light load resistor might work instead?
thank you,
Rüdiger
I don't know is it possible to bias AD797 in Class A I have never tried that mod, but if the answer is yes, you should always reference to -V, because you want to NPN output transistor to do the hard work, not PNP

Is it worth sonically?...Yes and no, you should try that for yourself, but be aware that the THD numbers will be up, but at the same time H2 will be up too, which can be pleasing to some ears.

Regarding standing current in AD797 output transistors, according to my simulation, it's about 0,5mA.
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Old 23rd May 2008, 10:39 PM   #5
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the datasheet claims 500uA output stage bias

push-pull operation operation means ~+/-1 mA load current would be delivered in Class A without additional bias, which turns out to be not much when using feedback impedance levels low enough to take advantage of the input noise performance

the AD797 has a unique local feedback option that essentially cancels the dominant pole compensation around the output stage local Cn loop, giving high loop gain for correction of output stage error:
http://www.diyaudio.com/forums/showt...28#post1513928

with the optimum Cn local feedback cap I doubt external Class A bias would give a measureable improvement

a possible difference between Class A and AB is that poor power supply routing could allow Class B rail currents to magnetically couple to low level signal loops - Self's "distortion #6:
http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm#2

I would suggest adding a buffer/gain stage with a CFA op amp in the feedback loop as shown in the datasheet fig 49 if ultimate low distortion performance was your goal
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Old 23rd May 2008, 10:47 PM   #6
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Quote:
Originally posted by jcx
the datasheet claims 500uA output stage bias

push-pull operation operation means ~+/-1 mA load current would be delivered in Class A without additional bias, which turns out to be not much when using feedback impedance levels low enough to take advantage of the input noise performance

the AD797 has a unique local feedback option that essentially cancels the dominant pole compensation around the output stage local Cn loop, giving high loop gain for correction of output stage error:
http://www.diyaudio.com/forums/showt...28#post1513928

with the optimum Cn local feedback cap I doubt external Class A bias would give a measureable improvement

a possible difference between Class A and AB is that poor power supply routing could allow Class B rail currents to magnetically couple to low level signal loops - Self's "distortion #6:
http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm#2

I would suggest adding a buffer/gain stage with a CFA op amp in the feedback loop as shown in the datasheet fig 49 if ultimate low distortion performance was your goal

This is correct, you rapidly get into touchy bypassing issues. You really have to think about where the harmonic energy in the supply current is going.
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Old 24th May 2008, 07:18 AM   #7
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Is it possible to tap the signal from the decompensation pin 8 (point B in the simplified schematic) and connect it to a discrete output stage and wrap it in the feedback loop instead of the built in diamond buffer, just like you can from the compensation pin in AD829 and AD744?
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Old 25th May 2008, 06:04 PM   #8
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Hi, JCX,

Quote:
with the optimum Cn local feedback cap I doubt external Class A bias would give a measureable improvement
Is this applicable to power amp too? To make a better classB/AB output stage?
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Old 25th May 2008, 06:07 PM   #9
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Quote:
You really have to think about where the harmonic energy in the supply current is going.
To the output node via Cbc?
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Old 25th May 2008, 07:28 PM   #10
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One good solution is to connect a constant current sink at the output. Very good results are possible with bipolar sink, in the range 8...12 mA. Fets exhibits poor linearity at low voltage, and thus are not usable.

PL.
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