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Ask for help... buffer add to curcuit with DC servo becomes unstable
Ask for help... buffer add to curcuit with DC servo becomes unstable
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Old 13th June 2015, 11:40 AM   #1
snowcatmaiden is offline snowcatmaiden  Hong Kong
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Join Date: Mar 2009
Default Ask for help... buffer add to curcuit with DC servo becomes unstable

dear everyone,

I am going to build an amplifier which can input differential signals.

Ask for help... buffer add to curcuit with DC servo becomes unstable-draft1-jpg

(sorry for very draft feel schematics)

Ask for help... buffer add to curcuit with DC servo becomes unstable-kma-servo-png

DC servo is the same as this one,
changed to +-60V power and adjusted the base voltage of cascode BJT to 24V. JFET using K246 J103.

I have tested the following:
1. the buffer itself worked well, with some DC output.
2. i tested the amplifying circuit with single ended signal to non-inverting input, NFB 1Kohm connected to GND, and worked well.
3. only one buffer which is for single ended signal, output to non-inverting input of amplifying circuit with NFB 1Kohm connected to GND. worked well.


But when I used both buffers for hot and cold then connect to amplifying circuit as the schematic,
1. there is a very unstable DC at the output.
2. the maximum output power is less than that with single ended input signal. It is easier for differential signal input to clip.
3. with differential input when it clips, the output tubes burn even without loading. It did not happen with single ended signal which just clip without burning.
4. slew rate is lower with differential signal, overshoot more.


I would like to ask for your helps. The questions are
1. Is that the DC servo and buffer conflict with each other? How to deal with this? I saw many amplifiers outside using about the same idea with no problem.
2. Tried hard to eliminate the DC output of the buffer but no results. Is it a must to have very low DC to make the buffer and the whole stable?
3. Too high voltage for the buffer?
4. many wishes for any and all other suggestions.

sorry for bad english
Attached Images
File Type: jpg draft1.JPG (108.0 KB, 227 views)
File Type: png kma-servo.png (34.4 KB, 223 views)
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Old 15th June 2015, 07:09 AM   #2
snowcatmaiden is offline snowcatmaiden  Hong Kong
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Join Date: Mar 2009
ask for help m(_ _)m
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Old 15th June 2015, 08:55 AM   #3
DF96 is offline DF96  England
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Join Date: May 2007
Have you studied and understand servo loop stability? Also parasitic oscillation.
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Old 16th June 2015, 05:17 AM   #4
snowcatmaiden is offline snowcatmaiden  Hong Kong
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Join Date: Mar 2009
thanks for your reply.

If I only use the amplifying circuit with the DC servo,
which the 1Kohm of NFB connected to the ground but not cold signal,
the whole is very stable.

and did not find any parasitic oscillation in amplification circuit.

How about the buffer? It would be a big problem if the buffer contains/generates noises?
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