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Old 27th April 2013, 08:28 PM   #4051
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Quote:
Originally Posted by maxente View Post
I manage borrow a o-scope
I mount 100 ohm trimmer + 47p on each gate to rails
If i adjunst trimmer from the positive rail nothing happens, if I adjust the trimmer from gate to negative rail more oscilation appear
How should i proceed next?
Both 50 pF capacitors must be trimmers otherwise calibration is nonsense. The whole aim is to set the amp's loop gain below 1 at frequency where phase shift is over 120 degrees. You can also try small capacitance from gate to GND if compensation to DC rails would be uneffective.
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Old 27th April 2013, 10:00 PM   #4052
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Originally Posted by ALAZ View Post
hi to all
Unfortunately 2sk1058/2sj162 not availble in my city but irfp240/9240 is availble.for use this irfp what modification is Necessary for ssa?
regard
Vertical mosfet's positive tempco compensation, meaning bias spreader has to have its semiconductor as a temperature sensor mounted on the main heatsink, to properly compensate temperature/output bias dependency.
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Old 27th April 2013, 10:21 PM   #4053
maxente is offline maxente  Romania
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Quote:
Originally Posted by Lazy Cat View Post
Both 50 pF capacitors must be trimmers otherwise calibration is nonsense. The whole aim is to set the amp's loop gain below 1 at frequency where phase shift is over 120 degrees. You can also try small capacitance from gate to GND if compensation to DC rails would be uneffective.
I follow your advice but it's worse
Thank you very, very much for your support, but I think i will not continue because it is very difficult for a simple dyer like me with no experience resolving this unstable amplifier
Thank you very much again and good luck !
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Last edited by maxente; 27th April 2013 at 10:24 PM.
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Old 27th April 2013, 10:50 PM   #4054
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Quote:
Originally Posted by maxente View Post
I follow your advice but it's worse
Thank you very, very much for your support, but I think i will not continue because it is very difficult for a simple dyer like me with no experience resolving this unstable amplifier
Thank you very much again and good luck !
OK, according to scope plots, there's also some other problem in your circuit than just an oscillation, as so complex scope signal indicates that. Usually theres HF sine or its modulation at large. I know it's difficult for beginer to start and solve such wild amp conditions, especially if nobody built it before. This SSA version for sure can be stable, if I'll catch some spare time maybe I'll do it, but at the moment everything from me is dedicated to VSSA.
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Old 28th April 2013, 07:46 PM   #4055
ALAZ is offline ALAZ  Iran
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Quote:
Originally Posted by Lazy Cat View Post
Vertical mosfet's positive tempco compensation, meaning bias spreader has to have its semiconductor as a temperature sensor mounted on the main heatsink, to properly compensate temperature/output bias dependency.
hi.your order is that bd139/140 mounted to main heatsink with irfp mosfets?
and which of ssa with dc servo or original ssa is better in sound qulity?
Grateful
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Old 7th May 2013, 10:04 PM   #4056
shaan is offline shaan  India
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Quote:
Originally Posted by maxente View Post
...this unstable amplifier...
Hi max.

You might try these to make it stable:

1. Install two 47pF in parallel with the 1K feedback resistors, one for each.

Or,

2. Replace 1K/100R feedback network with 2K2/220R.

I can't assure but one of these tricks might help. It will be sad to see you giving up after this much progress.

shaan

Last edited by shaan; 7th May 2013 at 10:07 PM.
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Old 7th May 2013, 10:18 PM   #4057
sonnya is offline sonnya  Denmark
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Quote:
Originally Posted by shaan View Post
Hi max.

You might try these to make it stable:

1. Install two 47pF in parallel with the 1K feedback resistors, one for each.

Or,

2. Replace 1K/100R feedback network with 2K2/220R.

I can't assure but one of these tricks might help. It will be sad to see you giving up after this much progress.

shaan
It is an current feedback design. So don't go for no.1!!

Place a millercap accross Basis and Collector on VAS transistor. 47pF.

Replace 1K/100R with 4K7/470R if this is stable -> 2K2/220R.
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Old 8th May 2013, 12:40 PM   #4058
shaan is offline shaan  India
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Quote:
Originally Posted by sonnya View Post
It is an current feedback design. So don't go for no.1!!
Coz doing so will make it oscillate, right?

Quote:
Place a millercap accross Basis and Collector on VAS transistor. 47pF.
Where are those pins?!

Anyway, VAS compensation has been tested by him. FAILED. See just a few posts back.

Quote:
Replace 1K/100R with 4K7/470R if this is stable -> 2K2/220R.
Yea, Step-jump is very healthy for math exams.

happy listening...
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Old 8th May 2013, 01:08 PM   #4059
sonnya is offline sonnya  Denmark
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See the red caps.
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Old 8th May 2013, 04:59 PM   #4060
maxente is offline maxente  Romania
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Hi shaan and sonnya
I will not give up just yet
I'm waiting for Lazy Cat to get some time to review the schematic but in the meantime if you give me some help i certainly appreciate it
I will try to do what you said an get back to you
Thank you shaan and sonnya
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