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Old 4th December 2012, 04:55 PM   #411
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Quote:
Originally Posted by studeb View Post
you could try to encase some of all of it
small metal or plastic box to test it out?
I am tempted to glue a piece of thin copper foil on the input transistors and see if a more even temperature spread will have an effect.
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Old 4th December 2012, 07:14 PM   #412
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Sorry for misleading everyone this but the oscillation issue remains. It behaved fine with an open input but as soon as the input is short circuited it goes bonkers.

DC condition at output is probably related. Will proceed with Hesener's suggestion to remove the helper transistors from the current mirrors
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Old 4th December 2012, 07:32 PM   #413
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Put a cap of ca. 1nF parallel to the input. That will of cause not help when the input is short circuited. I have more ideas.
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Old 5th December 2012, 07:35 AM   #414
hesener is offline hesener  Germany
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Quote:
Originally Posted by analog_sa View Post
It is not fully encased. The dc drift has a frequency in the region of 1-2Hz; isn't that too fast for thermal wandering? Approximately 120mV of wandering with servo on and 2v without the servo.
1-2Hz is OK for small thermal masses (like in this case); for power transistors it would be a very different story.....

Quote:
Originally Posted by analog_sa View Post
Apparently my beta matching is not so great as there is a constant disbalance around +2v at output. At max it goes up to maybe +4v, so not using the servo effectively limits the max output swing on one polarity - not so great.
It may not be HFE mismatch but base current mismatch. Changing offset voltage with different input impedance should reveal this... maybe its just one "bad apple"....

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Originally Posted by analog_sa View Post
Haven't made a final judgement about the servo yet. Maybe speeding it up will cure the drift. Will do some more listening with and without the servo but if output offset cannot get below a few mV there will have to be an output cap anyway.

How was the dc stability in the beta run?
Rock solid (in both my R1 and R2 builds). With your oscilloscope, can you see oscillation in the 40-50MHz range? (just checking....) Please also check the supply rails (AC coupling, of course)
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Old 5th December 2012, 08:05 AM   #415
carolus is offline carolus  Belgium
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Quote:
Originally Posted by Bksabath View Post
Carolus has same but opposite problem in the other Paradise tread
Paradise phono stage part GB post 216
I be happy to help PM on its way
Sorry for late response ..... I sent a PM ..
Thanks for help.
Karel
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Old 5th December 2012, 11:31 AM   #416
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Not need for apolologies this is Diy so time and waste of it are part of funn.

As on PM my batch of transistors from Fairchild mesure prety Hi (I have not oppened goodies bag so far)
I be happy to help as I have no space for 1800 transistor to sit on bench gatering dust.
But IMO start posting what you need on the tread and start swaps
There may be somebody near you with oposite problem as you have
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Old 5th December 2012, 11:33 AM   #417
RCruz is offline RCruz  Switzerland
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Quote:
Originally Posted by analog_sa View Post
Btw, RC, how stable is your offset?
It wonders slowly between +-5mV
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Old 5th December 2012, 12:52 PM   #418
vrun is offline vrun  Germany
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Using an unexpected day off I started stuffing my boards ...
(GB parts missing come from my parts Box and Reichelt (C's mainly))
However:
When I came to the the Heatsink I noticed that I have no more Isolation Pads...
...and the Manual only says "remember to put isolation if needed" <grin>
Question: Do I need to isolate the Power Transistors on the Heatsink?

(sry for beeing lazy but I am in for a late lunch break and hope the answer is ready when I am finished ... while the answer may be useful for all of us still soldering.)
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Old 5th December 2012, 01:22 PM   #419
RCruz is offline RCruz  Switzerland
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Quote:
Originally Posted by vrun View Post
Using an unexpected day off I started stuffing my boards ...
(GB parts missing come from my parts Box and Reichelt (C's mainly))
However:
When I came to the the Heatsink I noticed that I have no more Isolation Pads...
...and the Manual only says "remember to put isolation if needed" <grin>
Question: Do I need to isolate the Power Transistors on the Heatsink?

(sry for beeing lazy but I am in for a late lunch break and hope the answer is ready when I am finished ... while the answer may be useful for all of us still soldering.)
You need to isolate them if they are not the all plastic case build
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Old 5th December 2012, 01:27 PM   #420
FdW is offline FdW  Netherlands
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Originally Posted by RCruz View Post
You need to isolate them if they are not the all plastic case build
And you need to apply heat-conducting compound.
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