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Old 5th November 2011, 07:46 PM   #1301
Salas is offline Salas  Greece
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Quote:
Originally Posted by Lazy Cat View Post
Maybe it is a possible error or unsymmetry caused by the fet current source because of its voltage drop ...

Mine SSA BIGBT has no power on thump-pop whatsoever (scope shows 200mV positive jump for the first 0,2 s for no loaded amp) and its thermal DC offset drift inside +/-10 mV over cold to 70C.

More important issue is an output bias thermal stabilization which can be realized successfully in many ways, already suggested here and in TGM5 thread, how to hold the amp at a certain thermal working conditions.

According to my oppinion the most important feature by this design is its no frequency compensation need as a result of the SSA feedback nature and consequently the sound quality result.

Some times it goes up to 200-300mV at start, but sometimes not, shootin much. I am suspicious of the Zeners when cold or hot because DO-41 glass mini cases. About the Vgs (off) kick of the JFET CCS I have seen no difference in output pair Iq per side. Upper CCS resistor from common base cascode transistor base and Zener node being a trimmer is necessary anyway because the PNP input side has lower Vbe. And it centers OK in general. Could work without comp but with a step network it showed better square wave on rising and trail edge.
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Old 5th November 2011, 08:02 PM   #1302
Sheldon is offline Sheldon  United States
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Thanks for the review of my thermal comp question.

TC, I don't remember if it's been addressed here, but what is the outcome on the opposing rail if either rail fuse goes?

Sheldon
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Old 5th November 2011, 08:15 PM   #1303
Salas is offline Salas  Greece
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Quote:
Originally Posted by sonnya View Post
I have a solution to the drift...

1. You Can use it or not. The 2 input transistor should be feed by an current mirrors feed by a shared currentsource Like cr200..

2. the cr200 Will give an idle current of 2mA +/-10%.. this Will reduce the basiscurrent and remove the heat. The baseresistor for the next stage has to be 5x larger..

This solution is taken from my mirand AMP.

I have a drift starting at 300mV falling to ~50mV. And i Will ADD the circuit above
Can you show it schematically? Thanks.
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Old 5th November 2011, 09:38 PM   #1304
sonnya is offline sonnya  Denmark
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I have attached the frontend.
Attached Files
File Type: pdf Symetricfrontend-modified 05-11.pdf (12.9 KB, 273 views)
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Old 5th November 2011, 09:39 PM   #1305
sonnya is offline sonnya  Denmark
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My mirand schematic floats around somewhere in here....
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Old 5th November 2011, 09:46 PM   #1306
Salas is offline Salas  Greece
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Nice. You had CCSed the input current feed substituting the current feed resistors. Something that Lazy Cat had talked about as an option early in the thread, pointing out the PSRR benefit mainly. But it proves DC conditions stabilizing also. A few patches and this front end will be a true all rounder probably.
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Old 5th November 2011, 09:49 PM   #1307
sonnya is offline sonnya  Denmark
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In my point of view.

All those simpel circuits does need housekeeping!
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Old 5th November 2011, 09:53 PM   #1308
Salas is offline Salas  Greece
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True. Why did you degenerate the drivers with 22R by the way? Shaped the THD to your preference, or to shape the phase margin curve a bit for applying the Miller caps?
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Old 5th November 2011, 10:05 PM   #1309
sonnya is offline sonnya  Denmark
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What happend with the vBE voltage and Hfe as the temperature rises??

First The VBE falls of with ~2mV/K and second the gain rises.

So all extra current that does not go into R12 and R13 goes into the base of Q3 and Q6 and the Collector current goes up... When the collector current goes up the temperature rises more....

As the room temperatur goes up and down so does the the collector current alot!

This leads to an uncontrollable idle current in Q3 and Q6.. You will not be able to make to identical amps as you cannot get transistors with the same HFE and VBE drop....

The 22R emitter resistor is basic control of the current in an BJT.... Just basic.....
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Old 5th November 2011, 10:16 PM   #1310
Salas is offline Salas  Greece
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I see about getting THAT340 but no news yet.
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