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Old 7th April 2012, 09:19 PM   #21
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Logical evolution: TSSA with VAS.

One benefit this amp has, the 1N4148 bias potential diodes thermally compensates input BJT-s Vbe tempco, thus stabilize VAS bias current.
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Old 7th April 2012, 09:23 PM   #22
sonnya is offline sonnya  Denmark
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Quote:
Originally Posted by Lazy Cat View Post
Logical evolution: TSSA with VAS.

One benefit this amp has, the 1N4148 bias potential diodes thermally compensates input BJT-s Vbe tempco, thus stabilize VAS bias current.
It is a lot better. But i would recommend a cap across bias resistor.
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Old 7th April 2012, 09:31 PM   #23
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Hi sonnya

Yes and also input cap, some decoupling caps etc. Intention of this sch is to show basic principle more clearly, it is not final product sch.

Did you test TSSA first post sch yet? This feedback bridge doesn't have any injecting current, which imediately brings two benefits, thermal stability plus no modulation of injecting current, simply because it isn't present.
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Old 7th April 2012, 09:38 PM   #24
sonnya is offline sonnya  Denmark
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Go to post 19 and 20.
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Old 7th April 2012, 09:48 PM   #25
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Aha thanks, missed them, were posted in between I wrote ...

Hej not bad at all considering four trannies only and to be symmetrical. Input BJT-s, splitted feedback bridge and biasing are working properly. Lowering distortions is a matter of more sophisticated schematic.
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Old 7th April 2012, 10:00 PM   #26
sonnya is offline sonnya  Denmark
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No, it is not bad at all.

The same circuit could use njw0281/njw0302. With an higher idle in the input pair (10mA) and with good cooling it would be easy to do a class a amp with them also. Maybe even remove the collector resistors??!?!!!
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Old 7th April 2012, 10:12 PM   #27
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Something like this, all BJT simplest.
P.S. 1mF FB bridge cap can be also beneficial.
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Old 7th April 2012, 10:21 PM   #28
sonnya is offline sonnya  Denmark
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First sim of this says: use darlington sap15n/p

But sim was worse for tssa than measured.....
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Old 7th April 2012, 10:24 PM   #29
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TSSA - reality friendly amplifier!
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Old 8th April 2012, 12:47 AM   #30
Jay is offline Jay  Indonesia
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Default Why No DC Servo?

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Originally Posted by buzzforb View Post
Might have to try this one. See how the bjt compares to jfet.
I think I will prefer the bipolar if the distortion is close. But how close?

I'm curious why DC servo is not discussed with SSA circuits. It is hard to get a low THD without compensating other factors such as the DC offset (at least from simulation).

I will be building this TSSA but cannot yet decide on which circuit to build. Zero DC --> terrible THD. Good THD --> terrible DC.

I don't know what is the worse acceptable DC, 0.2 volt may be? But I think with such a simple circuit highish THD is okay, 0.3% may be?

I have tried a -0.2V offset with 0.02% THD (1kHz, 2V input sinewave, 2N5401/5551, tho low noise TR performs better). May be I need to aim for 0.1V?
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