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Old 16th November 2011, 03:05 PM   #1411
sonnya is offline sonnya  Denmark
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If the SSA was driving mosfet like my CFB with EXICON it would also be assymetric by a few volt. because of difference in the N and P channel.

So the few volt accross the JFET does not matter if the total voltage swing is +/-35V...

It will only be noticed by clipping....
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Old 16th November 2011, 03:06 PM   #1412
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Originally Posted by Salas View Post
For TO-126 at that power level (2mA or up to 10mA across 30V) don't expect much thermal wandering. Was moving around much more without. Its a solution for when some versions won't behave.
Yes it very much depends on topology. SSA front-end was for me first time tested in high performance mode (input cascodes + VAS cascodes).

I found out that this circuit is even faster as before (without cascodes). Phase shift in a SSA basic was 24° and now is 8,6° at 2 MHz, without any instability issue.

So based on this ground I can do a lot with this circuit in the future.
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Old 16th November 2011, 03:09 PM   #1413
Salas is offline Salas  Greece
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Quote:
Originally Posted by sonnya View Post
If the SSA was driving mosfet like my CFB with EXICON it would also be assymetric by a few volt. because of difference in the N and P channel.

So the few volt accross the JFET does not matter if the total voltage swing is +/-35V...

It will only be noticed by clipping....
There are ~0.6V pinch off Jfets to use. My latfets ain't matching also, just picked out off the tube.
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Old 16th November 2011, 03:13 PM   #1414
Salas is offline Salas  Greece
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Originally Posted by Lazy Cat View Post
Yes it very much depends on topology. SSA front-end was for me first time tested in high performance mode (input cascodes + VAS cascodes).

I found out that this circuit is even faster as before (without cascodes). Phase shift in a SSA basic was 24° and now is 8,6° at 2 MHz, without any instability issue.

So based on this ground I can do a lot with this circuit in the future.
Its a very worthy bridge front end from the moment you get to know its traits. Such stuff usually lives on miniscule silicone area, trimmed and thermally homogenous current feedback op amp world. Was a refreshing idea you brought it back to DIY discrete world power amps Andrej. Hats off for that.
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Old 16th November 2011, 03:16 PM   #1415
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Very nice. Will you do FFT on some computer card loop also? In which one specific schematic is the build referring to?
This one, IRF610/9610 as outputs, 2SA1987/2SC5359 omitted. Current capability limited with IRF SOAR, but nevertheless the signal conditions are relevant.
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File Type: jpg SSA BIGBT HP.jpg (762.7 KB, 468 views)
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Old 16th November 2011, 03:19 PM   #1416
Salas is offline Salas  Greece
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How much low do you trim the front BJTs current feeding resistors so they run 2.5mA Iq? ~?
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Old 16th November 2011, 03:53 PM   #1417
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How much low do you trim the front BJTs current feeding resistors so they run 2.5mA Iq? ~?
Well it goes like this:
- 15 V supply voltage on zeners
- 2,5 mA input BJT collector current
- 1,220 V SSA bridge DC voltage, that means 0,61 V on 93,7 ohm (100 ohm parallel to 1,5k) is another 6,5 mA
- so all together is 9 mA through current supply resistor, voltage on it is 14,39 V (15 V - 0,61 V) so the resistor-trimmer value is 1.597 ohm
- 15 V zener bias current is 4 mA
- VAS current 12 mA at start, stabilized at 15 mA later
- driver bias current fixed to 50 mA (iron fist Vbe multiplier)

Images:
- green LED side with input BJT's (below middle blue cap, BJT's glued together) plus cascodes
- VAS side plus cascodes
- complete amp
Attached Images
File Type: jpg SSA8.jpg (78.8 KB, 423 views)
File Type: jpg SSA9.jpg (99.7 KB, 367 views)
File Type: jpg SSA10.jpg (110.1 KB, 362 views)

Last edited by Lazy Cat; 16th November 2011 at 03:56 PM.
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Old 16th November 2011, 04:07 PM   #1418
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I found out that this circuit is even faster as before (without cascodes).
That's why God had created cascodes ;-)
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Old 16th November 2011, 04:10 PM   #1419
Salas is offline Salas  Greece
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Yes, around the 8-9mA mark the stronger side seemed good in mine too that I CCSed the feeds.

Your Vbe mult keeps bias well. In latfet CFP with drop over resistor for bias, the Vas warm up tips the bridge much more readily in my experience for output stage Iq, thus offset.
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Old 16th November 2011, 04:16 PM   #1420
Salas is offline Salas  Greece
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That's why God had created cascodes ;-)
Yep, the tube gods of yore.
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