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Old 28th February 2012, 02:07 PM   #2011
PauloPT is offline PauloPT  Portugal
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Here's my simulation files. Maybe it's problem with the models, I don't know.

Quote:
Me I modified it a lot.
Wanna share your modifications?

Thanks.
Attached Files
File Type: zip SSA.zip (2.7 KB, 125 views)
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Last edited by PauloPT; 28th February 2012 at 02:24 PM.
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Old 28th February 2012, 03:18 PM   #2012
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by Salas View Post
550/560 are much OK for input, but avoid for drivers, they may die during bias drifts in test phase many times, Vceo not enough for full swing also. Emitter resistors on drivers did not play a key stabilization role, those were BD139/140 I had experience with though, maybe doing better for your 2SA1360/2SC3423.
Thanks Salas. Yes, bias stability is tricky. If I understand correctly, both the input devices and drivers increase conduction with temperature rise, which amplifies the affect.

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Yes they can run <5mA and the amp still be fast. You may need other than 330R to turn the drivers on at the current you want. Don't assume that is too easy to stabilize VS BJT OPT though. That 330R between the output Mosfet gates need be something more elaborate IMHO, there it was a pain to keep bias in mine. Maybe a Vbe multiplier, but have your own test go to see also.
The Thermaltrak output devices are pretty well compensated by the internal diodes. They seem to be fairly stable to temperature changes, if I correct for the driver current. It's the driver current that is difficult to stabilize, so I guess that you are probably correct that a VBE multiplier would be needed to stabilize the FET version. In that case the output devices are self stabilizing, but the VBE multiplier would compensate for the driver drift. However, that means that the current through the drivers is still allowed to vary a lot - which could affect distortion. So how do we get the drivers to quickly stabilize to a desired current? It seems like that is the issue. Is LC's latest scheme the only way?
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Old 28th February 2012, 05:08 PM   #2013
Salas is offline Salas  Greece
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I used a JFET CCS under the upper driver and fixed their bias, some argued that goes against full symmetry, others argued its gonna effect it only near clip. To me the most important aspect is not to force too much base current to the upper driver, its going to suck it up from the respective input BJTs and be exhausting the drive ability non symmetrically with swing then.
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Old 28th February 2012, 07:45 PM   #2014
Bigun is offline Bigun  Canada
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As I mentioned before, I considered the NTC in the early days of designing the TM5 to 'fix' the bias current stability. The place I found worked best was in place of the resistor that feeds current from the zener diodes to the feedback node. I found suitable thermistors at digikey with the right coefficient to hold it stable. But I didn't use them in the end, I don't think they are necessary - and they are expensive. Perhaps for the big higher power amps it becomes necessary again.

LC - I like the way your build is coming along, nice mechanical layout. Did you orient the power trafo's so that their magnetic flux leakage cancels out ? (I saw that done in a Goldmund amp I think).
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Old 29th February 2012, 12:25 AM   #2015
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by Salas View Post
I used a JFET CCS under the upper driver and fixed their bias, some argued that goes against full symmetry, others argued its gonna effect it only near clip. To me the most important aspect is not to force too much base current to the upper driver, its going to suck it up from the respective input BJTs and be exhausting the drive ability non symmetrically with swing then.
Will this work with BJT outputs?

Sheldon
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Old 29th February 2012, 02:04 AM   #2016
Salas is offline Salas  Greece
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Don't know, I only did it in the Latfet amp. Not difficult you give it a try though.
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Old 29th February 2012, 02:27 AM   #2017
Sheldon is offline Sheldon  United States
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Don't know, I only did it in the Latfet amp. Not difficult you give it a try though.
Will do. But might be a couple of weeks before I can get to it.

Sheldon
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Old 29th February 2012, 12:01 PM   #2018
PauloPT is offline PauloPT  Portugal
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Can anyone please look at my simulation files on post #2011 and tell me what I'm doing wrong? Thank you.
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Old 29th February 2012, 12:59 PM   #2019
Jay is offline Jay  Indonesia
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Can anyone please look at my simulation files on post #2011 and tell me what I'm doing wrong? Thank you.
R6
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Old 29th February 2012, 11:13 PM   #2020
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LC, because I can't find any good transistor to replace BCs 550/560C at VAS (Cascode makes high output impedance, not good for BJT OPS) So I decide to limit the rail voltage, is it wise to do so ?
My plan is to use 20V zener regulation for whole input and VAS (exclude Output). Buy Zener Diode,Zener,BZX85C20 Magnatec BZX85C20V online from RS for next day delivery.
its datasheet : http://docs-asia.electrocomponents.c...6b800297e6.pdf
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