Simple Symetrical Amplifier - Page 202

PauloPT · 02:24 PM

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.

Sheldon · 28th February 2012, 03:18 PM

Quote:

Originally Posted by Salas

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.

Quote:

Originally Posted by Salas

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?

Salas · 28th February 2012, 05:08 PM

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.

Bigun · 28th February 2012, 07:45 PM

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).

Sheldon · 29th February 2012, 12:25 AM

Quote:

Originally Posted by Salas

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

Salas · 29th February 2012, 02:04 AM

Don't know, I only did it in the Latfet amp. Not difficult you give it a try though.

Sheldon · 29th February 2012, 02:27 AM

Quote:

Originally Posted by Salas

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

PauloPT · 29th February 2012, 12:01 PM

Can anyone please look at my simulation files on post #2011 and tell me what I'm doing wrong?

Thank you.

Jay · 29th February 2012, 12:59 PM

Quote:

Originally Posted by PauloPT

Can anyone please look at my simulation files on post #2011 and tell me what I'm doing wrong?

Thank you.

R6

guitar89 · 29th February 2012, 11:13 PM

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

28th February 2012, 07:45 PM	#2014
Bigun diyAudio Member Join Date: Jan 2009 Blog Entries: 2	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). __________________ "The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

29th February 2012, 12:01 PM	#2018
PauloPT diyAudio Member Join Date: Jan 2010 Location: Lisboa, Portugal	Can anyone please look at my simulation files on post #2011 and tell me what I'm doing wrong? Thank you. __________________ Kind Regards, Paulo.

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28th February 2012, 05:08 PM	#2013
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	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.

29th February 2012, 02:04 AM	#2016
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	Don't know, I only did it in the Latfet amp. Not difficult you give it a try though.

29th February 2012, 11:13 PM	#2020
guitar89 diyAudio Member Join Date: Dec 2010 Location: Malaysia, Selangor	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