Chistophe, the discussion is not about the closed loop gain but about the open loop gain to be raised to get more leeway with feedback to lower distortion.
It ocurred to me that when we "turn around" the lateral Fets ( sources to output ) and ground the output as a transimpedance stage like padamiecki there could be a possibility to set Ugs to the right value and also get more open loop gain.
Ah, thanks, Joachim, sorry for my misunderstanding.
My thoughts:
I suppose the only interest is increasing open loop gain is with keeping the open loop bandwidth equal or better ? Is-it possible ?
As i said in the SSA thread, slew rate is the goal.
... Yes, more feedback gives a better damping factor.
... But, in an other side, it will produce higher signal in the feedback path and that can lead to more TIM ?
I suppose that optimizing this aspect will require a lot of listening test ?
Interesting question to explore.
I am not sure, is that 3V input or output ?
If it is 3V input then his simulation shows much lesser harmonic distortion as other versions of the TSSA i have seen.
Christophe, would more open loop gain result not only in a higher damping factor but also less distortion ?
If it is 3V input then his simulation shows much lesser harmonic distortion as other versions of the TSSA i have seen.
Christophe, would more open loop gain result not only in a higher damping factor but also less distortion ?
with 10Vpp @input this topology became behave strange (-4Vdc offset)
but the wise men will do from this joke good follower
😀 I'm more confused. I thought, of course Joachim knew for sure that it was input. And of course it has lower distortion. But how can I supposed to see it "apple-to-apple" with other TSSA distortions? I mean, there are many ways to make it even lower distortion, especially when it doesn't need to work as a normal amplifier...
anyway it is better than this
http://www.diyaudio.com/forums/pass-labs/215958-ticle-zen-amp-3.html
(the last post)
Well, when we increase the gain factor of 1 stage in an amplifier (with no other change) we often increase its own distortion. (= We reduce its local feedback) In a global loop, increasing feedback to compensate this adding gain will reduce back the global distortion. Did the result better or worse than before ? Hard to predict !
I think we can have a more accurate answer looking at the slewrate.
Basicly the system closed loop gain is calculated with the following formula..
closed loop gain = A/(1+(A * Beta).
A is openloop gain.
Beta is the feedback network. For inverted configuration it is:
Beta = 1/(RF/RG)
For noninverted configuration it is:
Beta = 1/(1+(RF/RG)).
The TSSA as example. You will have a openloop gain A = 158.
Beta is 1/(1+(1K/100)) = 1/11 = .09091
Closedloop gain is: 158/(1+158*.09091) = 10.28.
In comparison with this one http://www.diyaudio.com/forums/soli...est-symmetrical-amplifier-5.html#post2996045: who has a A = 1.000.000..
It is really bad with the TSSA. But it is helped along with CFP input pair wich have local feedback that lowers distortion from the input pair.
Also the mosfet runs in class A .. So the most of the time we do not need to struggle with crossover distortion.
The SSA maybe be better in terms gain. but it also have some unlinear parameters the TSSA does not have...
There is several parameters we can work with.
Reduce RF and RG to 499R and 49.9R => A = ~300
use double lateral mosfet => A = ~600.
Raise the resistor R8 and R7 to 2K => A= ~1200.
Whe have now reduced the distortion 8 times. And Zout is also lowered 8 times.
The system gain means that we 2VRMS to get maximum power. Most DACS and CD players can manage that.
So lowering the feedback resistors raises the open loop gain. Is that the same mechanism as in CFB opamps ? Lowering too much would then cause instability.
Joachim:
Exactly.... Except there no such thing as a stable amp with out Cdom capacitors in one form or another. In my amp it is C12 and C13.
Thats why i asked if the SSA design are stable into 10uF load (I think Quad electrostates are close). My designs are not stable without extentions in the design...
I really dont think the SSA are completly stable... Go for Megahertz they all say. Some of it is true... But you need take into account what you pick up of noise and how close you will come to instabillity..
They are now 47pF but i would suggest 220 - 330pF with all the changes above....
Exactly.... Except there no such thing as a stable amp with out Cdom capacitors in one form or another. In my amp it is C12 and C13.
Thats why i asked if the SSA design are stable into 10uF load (I think Quad electrostates are close). My designs are not stable without extentions in the design...
I really dont think the SSA are completly stable... Go for Megahertz they all say. Some of it is true... But you need take into account what you pick up of noise and how close you will come to instabillity..
They are now 47pF but i would suggest 220 - 330pF with all the changes above....