If it made the VAS less linear it would increase distortion. Did you check? It just reduces voltage gain, which is irrelevant because this VAS is current driven.
SMT old eyes
For us old folks who struggle with surface mount soldering could the surface mount component be already soldered on boards when they ship? Yes that makes it harder to customize but might actually mean I don't need to buy 4 boards to get one soldered that is working. Yes likely more expensive. Oh how I struggled solderings Waynes linestage preamp board.
Single side pcb,diy friendly in the first phase..like in the past
For us old folks who struggle with surface mount soldering could the surface mount component be already soldered on boards when they ship? Yes that makes it harder to customize but might actually mean I don't need to buy 4 boards to get one soldered that is working. Yes likely more expensive. Oh how I struggled solderings Waynes linestage preamp board.
Hi Keantoken, thank you for trying to support me find the source of the oscillation. To be honest I haven't had much time to look into the effects of removing R23. I was more focused on the looking in the drivers and output stage. I think that I found a solution. Check post #2248
But I will take a look most likely over the weekend.
I appreciate that you have greater knowledge onbthis subject than I but.
I understand that the larger R23 is, it lowers the gain in the VAS but doesn't increasing R23 also increase how linear the VAS will be due to the reduction between the ratio of re' to RE?
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The ratio of re' to RE doesn't matter because it is the Ic swing that sets the harmonic generation across Re', and the Ic swing is the same regardless of RE.
If our VAS was voltage driven and the feedback loop did not keep the IC swing the same in all conditions in order to meet the output demands, then RE would decrease the harmonic generation of re' by reducing Ic swing (at the same time as reducing gain). But that is not the circuit that is on our bench.
The resistor does however increase the conversion of OPS drive current to voltage at the VAS input which CAN increase distortion, but luckily again our VAS is current driven so the effect is not very strong.
On the other hand, RE is actually equivalent to 136ohm resistors in series with the driver bases, and you already showed this is undesirable when you bypassed R34/35 to improve stability.
That is one useful features of RE, it can act like base stoppers for the drivers without interfering with the bias loop. But in the circuit on our bench, RE is acting like base stoppers that are far too large.
I have not once, ever, found the idea of "local feedback linearizing the VAS due to Miller capacitance or RE" to be helpful, and in fact every time I've seen it brought up it leads in totally the wrong direction. It's like a computer virus which hides itself by causing the CPU to skip over countless important error checks.
If our VAS was voltage driven and the feedback loop did not keep the IC swing the same in all conditions in order to meet the output demands, then RE would decrease the harmonic generation of re' by reducing Ic swing (at the same time as reducing gain). But that is not the circuit that is on our bench.
The resistor does however increase the conversion of OPS drive current to voltage at the VAS input which CAN increase distortion, but luckily again our VAS is current driven so the effect is not very strong.
On the other hand, RE is actually equivalent to 136ohm resistors in series with the driver bases, and you already showed this is undesirable when you bypassed R34/35 to improve stability.
That is one useful features of RE, it can act like base stoppers for the drivers without interfering with the bias loop. But in the circuit on our bench, RE is acting like base stoppers that are far too large.
I have not once, ever, found the idea of "local feedback linearizing the VAS due to Miller capacitance or RE" to be helpful, and in fact every time I've seen it brought up it leads in totally the wrong direction. It's like a computer virus which hides itself by causing the CPU to skip over countless important error checks.
It limits the current of the VAS buffer transistor so it cannot draw amps of current through the VAS transistor base.
Thank you so very much for taking the time to explain all that to me. I have never read about that interaction before. When you have time, could you please quote any further reference material that I can read over that again... I have access to most amplifier design books....
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I don't have any reference material on this specifically, I mostly figured it out myself in the course of investigation. The same is true every time I have seen it brought up, someone figured it out themselves, it is rarer that they actually found the answer in a textbook, as the right textbook is never available at the moment you actually need it.
Hi Keantoken,
Your completely right. The Ic swing does not change with any change in RE....
Wow... who would have thought... Thanks again for this precious tip.
One question though.. you said that
"RE is actually equivalent to 136ohm resistors in series with the driver bases"
How did you calculate that?
beta x Re, the external emitter resistor in series with the internal emitter resistor , that would be the impedance looking into the base of the trannie, i know that for small signals trannies, not sure how that works for power trannies with large signals..
but beta varies with temperature and internal emitter resistor varies with emitter current, so what now?
but beta varies with temperature and internal emitter resistor varies with emitter current, so what now?
No, it's actually simpler than that. At RF, the miller capacitor connects the VAS base to it's collector. Then the VAS acts equivalent to a diode and 68ohm resistor in series, re' in series with RE. This is a darlington VAS but that doesn't change how it works.
In startmp's schematic, this would be a diode in series with a 68ohm resistor. This is effectively in series with the driver bases. Since both bases share that resistor, then it's equivalent circuit when split into individual base stoppers is double it's original value. So 68ohm * 2 = 136ohm.
Let me clarify that re' means the dynamic resistance of the transistor emitter junction, and RE means the emitter resistor we add to it. I just adopted these terms as they were used by Stuartmp. Sorry for any confusion.
In startmp's schematic, this would be a diode in series with a 68ohm resistor. This is effectively in series with the driver bases. Since both bases share that resistor, then it's equivalent circuit when split into individual base stoppers is double it's original value. So 68ohm * 2 = 136ohm.
Let me clarify that re' means the dynamic resistance of the transistor emitter junction, and RE means the emitter resistor we add to it. I just adopted these terms as they were used by Stuartmp. Sorry for any confusion.
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Sorry keantoken,
I appreciate your efforts to explain that to me but I still don't quite understand how you got the 68 ohms.
If re' equals Vt/Ic and the VAS current is about 9mA we get 26/9 which equals approximately 3 ohms add that to RE of 47 and we get 50 then add the base stopper resistance of 22 and we get 77 ohms. What am I missing?