.Attached is the analysis.
I did it by hand, so I hope my handwriting is understandable...
Best, Sandro
Hi Sandro
Yes, your analysis is correct.
Stein
WOW! TPC is great. All these years I'd overlooked it; that was a mistake.
I took a stereo TMC amp, and swapped one channel to TPC. The compensation networks were set up to produce nearly identical loopgain bode plots on the sim.
TPC sounds clearer, less congested, with better separation between instruments. Complex sounds are startlingly accurate.
What could explain this? It's not likely to be the amount of feedback around the OPS which is identical (and ample!) in each case.
My guess is that TPC and TMC load the input stage differently. To the IPS, at audio frequencies, a TMC network looks exactly like a plain old Miller cap: it must be charged and discharged as the output slews, demanding currents up to 10s of microamps.
TPC draws orders of magnitude less current from the IPS.
Given all of Self's work showing the importance of precise current balance in the input stage pair to reduce distortion, it's intuitive that we should load the IPS with high impedance to draw as little current as we can. Miller compensation and TMC fail equally at this.
If that's the problem, why does the simulator report ultralow distortion for TMC amps? Is the simulator blind to the distortion effects of an unbalanced IPS? More research is needed...
It's possible that the TMC version of this amp was just marginal in some way, and its TPC incarnation happened to be healthier. Maybe a healthy TMC amp would sound identical to its TPC doppelganger.
I took a stereo TMC amp, and swapped one channel to TPC. The compensation networks were set up to produce nearly identical loopgain bode plots on the sim.
TPC sounds clearer, less congested, with better separation between instruments. Complex sounds are startlingly accurate.
What could explain this? It's not likely to be the amount of feedback around the OPS which is identical (and ample!) in each case.
My guess is that TPC and TMC load the input stage differently. To the IPS, at audio frequencies, a TMC network looks exactly like a plain old Miller cap: it must be charged and discharged as the output slews, demanding currents up to 10s of microamps.
TPC draws orders of magnitude less current from the IPS.
Given all of Self's work showing the importance of precise current balance in the input stage pair to reduce distortion, it's intuitive that we should load the IPS with high impedance to draw as little current as we can. Miller compensation and TMC fail equally at this.
If that's the problem, why does the simulator report ultralow distortion for TMC amps? Is the simulator blind to the distortion effects of an unbalanced IPS? More research is needed...
It's possible that the TMC version of this amp was just marginal in some way, and its TPC incarnation happened to be healthier. Maybe a healthy TMC amp would sound identical to its TPC doppelganger.
Another benefit of TPC: it can be used in a preamp gain stage with only a VAS and no separate OPS, where TMC is impractical. Again, the value is in presenting an easier load to the IPS than miller compensation.
Another benefit of TPC: if the compensation network capacitors are nonlinear, TMC makes the IPS drive that nonlinear capacitance and TPC doesn't. I modeled a nonlinear cap in spice, used it in the compensation networks. It severely impacts the distortion performance of a TMC amp and doesn't touch the performance of a TPC amp at all.
Maybe that's academic; C0G/NP0 caps are supposed to be very linear over a wide voltage range. But if you'd rather not rely on that, or if you only have crappy X7R caps available, go for TPC.
For that matter, ceramics are microphonic aren't they? It might be a good idea to spec a film cap facing the IPS for either TMC or TPC. Tiny error currents at the IPS output can corrupt the whole amp performance.
Another benefit of TPC: if the compensation network capacitors are nonlinear, TMC makes the IPS drive that nonlinear capacitance and TPC doesn't. I modeled a nonlinear cap in spice, used it in the compensation networks. It severely impacts the distortion performance of a TMC amp and doesn't touch the performance of a TPC amp at all.
Maybe that's academic; C0G/NP0 caps are supposed to be very linear over a wide voltage range. But if you'd rather not rely on that, or if you only have crappy X7R caps available, go for TPC.
For that matter, ceramics are microphonic aren't they? It might be a good idea to spec a film cap facing the IPS for either TMC or TPC. Tiny error currents at the IPS output can corrupt the whole amp performance.
The benefit of TPC for the input stage (which is not present in TMC), is due to TPC increasing the overall open loop gain of the amplifier (A_OL and not LG is what matters here) in the frequency region before the zero (i.e. in the flat gain and two pole region). This increased gain reduces the differencial signal across the input stage VD=VOUT/A_OL which keeps it in the linear range of the input stage for a larger frequency range.
Does the effect equate to lower loading of the IPS? It does, but that is a consequence rather than the main intent of TPC. So I would not think about TPC this way.
Regarding the benefit of TPC in the pre-amp you are correct, with no OPS you cannot do TMC.
Finally, if you use crappy capacitors for compensation you will exhbit distortion since the voltage signal is essentially changing the loop gain of the amplifier over frequency due to the voltage coefficient of the capacitor. Is TPC more forgiving? yes. Is TPC not affected at all? No. I think in your case, something else now dominates the distortion.
Distortion is an interesting since it sums up in an RSS fassion (SQRT(x1^2 + X2^2+...)). So it is a winner takes all type deal, what you see is the largest one, so if the capacitor distortion fell below the distortion of some other artifact, you won't see it.
"Tiny error currents at the IPS output can corrupt the whole amp performance". Every distortion artifact that is inherent to the amplifier design can be equated to a load current from the input stage, even cross-over distortion believe it or not.
BTW, "TPC sounds clearer, less congested, with better separation between instruments. Complex sounds are startlingly accurate." This is a subjectivist comment.
The engineering comment is: "TPC reduces distortion artifacts at both the input and output stages..." to which we can add "while only TMC only focuses on the OPS distortion artifacts".
Regards, Sandro
Does the effect equate to lower loading of the IPS? It does, but that is a consequence rather than the main intent of TPC. So I would not think about TPC this way.
Regarding the benefit of TPC in the pre-amp you are correct, with no OPS you cannot do TMC.
Finally, if you use crappy capacitors for compensation you will exhbit distortion since the voltage signal is essentially changing the loop gain of the amplifier over frequency due to the voltage coefficient of the capacitor. Is TPC more forgiving? yes. Is TPC not affected at all? No. I think in your case, something else now dominates the distortion.
Distortion is an interesting since it sums up in an RSS fassion (SQRT(x1^2 + X2^2+...)). So it is a winner takes all type deal, what you see is the largest one, so if the capacitor distortion fell below the distortion of some other artifact, you won't see it.
"Tiny error currents at the IPS output can corrupt the whole amp performance". Every distortion artifact that is inherent to the amplifier design can be equated to a load current from the input stage, even cross-over distortion believe it or not.
BTW, "TPC sounds clearer, less congested, with better separation between instruments. Complex sounds are startlingly accurate." This is a subjectivist comment.
The engineering comment is: "TPC reduces distortion artifacts at both the input and output stages..." to which we can add "while only TMC only focuses on the OPS distortion artifacts".
Regards, Sandro
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Sandro, you're right. I've also been thinking about this terms of loop gain at the IPS. That's one of the starkest ways to describe it: would you rather have 30db or 60db of loopgain around the IPS at 20kHz? That's the difference between TMC and TPC.
TPC has roughly the same loop-gain plot regardless of where you probe. It's one loop. TMC has separate local and global loops, and you see different loopgain plots depending on where you probe. I used to think this complexity and modality was "the cost of doing business" if you wanted high feedback around the OPS but it's not.
The simulator says TMC can perform just as well. But the simulator thinks that the LTP transistors are identical; it thinks the degeneration resistors are perfectly matched. It thinks the current mirror is equally perfect. In a real circuit with "lumps and bumps" we can be happy to have the extra 30db of loopgain. The more we can reduce the signal excursions at IPS, the more linear it will be within its narrowed range of travel, probably.
TPC has roughly the same loop-gain plot regardless of where you probe. It's one loop. TMC has separate local and global loops, and you see different loopgain plots depending on where you probe. I used to think this complexity and modality was "the cost of doing business" if you wanted high feedback around the OPS but it's not.
The simulator says TMC can perform just as well. But the simulator thinks that the LTP transistors are identical; it thinks the degeneration resistors are perfectly matched. It thinks the current mirror is equally perfect. In a real circuit with "lumps and bumps" we can be happy to have the extra 30db of loopgain. The more we can reduce the signal excursions at IPS, the more linear it will be within its narrowed range of travel, probably.
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