Actually CFP is MUCH worse than EF2 for this. I've described a test which shows THD goes up to maybe 1%; all nasty crossover stuff for a minute or two, until the amp cools to 'normal' temperature. Bob Cordell has reported similar results.
That's not to say it is impossible to design CFP O/P without this behaviour .... but I haven't found any in real life.
There's another evil in CFP O/Ps ... at least as Self does them.
In one version of his early books, Self shows EF2 THD drops with level but CFP THD tends to increase at low level below 1W. (This is on top of the usual measured 'increase' due to S/N) I asked him to investigate this further for a later edition but got no answer
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@PMA - I realized that making the simulator lopsided would be easier than straightening the amplifier.
I adjusted the parameters to match your 23mV measurements in post #47. I found that I could replicate the distortion spectrum through the fifth harmonic by using 0.1 ohm and 0.34 ohm emitter resistors. This mimics NPN/PNP Hfe mismatch times source impedance in the amplifier.
And the optimum bias is... more is better.
The curves are for different peak amplitudes in volts.
I feel good about the simple model (this model has negative feedback).
Ed
I adjusted the parameters to match your 23mV measurements in post #47. I found that I could replicate the distortion spectrum through the fifth harmonic by using 0.1 ohm and 0.34 ohm emitter resistors. This mimics NPN/PNP Hfe mismatch times source impedance in the amplifier.
Code:
harmonic % dB
0 0.004641 -86.668312
1 99.992177 -0.000679
2 0.001541 -96.245626
3 0.000031 -130.180912
4 0.000289 -110.772811
5 0.000033 -129.681139
6 0.000104 -119.652462
7 0.000033 -129.573269
8 0.000042 -127.470751
9 0.000025 -132.000111
thd 0.001573%And the optimum bias is... more is better.
The curves are for different peak amplitudes in volts.
I feel good about the simple model (this model has negative feedback).
Ed
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Gee, and I thought that measuring real life amplifiers with real equipment actually meant something. My god! Decades utterly wasted by not knowing how to drive a simulator!
Give me a break!
Give me a break!
This is very positive. The distortion being due to Ebers-Moll is confirmed and I did not have to buy an expensive analyzer. 🙂
Ed
Ed
Actually Ed ... you do. Or at least borrow one.
Ebers-Moll is obviously correct, but there are so many other factors and assumptions made in simulations. A simulator is much more useful in an integrated IC, like an op amp. Outside of that you have too many factors to take into account.
I matched the parts in a Symasym. It was a "utility amplifier" I didn't expect huge things from. I also used different devices for outputs and drivers. My minimum THD point was 5 mA of bias current at 1 watt into 8 ohms. The crossover notch disappeared before that value, I built four, they were all consistent. Non-inductive wire wound resistors for emitters. Many amplifiers I see work the best from 10 to 30 mA of bias current. Some need more. Some designs are simply never happy, typically "designed by ear" models. Matching the outputs (multi-output) and drivers reduces crossover notches.
That is real life, measured performance. No assumptions, no "should be". Real life.
Ebers-Moll is obviously correct, but there are so many other factors and assumptions made in simulations. A simulator is much more useful in an integrated IC, like an op amp. Outside of that you have too many factors to take into account.
I matched the parts in a Symasym. It was a "utility amplifier" I didn't expect huge things from. I also used different devices for outputs and drivers. My minimum THD point was 5 mA of bias current at 1 watt into 8 ohms. The crossover notch disappeared before that value, I built four, they were all consistent. Non-inductive wire wound resistors for emitters. Many amplifiers I see work the best from 10 to 30 mA of bias current. Some need more. Some designs are simply never happy, typically "designed by ear" models. Matching the outputs (multi-output) and drivers reduces crossover notches.
That is real life, measured performance. No assumptions, no "should be". Real life.
I am okay with getting the first-order effect right and designing in a manner that minimizes all other effects. Audio is just an old hobby and I already have enough amplifiers. 🙂
ETA: Here is another check. The simulator needed a loop gain of 700 to produce the right absolute values. @PMA - What was the actual loop gain?
Ed
ETA: Here is another check. The simulator needed a loop gain of 700 to produce the right absolute values. @PMA - What was the actual loop gain?
Ed
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