The LM1875 for best

[kokoriantz]

I will provide each channel its own regulator to be installed very near if not on board. The 50m ohm is wiring resistance. It requires only 10uF capacitor, both BDs are mounted on heatsink 1.5w each dissipation. I will add a dc detector by two opto couplers on the amp output, to trigger a scr and protect the speaker.
The output impedance is very uniform resistive.
42v supply.jpg

[kokoriantz]

Now adjusted, it flies high to 22Mhz. It needs a minimum 27p grounding the input.
COMPOSITE WB fr.JPG
I am dealing with the input coupling capacitor charge plop on start up. I must review all the start up mute.
As it is CFA, it also needs a dc servo.
Distortion estimation gives 0.00055% at mid power.

[kokoriantz]

Understanding the distortion estimation.
The CFA opamo now became a WB, wide band, amplifier has initially high open loop distortion. Of course the feedback decreases it but also the higher impedance of the negative input. sketch.JPG
This is the internal input stage in principle. The current generated by the difference of the inputs due to internal 100ohm and external negative impedance are harvested at collectors by current mirrors and by internal resistor transforms the differential current into voltage to be followed by unity gain diamond follower. The distortion is mainly due to non linear character of the base-emitters Ic/Vbe. If I apply a very high resistor in series with the 100ohm than the nonlinear character becomes less significant.
With 8K it measures at 1.5V out 0.02% only in open loop. The problem now, the power stage distortion of 0.01% to add, is no more negligible. If I increase the gain of the power from 9.2 to 20 lets say, the distortion becomes 0.02+0.02% but As I doubled the gain and deviding it by 8.5 I have extra NFB of 20/8.5=2.3 times and my total closed loop distortion is (0.02+0.02)/2.3=0.017%÷by the gain of the WB. In other words with gain of 20 I must multiply the measured by simulator distortion by 0.02/0.017=by 1.17. The sumulator is measuring 0.00018% hence my distortion neglecting noise issues is 0.00021%.

I am trying to limit the saturation. This is the output unlimited.saturated without.jpg
and this with limiter and a gain of 15.
saturated with diodes.jpg
As shown , already the power alone has bad saturation, with composite it becomes horrible. Also to note that this occurs in PBTL at 100w. My problem with diode limiters distortion becomes 100 times higher. I must find much sharper limiter than diodes.

[kokoriantz]

I found better to clip by feedback with zeners. By this any gain adjust is possible and it will clip at required output voltage determined by zeners and a less extent the series resistors. As it feedback it is possible also to provoke soft clipping starting from 80w to clipping.

The oscillograph shows the current in 100na scale drawn from the input, just at start of clipping.

20v clipping.JPG

[kokoriantz]

This is much simpler way of clipping. The Pfet has its drain on ground. I cannot clip it after the capacitor, as it clipps dissymetrical on first cycles due to capacitor charge .

[kokoriantz]

This is the best version I got. It will have four Zener diodes to determine the max power the user wants to limit. It is 90W, 50W,25w ,12W. The choiceof Pjfet is not yet determined.Must precise that it works in revers current.
The graph bellow shows the current the limiter pulling from the composite.
clipper3.JPG
I discovered from vast distortion curves of AD8008 on datasheet, This amp is designed to function at low impedance of 150ohm where it has it lowest distortion. The data is given for Mhz range. On simulator the lowest distortion about 50% lower than high impedance I have at 420 ohm.

[mjona]

Originally Posted by kokoriantz

Understanding the distortion estimation.
The CFA opamo now became a WB, wide band, amplifier has initially high open loop distortion. Of course the feedback decreases it but also the higher impedance of the negative input. Attachment 899249
This is the internal input stage in principle. The current generated by the difference of the inputs due to internal 100ohm and external negative impedance are harvested at collectors by current mirrors and by internal resistor transforms the differential current into voltage to be followed by unity gain diamond follower. The distortion is mainly due to non linear character of the base-emitters Ic/Vbe. If I apply a very high resistor in series with the 100ohm than the nonlinear character becomes less significant.
With 8K it measures at 1.5V out 0.02% only in open loop. The problem now, the power stage distortion of 0.01% to add, is no more negligible. If I increase the gain of the power from 9.2 to 20 lets say, the distortion becomes 0.02+0.02% but As I doubled the gain and deviding it by 8.5 I have extra NFB of 20/8.5=2.3 times and my total closed loop distortion is (0.02+0.02)/2.3=0.017%÷by the gain of the WB. In other words with gain of 20 I must multiply the measured by simulator distortion by 0.02/0.017=by 1.17. The sumulator is measuring 0.00018% hence my distortion neglecting noise issues is 0.00021%.

I am trying to limit the saturation. This is the output unlimited.Attachment 899248
and this with limiter and a gain of 15.
Attachment 899247
As shown , already the power alone has bad saturation, with composite it becomes horrible. Also to note that this occurs in PBTL at 100w. My problem with diode limiters distortion becomes 100 times higher. I must find much sharper limiter than diodes.

If you want say 20W into 8R multiply these together and find the square root and multiply that by root 2. That will give the peak positive and negative voltages you need to swing. That gives 17.9V peak or 35.8V peak to peak.

To avoid clipping you need higher voltage on your supply rails to allow for the voltage drops across the power transistors in the LM1875 chip.

This device has internal protection and it seems a better idea to increase the supply rails than having the dilemma of doing this by complex other means.

[kokoriantz]

The clipping problem has been resolved. It needs a Led indicator that triggers for half a second and find a suitable switching Pfet.
There are two issues that needs to be treated together, The charge of the input capacitor on start up and the DC servo that necessary for CFA. For DC servo I need a differential amplifier plus an integrator, similar to my error corrector. So, why not make it both functions with the same circuit. This is the subject simmering in my thoughts.

[kokoriantz]

Here is the differential servo installed. I adjusted 5Hz -3db. Despite the input WB has dissymetric inputs dc impedances to provoke offset, the servo brings it to 5uV on the output. By this the input capacitor loading high impedance, high quality non polarized capacitor can be used.
I can now simplify my complicated start/stop plop circuit.

[kokoriantz]

The OPA2134 is excellent but over 6$ from Mouser. It can be replaced by LM4562 for 1.6$ the output offset measures 5mV instead of 5uV.