Abnormal Clipping Need Help~~~

[elvischan917]

Hello everyone!

I just built this circuit and get this kind of abnormal clipping, can someone tell me what's wrong? What should I check?
One more question, should the max output voltage be higher than this level?

Supply Voltage just before clip is +/-47.5V

Schematic


Clipping Waveform


Waveform just before clip

[elvischan917]

Just thanks for your help in advance ^_^

[astankov]

Probably problem in your VAS. Decrease if possible the value of R1 - make it as low as possible or remove it. R3 value also must be decreased. I had the same problem in one schematic and this solve it.

[elvischan917]

Originally Posted by astankov

Probably problem in your VAS. Decrease if possible the value of R1 - make it as low as possible or remove it. R3 value also must be decreased. I had the same problem in one schematic and this solve it.

Hi astankov~

Would you mind to explain how R1 and R3 affect this circuit? Would decreasing values of these two causing other issues?

Thanks!
Alvis

[AndrewT]

649/669 are excellent driver transistors. They are not suitable where low current, high speed, low Cog, high hFE devices will allow better performance.
Don't use them in VAS, nor in Vbe multiplier.
You may need to increase Cdom, to account for the lower capacitance reflected through Q1

Q1 needs protection. Add a resistor between collector and ground.

124p ?
Is that 120nF?
Why won't 100nF or 150nF do in those decoupling locations?

The clipping shown reminds me of the odd behaviour of opamps that have their input taken outside the specification range. Some opamps reverse phase and other oddities.

[astankov]

When I was first time simulating this schematic - My new design or updated old one I was using 68R resistor in the emitter of the main VAS transistor (first picture) and I had perfect sine and square wave response but in clipping just like your problem but in negative half of the wave. In your case is in positive wave because of the different polarity of the transistors. I started searching the problem and found it - when you decrease the value of 68R resistor in the emitter of the main VAS problem started to disappear and it disappears completely when resistor is removed (second picture). That was my experience with this problem. I can't technically explain you why this happens only to point possible solution. You can try it or not this is your decision.

Regards!
Andrey.

[xianrenb]

Originally Posted by elvischan917

Hello everyone!

I just built this circuit and get this kind of abnormal clipping, can someone tell me what's wrong? What should I check?
One more question, should the max output voltage be higher than this level?

Supply Voltage just before clip is +/-47.5V

Schematic


Clipping Waveform


Waveform just before clip

I think Q3 and Q4 do not form a proper current mirror.
A resistor should be added between base and collector of Q3.

[AndrewT]

Q3 & Q4 are shown as a correct current mirror.
The trimmer VR @ 200r is higher than normal, but it will still work.

Have a look at what voltages and currents exist around the VAS & EF when clipping.
Compare those voltages and currents to what happens around the input LTP when clipping.

[elvischan917]

Originally Posted by astankov

When I was first time simulating this schematic - My new design or updated old one I was using 68R resistor in the emitter of the main VAS transistor (first picture) and I had perfect sine and square wave response but in clipping just like your problem but in negative half of the wave. In your case is in positive wave because of the different polarity of the transistors. I started searching the problem and found it - when you decrease the value of 68R resistor in the emitter of the main VAS problem started to disappear and it disappears completely when resistor is removed (second picture). That was my experience with this problem. I can't technically explain you why this happens only to point possible solution. You can try it or not this is your decision.

Regards!
Andrey.

Thanks for your help. I tried to short R1 but then there is an oscillation around 7MHz at the output of VAS. I did not check the final output yet. I will try it further then.

Thanks!

Alvis

[elvischan917]

Originally Posted by AndrewT

Q3 & Q4 are shown as a correct current mirror.
The trimmer VR @ 200r is higher than normal, but it will still work.

Have a look at what voltages and currents exist around the VAS & EF when clipping.
Compare those voltages and currents to what happens around the input LTP when clipping.

Hi Andrew,
Thanks for your advice! But what does EF and LTP means?
By the way, I have measured the positive supply voltage at clipping, please check and see any clues. Its absolute value is about 1~2V lower than the negative supply voltage, and, the negative side voltage supply waveform is not distorted.



Thanks!
Alvis