Class AB amp clipping at 55v under no load, help

[jxdking]

Did you verify your signal source can output clean 4Vp by its own?

 

[hyperaudio_121]

If it is not going over 55v something is wrong. Agree?
It can be a self oscillation that appears at 55v. Not likely as the output transistors dont get hot. Agree?
It can be a faulty component. Agree?
For instance if you have got a fake Q8. Where did you buy it? A 100v transistor can have a breakdown at 135v.
If you glue a coin as a cooler on a 2n 5551 it can do the job.

ok i'll try replacing Q8 it with MJE1032 and will check again ,,,
and cooling fan is already there for 2n5551

 

[hyperaudio_121]

Did you verify your signal source can output clean 4Vp by its own?

yes

 

[hyperaudio_121]

Then again, since it clips at no load, it’s probably not the power supply. I’m siding with Stig: You probably have a faulty component somewhere. The schematic is standard blameless style and the component values seem fine, so it would perform well!

its still confusing cuz amp is working fine ,, i used it for almost 5hr continously ,

 

[Mooly]

Ok. Have you checked your power supply? What happens if you put a scope on the rails? Does it bog down?If it’s running fine at 800W, that’s kind of the only place left to look.

@hyperaudio_121 have you done the check njswede suggested and looked at both the rails on a scope when it clips?

 

[jxdking]

Is the clipping symmetrical, or only at top/bottom?

 

[wahab]

Try to decrease R13 from 1k to 470R, this will increase the gain by 6dB and improve stability if ever the amp is oscillating,
a gain of only 23 is not cautious given the IPS current and the 68pF compensation capacitor, another possibility to increase the gain is to just increase the 22k R12 resistance to 47k, this would be a better solution since at full power a 22k return resistance
has to withstand quite a lot of dissipation.

 

[hyperaudio_121]

@hyperaudio_121 have you done the check njswede suggested and looked at both the rails on a scope when it clips?

yes

 

[hyperaudio_121]

Is the clipping symmetrical, or only at top/bottom?

its symmetrical but starting from positive

 

[jxdking]

I don’t find any information regarding K4793/K1837. What’s the voltage rating of them?
Just use MJE340/MJE350 instead.

 

[bogdan_borko]

You measured 55V, with DVM? If so, you are maybe measuring 55V RMS and that is 77.78V peak.
Your rail voltage is?

 

[hyperaudio_121]

I don’t find any information regarding K4793/K1837. What’s the voltage rating of them?
Just use MJE340/MJE350 instead.

its alternate of 2sc4793 and 2sa1837 ,,,,,,,, what about performance of mje340/350

 

[hyperaudio_121]

You measured 55V, with DVM? If so, you are maybe measuring 55V RMS and that is 77.78V peak.
Your rail voltage is?

its +- 80v

 

[bogdan_borko]

its +- 80v

And you measured 55V with what?

 

[njswede]

The suspense is killing me. Based on how close 77.55V is to 80 minus two diode drops, I’m betting on “multimeter in AC mode”. 😀

 

[hyperaudio_121]

with oscilloscope , 55v rms

 

[hyperaudio_121]

The suspense is killing me. Based on how close 77.55V is to 80 minus two diode drops, I’m betting on “multimeter in AC mode”. 😀

its oscilloscope

 

[njswede]

55V RMS = 77.55V peak. You don’t have a problem. That’s the voltage you’re supposed to see. Case closed!

 

[njswede]

Your driver and output devices form a Darlington pair, so you lose two diode drops. 80-1.2=78.8V, minus some losses to parasitics.

 

[hyperaudio_121]

55V RMS = 77.55V peak. You don’t have a problem. That’s the voltage you’re supposed to see. Case closed!

ohh got it , the torroid is 60-0-60 /1200va ,, peak is +- 80 ,,, I had forgotten that ±80V is the peak voltage, and when we calculate the RMS value, it comes out to be around 55V,,, thankks