DIY Class A Headphone Amp suggestion

Made series of measurements with no load (just soundcard, had to briefly short the output to remove dc), 10k load, 1k load and 50 ohm load.
Seen no difference in frequency response, but i see no rolloff anymore.
Observed slight distortion increase with lower load.
 

Attachments

  • 1000032825.jpg
    1000032825.jpg
    372.9 KB · Views: 65
  • Like
Reactions: Michelag
Thank you Adason, you have done a lot of heavy work and verified my simulations. I have never seen any rolloff with old caps; if any, particularly electrolytics, they just lose capacitance and go leaky.
I certainly see that loading is the greatest influence on performance, particularly the rise of all hamonic (though profile is unchanged) with increasing load. My own thoughts are that a HPA for 16R cans might be better done with a pmos slave, rather than bipolar. Then the master current is little changed over the range.


HD
 
Made series of measurements with no load (just soundcard, had to briefly short the output to remove dc), 10k load, 1k load and 50 ohm load.
Seen no difference in frequency response, but i see no rolloff anymore.
Observed slight distortion increase with lower load.
So what changed physically between rolloff and non-rolloff measurements? No rolloff without load? or new components?
 
So Adason, regardless of the roll-off, is what you hear acceptable or rejectable. Keep in mind that extending the frequency range will also increase the distortion higher up. Does your hearing still conform to your youth or is it just that the measurement hinders your ultimate enjoyment? My hearing is stuffed, I am seventy and listening to my signal generator, I cannot tell if there is anything above 9 kHz. The fact that my amps have a flat response between DC and daylight makes no difference at all. My question remains, if you cannot hear it, does it matter? I like music as much or even more now than when I was 16 years old.
 
  • Like
Reactions: widea
Well you have a task now, it is to find a solution to the amp roll-off. Normally it should not be difficult. One should investigate the GBP (Gain Bandwidth Product) looking at the whole schematic holistically. It may be that the BW is gain limited and requires using transistors that has higher hfe (not HFE). I will really be interested in your findings. Since I do not have access to an extensive lab like in the old days, it becomes difficult to theoretically tinker and subjectively correct things only by ear.
 
The phase inversion could be caused by the output capacitor and inductance of the headphones and form a tank oscillator which has very high Q. Try placing a series resistor and analyse the system again to establish which direction it is going. You may be better off looking at this on an oscilloscope not a spectrum analyser.
 
So what changed physically between rolloff and non-rolloff measurements? No rolloff without load? or new components?
Nothing changed. That's why it remains a mystery. Except early measurement vs later measurement. Nothing, absolutely nothing changed on the board.
I made the brick power supply and used it for few days.
But the measurements were done with original very well stabilized power supply, just like in first measurement. For consistency, plus it can be adjusted, so it allows measurement at any power supply voltage.
There was rolloff, now there is none. I can not explain it. Hence my theory.
Michelag, do you still measure rolloff?
 
  • Like
Reactions: Michelag