strange distortion, cap needed?

[qweets]

hi : )

i made up this little headphone amp circuit below. it sounds really nice, clean , low noise.
with lots of gain.
However, when i turn up the volume on the variable resistor. it gets louder and louder ( not distorted at all)
but then suddenly at max volume it becomes highly distorted , maybe DC had got in somehow? im not sure,
maybe a capacitor somewhere might help?

here is the circuit. i had to change a few things from the orignal, as i did not have the parts required, i replaced the transistors as well as a few caps values.
I added a resistor also.

i have posted the original below it too, for comparison

thanks for your help : )

here is the original

thanks : )

 

[(*steve*)]

quite possibly the amplifier is clipping as the amplitude of the output nears the supply rails (which it cannot exceed)

 

[qweets]

sounds interesting, steve : ) i tried to modifiy the circuit thinking along those lines.
in the first jpeg above, i added a 100k resistor inline with the variable in hope to limit its range, maybe this is what you mean?

 

[davenn]

why even take signal from the output transistors and feed it back to the input of Op-amp... all I can see is thats a recipe for disaster as you will get to a point when the feedback loop is too high in gain and the circuit will start to oscillate

looking at a few designs, it seems common for the feedback resistor from the output of the opamp to the input be much higher than what you have (560 Ohms) 5k6 or more seems more common.
Also you have no decoupling cap on the output of the opamp going into the transistor stage for the style of single rail PSU comments on various articles state that it is necessary

here's a cct variation for you to consider ....

note the changes where the * are


I'm sure steve will point out any screwups haha

Dave

 

[davenn]

The other variation you could consider, which seems also to be common, is to feed the input signal direct to the opamp and stick the gain control on the output of the opamp
The output of the gain control would of course go to the transistor stage
it doesnt have to be a TL081, can be any of the others in the list in your diagrams
Just note tho that the TL081 has Vcc on pin 7, not pin 8 as with most other opamps

cheers
Dave

 

[(*steve*)]

I'm sure steve will point out any screwups haha

You'll get some pretty evil crossover distortion there.

That was the purpose of placing the transistors within the feedback loop.

 

[davenn]

You'll get some pretty evil crossover distortion there.

That was the purpose of placing the transistors within the feedback loop.

Ahhhh ok
it just looked a little unusual to me, but not really my forte.
so if we restore the link to the output of the transistors should we really have some resistance in there to limit the amount of that feedback ?

Dave

 

[(*steve*)]

probably a better answer to all of these audio amplifier questions is something like the LM380.

This is really what they're designed for,

There are even lower powered varieties designed as headphone amplifiers (can't pull the part number off the top of my head at the moment -- LM386 perhaps?)

 

[qweets]

hello and thanks for your ideas.

I tried the adjustments , and a few variations , but had no luck , there was either distortion or the same problem.

i tried the tl071 idea a few days ago. but there was not enough gain/level.

i am trying to amplify line level signals , but some times the signals will be very quiet so im trying to have as much gain control as possible.

i shal look into the LM380. thank you : )