HP amp circuit just for interest

[M Gregg]

I found this old circuit,

I remember building it years ago.
I have no idea where it came from..perhaps someone may know?
It was just drawn on a scrap of paper.
I thought it might be interesting for someone.
No coupling caps.
You can use it to drive cables as well (Buffer)

Regards
M. Gregg

 

[geraldfryjr]

Class A output stage with a 45ma. of bias current.

jer

 

[AndrewT]

45mA of bias current? Where to? Through the headphones?

Have I fallen asleep with my eyes open, again?

Add A DC blocking capacitor to protect the headphones.
Add a load so that the output transistor can flow some bias current.

 

[00940]

45mA of bias current? Where to? Through the headphones?

Have I fallen asleep with my eyes open, again?

Add A DC blocking capacitor to protect the headphones.
Add a load so that the output transistor can flow some bias current.

Through the 330r resistor which goes to -15V.

There should be no significant offset.

It's a very common design, but has way too much gain for most applications.

 

[mat02ah]

45mA of bias current? Where to? Through the headphones?

No. +15V -> BD135 -> 330 Ohms -> -15V.

The weak points are:

- (Very) prone to (V)HF oscillation. Add at least a base stopper resistor (~100 Ohms).

- Asymmetric clipping produces DC offset at clipping.

- Make sure the BD135 has high current gain (BD135-16)

- No DC protection for the headphones in case of component failure or DC at the input.

 

[M Gregg]

No. +15V -> BD135 -> 330 Ohms -> -15V.

The weak points are:

- (Very) prone to (V)HF oscillation. Add at least a base stopper resistor (~100 Ohms).

I can't remember which Op amp I used Lm833 possibly I remember trying a few different ones..and a small Heat Sink on the BD135...

I'm sure I had another version of this as well...

Regards
M. Gregg

 

[M Gregg]

Any ideas for possible improvement?

The second circuit uses a single rail.
I guess the DC coupled version would benefit from a current limit circuit.

Regards
M. Gregg

 

[AndrewT]

I can't remember which Op amp I used Lm833 possibly I remember trying a few different ones..and a small Heat Sink on the BD135...

I'm sure I had another version of this as well...

Regards
M. Gregg

Now I see a connection via 150r to Vee.
But the 330r looked to me as open ended.
Now I can see why I misread it.

 

[cbdb]

Shouldn't the cold side of the Hps be grounded? Where's the current go?