JLM Hybrid Op Amp

[Gearsix]

Hi, yesterday I have relized a couple of JLM Audio Hybrid op amp  because intrigued by the possibility to switch  them from class A to class AB
The circuits sound really good but I noticed that when they are switched to class A the transistors (BD139 and BD140) become really hot ... although they don't smoke
If I switch the circuit to class AB the transistors do not heat too
Someone has experienced the same thing?

 

[[silent:arts]]

Class A is very inefficient and produces a lot of heat. This is normal.

 

[JohnRoberts]

Hi, yesterday I have relized a couple of JLM Audio Hybrid op amp  because intrigued by the possibility to switch  them from class A to class AB
The circuits sound really good but I noticed that when they are switched to class A the transistors (BD139 and BD140) become really hot ... although they don't smoke
If I switch the circuit to class AB the transistors do not heat too
Someone has experienced the same thing?

Yes, that is exactly why they developed class AB.

The "A" in class AB is class A for part of the signal swing, so you can get most of the benefit for a small fraction of the wasted power.

JR

 

[Gearsix]

To avoid to damage the circuit (when I use it in class A) it may be helpful to put one heatsink, considering that to have the best headroom I am using a +/- 24 Volt power supply ?

 

[pucho812]

yes a heatsink and thermal grease would assit you there.  Depending on the package make sure you use  a pad to isolate it from the heatsink.

 

[Gearsix]

I have just put some Heatsink and now it seems to get better!

 

[PRR]

> become really hot ... although they don't smoke

They run near 0.6 Watts and 80 degrees C. This is within spec, though not by much. I am sure they will run many years this way. I agree that I like my parts a bit cooler.

 

[gswan]

Class A amplifier efficiency is usually less than 30%, so expect 70% of your power to end up as heat. This is because the driver transistor is biased on all the time.  With transformer coupling you can get up to around 40% efficiency.
Class AB is far better as the bias currents are a lot less, however distortion increases.

 

[joaquins]

...
Class AB is far better as the bias currents are a lot less, however distortion increases.

X-over distortion increases (starts to exist actually).

  Class AB have higher THD at low levels than class A (right over the class A operation of the class AB). Under that level it works just as a class A. At higher level the story is other, and at some point might not have higher THD figures than a similar class A.

  Biasing the class AB so they are A under normal conditions and goest to B operation under extreme peaks or loads might be a good compromise, if the option of adjusting for an specific application is possible. Internal discrete opamps could be tweaked for that. Remember that your IC in the hybrid application could be running AB already, so you wouldn't be pure class A even if running 1A bias on your output devices.

JS