So ... i built this headphone amp and one transistor pair BD139/BD140 were getting very hot ...
There were issues with a magnetic pulse when i turned the power supply off. So i rebuilt the rectifier/regulator and put all the extra diodes to handle transients or backward voltage (discharge). Seems a little better.
The previous board was making 15v and -15.2v - the new one making 15.1/-15.1. Now its not getting what seemed before like thermal runaway (which yes, i accept that's about heatsinks).
So my real question is if this imbalance between +/- could have made the biasing cause more current and therefore heat - or some kind of DC that went away.
All the base were showing about 0.65v +/-
Here is the circuit and I've got a bunch of caps for all the power rails to stabilize (and I run 100 ohms between power to opamp power just because I saw it on a circuit and when I do that my opamps seem to be very stable).
I also added another filter stage to the rectifier with 3.3 ohm between caps (which I imagine only has 0.3v drop across that resistor on each rail) to reduce the ripple. So maybe that helped with the pre-regulated power that is feeding the output amplification stage with the BD139/140. The original power supply suggested just had 1000uF filter caps now its got about 1200uF and then another 470uF after the resistor and more smaller poly caps. One would think it didn't matter due to the caps at on the headphone amp bypassing all the power rails all over the place ...
I don't have an oscilloscope so I'm limited to a multi-meter and my imagination. Any diagnostic suggestions
There were issues with a magnetic pulse when i turned the power supply off. So i rebuilt the rectifier/regulator and put all the extra diodes to handle transients or backward voltage (discharge). Seems a little better.
The previous board was making 15v and -15.2v - the new one making 15.1/-15.1. Now its not getting what seemed before like thermal runaway (which yes, i accept that's about heatsinks).
So my real question is if this imbalance between +/- could have made the biasing cause more current and therefore heat - or some kind of DC that went away.
All the base were showing about 0.65v +/-
Here is the circuit and I've got a bunch of caps for all the power rails to stabilize (and I run 100 ohms between power to opamp power just because I saw it on a circuit and when I do that my opamps seem to be very stable).
I also added another filter stage to the rectifier with 3.3 ohm between caps (which I imagine only has 0.3v drop across that resistor on each rail) to reduce the ripple. So maybe that helped with the pre-regulated power that is feeding the output amplification stage with the BD139/140. The original power supply suggested just had 1000uF filter caps now its got about 1200uF and then another 470uF after the resistor and more smaller poly caps. One would think it didn't matter due to the caps at on the headphone amp bypassing all the power rails all over the place ...
I don't have an oscilloscope so I'm limited to a multi-meter and my imagination. Any diagnostic suggestions
+-15VDC lines voltage directly influence the output transistors bias. But the influence is largely decreased by voltage divider 10k / (68R + VD). So the answer is - yes, +-15V voltage affects bias current. And, in general, for a similar circuits - bias current has to be checked and set after each change.
If runaway is too large then there is some problem: either heatsink is too small, either you need to increase 4R7 resistors value, either you need to decrease a bias current, or there is some parasitic oscillation.
You can make a simple HF-detector with a small signal diode and a cap (or two caps). Look at the internet how to do it.
If runaway is too large then there is some problem: either heatsink is too small, either you need to increase 4R7 resistors value, either you need to decrease a bias current, or there is some parasitic oscillation.
You can make a simple HF-detector with a small signal diode and a cap (or two caps). Look at the internet how to do it.
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