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The scenario went like this: I used to run this amp with +-20v DC, and it seemed to work fine except for a little bit of clipping. Now, I changed my mind to make it work with bigger voltage!!! I've increased the voltage to +-50V DC. After powering the amp, a nice smell got out but it was working!!! Now I didn't cut the power to experiment what actually happens. Funny isn't it? Anyway, so, after playing the amp for around a minute things got out of control with a little burst!!

Therefore I've opened the amp and found that the supply decoupling caps got fired along with a few small transistors and one Output transistor from one channel only!!! My amp got 4 channels!!!

So, I took out the burned parts and replaced only the decoupling caps. Problem arised when I powered it after replacing the damaged parts. The divider resistors in the diagram (marked red) are getting hot and funny smell is coming again. Now, these resistors have been changed when I opened the amp because they got burned out too.

I've checked all the components connected to that divider network but failed to find any visual fault.

What can cause the resistors getting too hot?

Any suggestions would be greatly appreciated!!! enter image description here


I've just replaced the 220 Ohms resistors with 1.2k. Still it's getting hot!!!

I think 1.2k won't hurt according to the formula!!

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    \$\begingroup\$ Problem arised when I powered it after replacing the damaged parts How sod you know you replaced all damaged parts? Or did you only replaced the visually damaged parts? \$\endgroup\$ – Huisman Jan 13 '20 at 20:02
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The resistors circled in your schematic are not potential dividers, they are potential dropping or voltage dropping resistors feeding voltage regulators formed by ZD1 and ZD2. If you feed them with 20 V and the Zeners are 15 V devices then the current through them will be given by \$ I = \frac {20 - 15}{220} = 22 \ \text {mA} \$ and the maximum power dissipated in them will be given by \$ P = VI = 5 \times 0.022 = 110 \ \text {mW} \$.

You have applied 50 V so \$ I = \frac {50 - 15}{220} = 160 \ \text {mA} \$ and the maximum power dissipated in them will be given by \$ P = VI = 35 \times 0.160 = 5.6 \ \text {W} \$. Note the drawing says the resistors are rated for 1 W.

Therefore I've opened the amp and found that the supply decoupling caps got fired ...

This shouldn't have come as a surprise to you. The engineer that designed the amplifier would have chosen components rated for a 20 V supply, not a 50 V supply.

What can cause the resistors getting too hot?

We've covered that.

Any suggestions would be greatly appreciated!

Yes. Gain understanding first. Learn how the circuits work, why the designer chose certain components, do the calculations and then test your modifications.

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    \$\begingroup\$ Your 5.6 mW should be 5.6 W. \$\endgroup\$ – Peter Bennett Jan 13 '20 at 20:19
  • \$\begingroup\$ MathJAX copy'n'paste error! Fixed, thanks, Peter. \$\endgroup\$ – Transistor Jan 13 '20 at 20:22
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You probably damaged ZD1 and/or some other components connected to +15V.

ZD1 is a zener which tries to make 15V from the input voltage.
With 20V input, R19 will draw \$ \frac{20V-15V}{220 \Omega} \$ = 23 mA. R19 will dissipate 220&Omega * 23 mA = 115 mW.
This 23 mA is drawn partially by the zener (does typically 5 mA) and for the rest by the components connected to +15V. If the Zener diode would draw all the 23 mA, it would dissipate at most 15V * 23mA = 351 mW.

With 50V input, R19 will draw \$ \frac{50V-15V}{220 \Omega} \$ = 159 mA. R19 will dissipate 220&Omega * 159 mA = 5.57 W (5.5 its rated power!!)
If in the normal situation (Vcc=20V) the components connected to +15V would draw all the 23 mA calculated above, the zener still had to draw the remaining current (159 mA - 23 mA =) 136 mA trying to maintain 15V, so, it has to dissipate 2 W, which it probably doesn't survive as it is twice its rated power.

If ZD1 fails open, all other components connected to this +15V will get damaged due to overvoltage as well.

What can cause the resistors getting too hot?

So, either ZD1 failed short and causes R19 to become very hot.
Or, ZD1 failed open and one or some other components connected to this +15V are damaged which causes R19 to draw too much current.

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