Lm1875 stereo amplifier

[add]

i built a stereo amplifier on 2 LM 1875. WHEN I START THE AMPLIFIER IT WORKS ONLY FOR 15 SECONDS NICELY AND THEN SHUTSDOWN, WHEN I SWITCH IT OFF AND SWITCH ON AGAIN IT DOES THE SAME WHAT COULD BE THE PROBLEM

 

[mickeymoose]

Why are you shouting? What is the size of your heatsink? E

 

[purplepeople]

Heat sinks?

ensen.

 

[mickeymoose]

Yea, Bob Pease used old VW cylinder heads. Thermal resistance is supposed to be less than 0.5C/W
E

 

[mickeymoose]

Hey, that woul make for a great converation piece for the living room! An amp, VW cylinder heads left and right. Better talking about than that sofa you made out of that old whiskey barrel I helped you empty! E

 

[add]

is the problem because of heat sink

 

[lineup]

is the problem because of heat sink

It could be there is not enough heatsink to cool down.
There is a protection against overheat.

Otherwise check you have everything put together right.
Normaly LM1875 is easy to get to work.

What is your power supply? VDC?

 

[add]

LM 1875

when i switch on the circuit it works fine but after few seconds it goes dead ,
when i switch it off and switch on it immediately then also it's dead but i switch on after 5 - 10 seconds then it works finely but again same thing happens

i have attached a thick aluminum plate for heat sink

can the problem be because of slew rate of TL072 ic
(the slew rate is 13V/us ) or whether the circuit starts to oscillating.

 

[lineup]

It is nothing to do with TL072.

Are both channels doing the same?

I should checkout that the supply rails and ground are correctly attached.
LM1875 should take +/-21 Volt, I think.

Another thing that can switch off the amp is output shortcircuit or too heavy load.
Use one 8 Ohm or 10 Ohm power resistor as dummy load.
Checkout what you have connected to output pins.

 

[mickeymoose]

The pcb lay-out may be such that it causes oscillation. Show us your pcb lay-out and check the output of the amp with an oscilloscope. E

 

[Juergen Knoop]

why the 6V DC bias at IC1?

 

[add]

yes both the channels do the same
the speakers are of 4 ohm each

 

[Juergen Knoop]

...you shall not put any DC onto the inputs...
6V at the noninverting input, amplified by three, forces the TL072 faster into the supply voltage rails, than you can say ooups!
Remove R3 and R17 and see what happens!

 

[lineup]

REmove R3 and R17. They are both 270k

 

[add]

thanks Juergen Knoop , lineup both channels working but the
cap c4 has developed a bulge and that channel is dead now
before also this problem of cap occured, the voltage at ic TL072 at Vcc is around 9.6v but the other -Vee is fine i have checked the supply voltage +21/-21 both fine .
what was the reason to remove r3 and r17 can u explain to me.

 

[macboy]

thanks Juergen Knoop , lineup both channels working but the
cap c4 has developed a bulge and that channel is dead now
before also this problem of cap occured, the voltage at ic TL072 at Vcc is around 9.6v but the other -Vee is fine i have checked the supply voltage +21/-21 both fine .
what was the reason to remove r3 and r17 can u explain to me.

You probably installed C4 backwards, and the reverse voltage caused it to fail. It is also possible that it was just a bad component in the first place.

p.s. why are you double posting to here and to the solid state forum? Don't do that.

 

[add]

i will continue this thread in chip amps

 

[radio]

Hi all!
Where is the PCB for this amplifier, and where the test measurements, if any?
Thanks and cheers!